Sample records for diiodothyronine

  1. Metabolism of 3,3'-diiodothyronine in rat hepatocytes: interaction of sulfation with deiodination

    Otten, M.H.; Hennemann, G.; Docter, R.; Visser, T.J.


    Production of 3,3'-diiodothyronine (3,3'-T2) is an important step in the peripheral metabolism of thyroid hormone in man. The rapid clearance of 3,3'-T2 is accomplished to a large extent in the liver. The mechanisms of this process have been studied in detail using monolayers of freshly isolated rat hepatocytes. After incubation with 3,(3'-/sup 125/I)T2, chromatographic analysis of the medium revealed two major metabolic routes: outer ring deiodination and sulfation. The authors recently demonstrated that sulfate conjugation precedes and in effect accelerates deiodination of 3,3'-T2. In media containing different serum concentrations the cellular clearance rate was determined by the nonprotein-bound fraction of 3,3'-T2. At substrate concentrations below 10(-8) M /sup 125/I- was the main product observed. At higher concentrations deiodination became saturated, and 3,3'-T2 sulfate (T2S) accumulated in the medium. Saturation of 3,3'-T2 clearance was found to occur only at very high (greater than 10(-6)M) substrate concentrations. The sulfating capacity of the cells exceeded that of deiodination by at least 20-fold. Deiodination was completely inhibited by 10(-4) M propylthiouracil or thiouracil, resulting in the accumulation of T2S while clearance of 3,3'-T2 was little affected. Hepatocytes from 72-h fasted rats showed a significant reduction of deiodination but unimpaired sulfation. Other iodothyronines interfered with 3,3'-T2 metabolism. Deiodination was strongly inhibited by 2 microM T4 and rT3 (80%) but little by T3 (15%), whereas the clearance of 3,3'-T2 was reduced by 27% (T4 and rT3) and 12% (T3). It is concluded that the rapid hepatic clearance of 3,3'-T2 is determined by the sulfate-transferring capacity of the liver cells. Subsequent outer ring deiodination of the intermediate T2S is inhibited by propylthiouracil and by fasting, essentially without an effect on overall 3,3'-T2 clearance.

  2. Radioimmunoassay for 3',5'-diiodothyronine. [/sup 125/I tracer technique

    Burman, K.D.; Wright, F.D.; Smallridge, R.C.; Green, B.J.; Georges, L.P.; Wartofsky, L.


    The present report describes a RIA for 3',5'-diiodothyronine (T/sub 2/) that can be performed on unextracted serum and which has a lower limit of detectability of 2 ng/dl. Cross-reactivity with other iodothyronines was negligible, except for rT/sub 3/ which began to demonstrate cross-reactivity when rT/sub 3/ levels were elevated to 180 ng/dl. Employing this RIA for T/sub 2/, we have determined that 83 healthy individuals had a mean (+-SE) serum T/sub 2/ concentration of 5.0 +- 0.3 ng/dl, thyrotoxic subjects (n = 12) had a mean T/sub 2/ level that was elevated to 10.8 +- 0.8 ng/dl, and each of 6 hypothyroid subjects had undetectable (<2 ng/dl) concentrations. Athyreotic patients (n = 8), receiving 0.4 mg T/sub 4/ daily, had serum T/sub 2/ concentrations of 15.0 +- 3.0 ng/dl. Fasting in obese subjects was associated with an increase in serum T/sub 2/ to 6.9 +- 0.6 ng/dl from a basal level of 4.4 +- 0.4 ng/dl in the fed state (P < 0.01). Despite the fact that rT/sub 3/ levels may be elevated in amniotic fluid and that rT/sub 3/ is expected to represent the major source from which extrathyroidal T/sub 2/ arises, T/sub 2/ levels were low in amniotic fluid, being undetectable (<2 ng/dl) in 9 of 19 samples; the mean (+-SE) T/sub 2/ concentration in the 10 detectable samples was 5.4 +- 1 ng/dl. These data indicate T/sub 2/ is a normal component of serum and that the majority of serum T/sub 2/ is probably derived from peripheral conversion. Furthermore, these observations suggest that situations associated with elevated rT/sub 3/ levels (e.g., thyrotoxicosis and fasting) may also have increased T/sub 2/ values.

  3. 3,5-di-iodothyronine stimulates tilapia growth through an alternate isoform of thyroid hormone receptor β1.

    Navarrete-Ramírez, Pamela; Luna, Maricela; Valverde-R, Carlos; Orozco, Aurea


    Recent studies in our laboratory have shown that in some teleosts, 3,5-di-iodothyronine (T2 or 3,5-T2) is as bioactive as 3,5,3'-tri-iodothyronine (T3) and that its effects are in part mediated by a TRβ1 (THRB) isoform that contains a 9-amino acid insert in its ligand-binding domain (long TRβ1 (L-TRβ1)), whereas T3 binds preferentially to a short TRβ1 (S-TRβ1) isoform that lacks this insert. To further understand the functional relevance of T2 bioactivity and its mechanism of action, we used in vivo and ex vivo (organotypic liver cultures) approaches and analyzed whether T3 and T2 differentially regulate the S-TRβ1 and L-TRβ1s during a physiological demand such as growth. In vivo, T3 and T2 treatment induced body weight gain in tilapia. The expression of L-TRβ1 and S-TRβ1 was specifically regulated by T2 and T3 respectively both in vivo and ex vivo. The TR antagonist 1-850 effectively blocked thyroid hormone-dependent gene expression; however, T3 or T2 reversed 1-850 effects only on S-TRβ1 or L-TRβ1 expression, respectively. Together, our results support the notion that both T3 and T2 participate in the growth process; however, their effects are mediated by different, specific TRβ1 isoforms.

  4. Radioimmunoassay for 3,3'-L-diiodothyronine (3,3'T/sub 2/)

    Burman, K.D.; Strum, D.; Dimond, R.C.; Djuh, Y.Y.; Wright, F.D.; Earll, J.M.; Wartofsky, L.


    The report describes the development of a radioimmunoassay for 3,3'-L-diiodothyronine (3,3'T/sub 2/) which may be performed on unextracted serum. Utilizing a specific antiserum to 3,3'-L-T/sub 2/-bovine serum albumin conjugates developed in rabbits, cross-reactivity was less than 0.5% with 3,5,3'-triiodothyronine (T/sub 3/) and 3,3',5'-triiodothyronine (reverse T/sub 3/) and less than 0.01% with thyroxine (T/sub 4/). Intra-assay variation averaged 2.9% and inter-assay variation was 7.8% and 18.5% when serum samples with 3,3'T/sub 2/ concentrations of 8 ng/dl and 12 ng/dl, respectively, were analyzed. Assay sensitivity was considered to be 6 ng/dl by statistical criteria. The data suggest that 3,3'T/sub 2/ circulates in the serum of normal individuals and tends to parallel serum concentrations of T/sub 3/, T/sub 4/ and rT/sub 3/ in various states of thyroid function.

  5. Radioimmunoassay for 3,5-diiodothyronine and evidence for dependence on conversion from 3,5,3'-triiodothyronine

    Pangaro, L.; Burman, K.D.; Wartofsky, L.; Cahnmann, H.J.; Smallridge, R.C.; O' Brian, J.T.; Wright, F.D.; Latham, K.


    The present report describes a RIA for 3,5-diiodothyronine (3,5T/sub 2/) which uses inner ring-labeled 3,5-(/sup 125/I)T/sub 2/ as the ligand and has a lower limit of detectability of 0.5 ng/dl. Cross-reaction was 0.14% with T/sub 3/, less than 0.001% with T/sub 4/, 1.2% with 3,3',5-triiodothyroacetic acid, and 6.1% with 3,5-diiodothyroacetic acid. No cross-reaction was detectable for iodothyronines within their physiological ranges. Intraassay variation ranged from 2.2 to 7.8%, and interassay variation ranged from 12.7 to 14%. The mean (+-SE) serum 3.5T/sub 2/ concentration in 70 normal subjects was 4.3 +- 0.2 ng/dl. The mean (+-SE) 3.5T/sub 2/ in 14 hyperthyroid patients was increased to 18.4 +- 2.3 ng/dl (P < 0.001), and all but 1 patient had an elevated level. In 10 hypothyroid patients the mean (+-SE) 3,5T/sub 2/ level was decreased to 1.4 +- 0.3 ng/dl (P < 0.001). In 4 patients, levels overlapped with the normal range. In 4 hypothyroid subjects treated with L-T/sub 1/, 3,5T/sub 2/ levels were normal, suggesting that the majority of 3,5T/sub 2/ originates from extrathyroidal conversion from T/sub 3/. Studies in fasting obese subjects demonstrated that serum 3,5T/sub 2/ (mean +- SE) levels fell from 3.4 +- 0.3 to 2.5 +- 0.7 ng/dl during fasting. This fall was significant (P < 0.001) and in parallel with the fall in T/sub 3/ levels of 182 +- 20 to 126 +- 12 ng/dl. In fasting subjects given 100 oral L-T/sub 3//day T/sub 3/ levels rose from 138 +- 11 to 362 +- 26 ng/dl. 3,5T/sub 2/ levels (corrected for cross-reaction and for contamination of oral T/sub 3/ with 3,5T/sub 2/) rose from 2.2 +- 0.7 to 6.4 +- 1.0 ng/dl. In fasting subjects given 25 oral L-T/sub 3//day, T/sub 3/ levels fell from 165 +- 5.1 to 139 +- 6.9 ng/dl. Corrected 3,5T/sub 2/ levels changed from 3.7 +- 0.4 to 2.5 +- 0.3 ng/dl. Neither change were significant.

  6. Serum 3,3'-L-diiodothyronine, a direct radioimmunoassay in human serum: method and clinical results. [/sup 125/I tracer technique

    Burger, A.; Sakoloff, C.


    A specific radioimmunoassay for 3,3'-diiodothyronine (T/sub 2/) is described which is capable of detecting as little as 1.3 ng/dl. The antiserum recognizes mainly T/sub 2/; biliary conjugates of T/sub 2/ bind slightly to the antibody. The intraassay and interassay coefficients of variation were, respectively, 5.7% and 13.1%. T/sub 2/ was detected in the serum of hypothyroid patients treated with triiodothyronine (T/sub 3/) and in euthyroid subjects treated with reverse triiodothyronine (rT/sub 3/). These results suggest that both T/sub 3/ and rT/sub 3/ are deiodinated to T/sub 2/. Serum concentrations of T/sub 2/ in normal subjects decreased with age. Between 20 and 40 years the mean concentration was 4.3 +- 2.0 ng/dl (2 SD), between 41 and 60 years it varied from 1.9 to 5.8 ng/dl (3.8 +- 0.3 ng/dl, SE) and in elderly subjects have 60 years concentrations varied from unmeasurable to 4 ng/dl (2.9 +- 0.4 ng/dl, SE). Low serum T/sub 2/ concentrations were found in anorexia nervosa (2.5 +- 0.3 ng/dl, SE). In hypothyroidism the serum concentrations were low or unmeasurable. As most of the hypothyroid subjects were elderly their serum T/sub 2/ concentrations overlapped with the low values found in the elderly euthyroid subjects. In classical hyperthyroidism serum T/sub 2/ concentrations were greatly increased (3.3 to 31 ng/dl (11.8 +- 2.7 ng/dl, SE) but in ''T/sub 3/ toxicosis'' the concentrations were only modestly increased (2.4 to 8.8 ng/dl, 5.2 +- 0.8 ng/dl, SE).

  7. 3,5-Diiodothyronine in vivo maintains euthyroidal expression of type 2 iodothyronine deiodinase, growth hormone, and thyroid hormone receptor beta1 in the killifish.

    García-G, C; López-Bojorquez, L; Nuñez, J; Valverde-R, C; Orozco, A


    Until recently, 3,5-diiodothyronine (3,5-T(2)) has been considered an inactive by-product of triiodothyronine (T(3)) deiodination. However, studies from several laboratories have shown that 3,5-T(2) has specific, nongenomic effects on mitochondrial oxidative capacity and respiration rate that are distinct from those due to T(3). Nevertheless, little is known about the putative genomic effects of 3,5-T(2). We have previously shown that hyperthyroidism induced by supraphysiological doses of 3,5-T(2) inhibits hepatic iodothyronine deiodinase type 2 (D2) activity and lowers mRNA levels in the killifish in the same manner as T(3) and T(4), suggesting a pretranslational effect of 3,5-T(2) (Garcia-G C, Jeziorski MC, Valverde-R C, Orozco A. Gen Comp Endocrinol 135: 201-209, 2004). The question remains as to whether 3,5-T(2) would have effects under conditions similar to those that are physiological for T(3). To this end, intact killifish were rendered hypothyroid by administering methimazole. Groups of hypothyroid animals simultaneously received 30 nM of either T(3), reverse T(3), or 3,5-T(2). Under these conditions, we expected that, if it were bioactive, 3,5-T(2) would mimic T(3) and thus reverse the compensatory upregulation of D2 and tyroid receptor beta1 and downregulation of growth hormone that characterize hypothyroidism. Our results demonstrate that 3,5-T(2) is indeed bioactive, reversing both hepatic D2 and growth hormone responses during a hypothyroidal state. Furthermore, we observed that 3,5-T(2) and T(3) recruit two distinct populations of transcription factors to typical palindromic and DR4 thyroid hormone response elements. Taken together, these results add further evidence to support the notion that 3,5-T(2) is a bioactive iodothyronine.

  8. 3'-L-monoiodothyronine: development of a radioimmunoassay and demonstration of in vivo conversion from 3',5'-diiodothyronine. [/sup 125/I tracer technique

    Smallridge, R.C.; Wartofsky, L.; Green, B.J.; Miller, F.C.; Burman, K.D.


    A sensitive, reproducible RIA for the measurement of 3'-L-monoiodothyronine (3'T/sub 1/) is described. Mean intra- and interassay coefficients of variation were 2.4% and 22.5%, respectively. Cross-reactivity with other iodothyronines was negligible, except for 3,3'-L-diiodothyronine (3,3'T/sub 2/) which started to demonstrate cross-reactivity when 3,3'T/sub 2/ levels were elevated above 35 ng/dl. Fifty percent displacement occurred when 500 pg 3,3'T/sub 2/ were added to the 3'T/sub 1/ assay. Employing this assay, 11 normal subjects and 7 pregnant women had serum 3'T/sub 1/ levels below the limits of detectability of the assay (<2.5 ng/dl), whereas 17 hyperthyroid patients had elevated levels of 3'T/sub 1/, with the mean (+-SD) values being 6.5 +- 3.0 ng/dl. Serum 3'T/sub 1/ levels were present in all cord sera measured (7.3 +- 2.3 ng/dl; n = 19), and the highest levels of 3'T/sub 1/ observed were in 38- to 40-week gestation amniotic fluid specimens (15.4 +- 8.4 ng/dl; n = 20). Compared to other iodothyronines, it seems that a relatively low proportion of 3'T/sub 1/ is bound to circulating proteins, as the mean percentage of dialyzable 3'T/sub 1/ in 12 normal subjects was 5.7 +- 1.2%. An oral dose of 3'T/sub 1/ (120 given to 2 euthyroid individuals resulted in peak serum levels of 28 ng/dl 2 h after ingestion. After iv administration of 3'5'T/sub 2/ to 2 athyreotic patients (1 hypothyroid and the other euthyroid on replacement T/sub 4/), 3'T/sub 1/ levels rose from undetectable levels to 20 ng/dl. It was concluded that 3'T/sub 1/ is routinely detectable in the serum of hyperthyroid but not normal individuals, and even higher levels are present in cord sera and amniotic fluid. Moreover, the study demonstrates that in vivo conversion of 3',5'T/sub 2/ to 3'T/sub 1/ may occur.

  9. Effect of 3,3',5-triiodothyronine and 3,5-diiodothyronine on progesterone production, cAMP synthesis, and mRNA expression of STAR, CYP11A1, and HSD3B genes in granulosa layer of chicken preovulatory follicles.

    Sechman, A; Pawlowska, K; Hrabia, A


    In vitro studies were performed to assess whether stimulatory effects of triiodothyronine (T3) on progesterone (P4) production in a granulosa layer (GL) of chicken preovulatory follicles are associated with 3',5'-cyclic adenosine monophosphate (cAMP) synthesis and mRNA expression of STAR protein, CYP11A1, and HSD3B. Effects of 3,5-diiodothyronine (3,5-T2) on steroidogenic function in these follicles were also investigated. The GL of F3 to F1 follicles was incubated in medium supplemented with T3 or 3,5-T2, LH, or forskolin (F), and a combination of each iodothyronine with LH or F. Levels of P4 and cAMP in culture media were determined by RIA. Expression of genes involved in P4 synthesis (ie, STAR protein, CYP11A1, and HSD3B) in the GL of F3 to F1 follicles incubated in medium with T3 or 3,5-T2 and their combination with LH was performed by real-time PCR. Triiodothyronine increased basal and LH- and F-stimulated P4 secretion by preovulatory follicles. The 3,5-T2 elevated P4 synthesis by F3, had no effect on F2 follicles, and diminished P4 production by the GL of F1 follicles. It had no effect on LH-stimulated P4 production; however, it augmented F-stimulated P4 production by F2 and F1 follicles. Although T3 did not affect basal and F-stimulated cAMP synthesis by the GL of preovulatory follicles, it increased LH-stimulated synthesis of this nucleotide. However, 3,5-T2 elevated F-stimulated cAMP synthesis in F3 and F2 follicles; it did not change basal and LH-stimulated cAMP production. Triiodothyronine decreased basal STAR and CYP11A1 mRNAs in F3 follicles, increased them in F1 follicles, and elevated HSD3B mRNA levels in F1 follicles. Triiodothyronine augmented LH-stimulated STAR, CYP11A1, and HSD3B mRNA levels in F2 and CYP11A1 in F1 follicles. However, T3 decreased LH-stimulated STAR and HSD3B mRNA levels in F1 follicles. The 3,5-T2 did not affect basal STAR and CYP11A1 mRNA expression in all investigated follicles; however, it decreased LH-stimulated STAR

  10. 3',5'-diiodothyronine in health and disease: studies by a radioimmunoassay

    Chopra, I.J.; Geola, F.; Solomon, D.H.; Maciel, R.M.B.


    An RIA has been developed for 3'5'-diiiodothyronine (3',5'-T/sub 2/) in unextracted serum. Interference in binding of radioactive 3',5'-T/sub 2/ to anti-3',5'-T/sub 2/ was minimized by using phosphate buffer and merthiolate. The detection threshold of the RIA was 2.5 ng/dl. Recovery of nonradioactive 3',5'-T/sub 2/ averaged 99%. T/sub 4/, T/sub 3/, and rT/sub 3/ cross-reacted with anti-3',5'-T/sub 2/ antibody 0.0025, < 0.0004, and 0.22%, respectively. The 3'-monoiodothyronine cross-reacted 1.7%. Mean serum 3',5'-T/sub 2/ concentrations in ng/dl were 2.4 in 53 normal subjects, 4.2 in 7 hypothyroid patients, 14.9 in 34 hyperthyroid patients, 13.5 in 25 patients with hepatic cirrhosis, and 14.3 in 31 newborns' cord blood serum. The values for the latter four groups were significantly different from normal. The serum 3',5'-T/sub 2/ concentration of 7.7 ng/dl in eight subjects in the third trimester of pregnancy did not differ from normal when serum T/sub 4/ and T/sub 3/ were elevated. Oral administration of 300 rT/sub 3/ to 9 normal subjects led to an increase in serum 3'5'-T/sub 2/ concentration of 45% at 1h. Total fasting in 3 obese subjects was associated with an increase in serum 3',5'-T/sub 2/ from 8.6 to 16.3 ng/dl at 6 to 8 days; rT/sub 3/ increased similarly, while T/sub 3/ decreased and T/sub 4/ did not change. Administration of dexamethasone to 4 hyperthyroid patients was associated with nearly parallel increases in serum 3',5'-T/sub 2/ and rT/sub 3/ and a decrease in T/sub 3/. The 3',5'-T/sub 2/ concentrations in amniotic fluids were 15.2 ng/dl at 15 to 20 weeks gestation and 5.8 ng/dl at 33 to 40 weeks. Pronase hydrolysates of 9 normal thyroid glands contained 350 4/ and 0.24 3',5'-T/sub 2//g wet wt. It was estimated that thyroidal secretion contributes < 1% of 3',5'-T/sub 2/ in serum of normal man.

  11. The effect of a diiodothyronine mimetic on insulin sensitivity in male cardiometabolic patients: a double-blind randomized controlled trial.

    Fleur van der Valk

    Full Text Available BACKGROUND AND AIMS: Obesity and its associated cardiometabolic co-morbidities are increasing worldwide. Since thyroid hormone mimetics are capable of uncoupling the beneficial metabolic effects of thyroid hormones from their deleterious effects on heart, bone and muscle, this class of drug is considered as adjacent therapeutics to weight-lowering strategies. This study investigated the safety and efficacy of TRC150094, a thyroid hormone mimetic. MATERIALS AND METHODS: This 4-week, randomized, placebo-controlled, double-blind trial was conducted in India and The Netherlands. Forty subjects were randomized at a 1:1 ratio to receive either TRC150094 dosed at 50 mg or placebo once daily for 4 weeks. Hyperinsulinemic euglycemic clamp and (1H-Magnetic Resonance Spectroscopy (MRS were performed before and after treatment. RESULTS: At baseline, subjects were characterized by markedly impaired hepatic and peripheral insulin sensitivity. TRC150094 dosed 50 mg once daily was safe and well tolerated. Hepatic nor peripheral insulin sensitivity improved after TRC150094 treatment, expressed as the suppression of Endogenous Glucose Production from 59.5 to 62.1%; p = 0.477, and the rate of glucose disappearance from 28.8 to 26.4 µmol kg(-1min(-1, p = 0.185. TRC150094 administration did not result in differences in fasting plasma free fatty acids from 0.51 to 0.51 mmol/L, p = 0.887 or in insulin-mediated suppression of lipolysis from 57 to 54%, p = 0.102. Also, intrahepatic triglyceride content was unaltered. CONCLUSION: Collectively, these data show that, in contrast to the potent metabolic effects in experimental models, TRC150094 at a dose of 50 mg daily does not improve the metabolic homeostasis in subjects at an increased cardiometabolic risk. Further studies are needed to evaluate whether TRC150094 has beneficial effects in patients with more severe metabolic derangement, such as overt diabetes mellitus and hypertriglyceridemia. TRIAL REGISTRATION: NCT01408667.

  12. 3,5-Diiodothyronine-mediated transrepression of the thyroid hormone receptor beta gene in tilapia. Insights on cross-talk between the thyroid hormone and cortisol signaling pathways.

    Hernández-Puga, Gabriela; Navarrete-Ramírez, Pamela; Mendoza, Arturo; Olvera, Aurora; Villalobos, Patricia; Orozco, Aurea


    T3 and cortisol activate or repress gene expression in virtually every vertebrate cell mainly by interacting with their nuclear hormone receptors. In contrast to the mechanisms for hormone gene activation, the mechanisms involved in gene repression remain elusive. In teleosts, the thyroid hormone receptor beta gene or thrb produces two isoforms of TRβ1 that differ by nine amino acids in the ligand-binding domain of the long-TRβ1, whereas the short-TRβ1 lacks the insert. Previous reports have shown that the genomic effects exerted by 3,5-T2, a product of T3 outer-ring deiodination, are mediated by the long-TRβ1. Furthermore, 3,5-T2 and T3 down-regulate the expression of long-TRβ1 and short-TRβ1, respectively. In contrast, cortisol has been shown to up-regulate the expression of thrb. To understand the molecular mechanisms for thrb modulation by thyroid hormones and cortisol, we used an in silico approach to identify thyroid- and cortisol-response elements within the proximal promoter of thrb from tilapia. We then characterized the identified response elements by EMSA and correlated our observations with the effects of THs and cortisol upon expression of thrb in tilapia. Our data show that 3,5-T2 represses thrb expression and impairs its up-regulation by cortisol possibly through a transrepression mechanism. We propose that for thrb down-regulation, ligands other than T3 are required to orchestrate the pleiotropic effects of thyroid hormones in vertebrates.

  13. Rapid sulfation of 3,3',5'-triiodothyronine in native Xenopus laevis oocytes

    E.C.H. Friesema (Edith); R. Docter (Roel); E.P. Krenning (Eric); M.E. Everts (Maria); G. Hennemann; T.J. Visser (Theo)


    textabstractSulfation is an important metabolic pathway facilitating the degradation of thyroid hormone by the type I iodothyronine deiodinase. Different human and rat tissues contain cytoplasmic sulfotransferases that show a substrate preference for 3,3'-diiodothyronin

  14. Cloning and characterization of type III iodothyronine deiodinase from the fish Oreochromis niloticus

    J.P. Sanders (Jo); S. van der Geyten; E. Kaptein (Ellen); V.M. Darras (Veerle); E.R. Kuhn; J.L. Leonard; T.J. Visser (Theo)


    textabstractType III iodothyronine deiodinase (D3) catalyzes the inner ring deiodination (IRD) of T4 and T3 to the inactive metabolites rT3 and 3,3'-diiodothyronine (3,3'-T2), respectively. Here we describe the cloning and characterization of complementary DNA (cDNA) co

  15. Effects of substitution and high-dose thyroid hormone therapy on deiodination, sulfoconjugation, and tissue thyroid hormone levels in prolonged critically ill rabbits

    Y. Debaveye (Yves); B. Ellger (Björn); L. Mebis (Liese); T.J. Visser (Theo); V.M. Darras (Veerle); G. van den Berghe (Greet)


    textabstractTo delineate the metabolic fate of thyroid hormone in prolonged critically ill rabbits, we investigated the impact of two dose regimes of thyroid hormone on plasma 3,3′-diiodothyronine (T2) and T4S, deiodinase type 1 (D1) and D3 activity, and tissue iodothyronine levels in liver and kidn

  16. Biochemical mechanisms of thyroid hormone deiodination.

    Kuiper, George G J M; Kester, Monique H A; Peeters, Robin P; Visser, Theo J


    Deiodination is the foremost pathway of thyroid hormone metabolism not only in quantitative terms but also because thyroxine (T(4)) is activated by outer ring deiodination (ORD) to 3,3',5-triiodothyronine (T(3)), whereas both T(4) and T(3) are inactivated by inner ring deiodination (IRD) to 3,3',5-triiodothyronine and 3,3'-diiodothyronine, respectively. These reactions are catalyzed by three iodothyronine deiodinases, D1-3. Although they are homologous selenoproteins, they differ in important respects such as catalysis of ORD and/or IRD, deiodination of sulfated iodothyronines, inhibition by the thyrostatic drug propylthiouracil, and regulation during fetal and neonatal development, by thyroid state, and during illness. In this review we will briefly discuss recent developments in these different areas. These have resulted in the emerging view that the biological activity of thyroid hormone is regulated locally by tissue-specific regulation of the different deiodinases.

  17. Metabolic effects of thyroid hormone derivatives.

    Moreno, Maria; de Lange, Pieter; Lombardi, Assunta; Silvestri, Elena; Lanni, Antonia; Goglia, Fernando


    The processes and pathways mediating the intermediary metabolism of carbohydrates, lipids, and proteins are all affected by thyroid hormones (THs) in almost all tissues. Particular attention has been devoted by scientists to the effects of THs on lipid metabolism. Among others, effects related to cholesterol, lipid handling, and cardiac performance have been the subject of study. Many reports are present in the literature concerning the calorigenic effect of THs, with most of them aimed at identifying the molecular basis of this effect. However, at the moment the mechanism(s) underlying the metabolic effects of THs remain to be elucidated. THs exert most of their effects though TH receptors (TRs). However, some effects of THs cannot be explained by a nuclear-mediated pathway, and recently an increasing number of nonnuclear actions have been described, which can provide a regulatory system of which the effects differ from those mediated on the transcriptional level by TRs. Some of the TH derivatives (naturally occurring metabolites and analogs) possess biological activities. TH-related biological effects have been described for physiological products such as tetraiodothyroacetic acid (Tetrac) and triiodothyroacetic acid (Triac) (via oxidative deamination and decarboxylation of thyroxine [T4] and triiodothyronine [T3] alanine chain), 3,3',5'-triiodothyronine (rT3) (via T4 and T3 deiodination), 3,3'-diiodothyronine (3,3'-T2) and 3,5-diiodothyronine (T2) (via T4, T3, and rT3 deiodination), and 3-iodothyronamine (T1AM) and thyronamine (T0AM) (via T4 and T3 deiodination and amino acid decarboxylation), as well as for TH structural analogs, such as 3,5,3'-triiodothyropropionic acid (Triprop), 3,5-dibromo-3-pyridazinone-l-thyronine (L-940901), N-[3,5-dimethyl-4-(4'-hydroxy-3'-isopropylphenoxy)-phenyl]-oxamic acid (CGS 23425), 3,5-dimethyl-4[(4'-hydroxy-3'-isopropylbenzyl)-phenoxy] acetic acid (GC-1), 3,5-dichloro-4[(4-hydroxy-3-isopropylphenoxy)phenyl] acetic acid (KB-141

  18. 3,5 Diiodo-L-Thyronine (T2 Does Not Prevent Hepatic Steatosis or Insulin Resistance in Fat-Fed Sprague Dawley Rats.

    Daniel F Vatner

    Full Text Available Thyroid hormone mimetics are alluring potential therapies for diseases like dyslipidemia, nonalcoholic fatty liver disease (NAFLD, and insulin resistance. Though diiodothyronines are thought inactive, pharmacologic treatment with 3,5- Diiodo-L-Thyronine (T2 reportedly reduces hepatic lipid content and improves glucose tolerance in fat-fed male rats. To test this, male Sprague Dawley rats fed a safflower-oil based high-fat diet were treated with T2 (0.25 mg/kg-d or vehicle. Neither 10 nor 30 days of T2 treatment had an effect on weight, adiposity, plasma fatty acids, or hepatic steatosis. Insulin action was quantified in vivo by a hyperinsulinemic-euglycemic clamp. T2 did not alter fasting plasma glucose or insulin concentration. Basal endogenous glucose production (EGP rate was unchanged. During the clamp, there was no difference in insulin stimulated whole body glucose disposal. Insulin suppressed EGP by 60% ± 10 in T2-treated rats as compared with 47% ± 4 suppression in the vehicle group (p = 0.32. This was associated with an improvement in hepatic insulin signaling; insulin stimulated Akt phosphorylation was ~2.5 fold greater in the T2-treated group as compared with the vehicle-treated group (p = 0.003. There was no change in expression of genes thought to mediate the effect of T2 on hepatic metabolism, including genes that regulate hepatic lipid oxidation (ppara, carnitine palmitoyltransferase 1a, genes that regulate hepatic fatty acid synthesis (srebp1c, acetyl coa carboxylase, fatty acid synthase, and genes involved in glycolysis and gluconeogenesis (L-pyruvate kinase, glucose 6 phosphatase. Therefore, in contrast with previous reports, in Sprague Dawley rats fed an unsaturated fat diet, T2 administration failed to improve NAFLD or whole body insulin sensitivity. Though there was a modest improvement in hepatic insulin signaling, this was not associated with significant differences in hepatic insulin action. Further study will be

  19. Inner ring deiodination of thyroxine and 3,5,3'-triiodothyronine by human fetal membranes

    Roti, E.; Fang, S.L.; Green, K.; Braverman, L.E.; Emerson, C.H.


    Indirect evidence, based on injection of thyroxine (T4) into the amniotic cavity of humans, and maternal thyroidectomy in the rat, suggests that fetal membranes might be capable of converting T4 to 3,3',5'-triiodothyronine (rT3) by virtue of inner ring iodothyronine deiodinase activity. The present study was undertaken to provide direct evidence that human fetal membranes contain inner ring iodothyronine deiodinase activity directed toward T4 and 3,5,3'-triiodothyronine (T3). Homogenates of human fetal membranes were incubated with 125I-labeled T4, rT3, and T3, and with stable T4. Conversion of 125(I)-T4 to 125(I)-rT3 was noted in chorion and amnion. 125I-T3 was converted to 125(I)-3,3'-diiodothyronine (T2) in chorion and amnion. 125(I)-rT3 was stable in fetal membranes under the incubation conditions employed. Time-, temperature-, pH-, and protein content-dependent conversion of stable T4 to rT3 was found in fetal membranes. Iodothyronine metabolism did not occur in the absence of dithiothreitol. These studies indicate that human fetal membranes contain an inner ring deiodinase enzyme. Because of its intimate contact with the amniotic cavity, this enzyme may generate a portion of the rT3 found in amniotic fluid.

  20. Rapid and selective inner ring deiodination of thyroxine sulfate by rat liver deiodinase

    Mol, J.A.; Visser, T.J.


    Previous studies have shown that the inner ring deiodination (IRD) of T3 and the outer ring deiodination (ORD) of 3,3'-diiodothyronine are greatly enhanced by sulfate conjugation. This study was undertaken to evaluate the effect of sulfation on T4 and rT3 deiodination. Iodothyronine sulfate conjugates were chemically synthetized. Deiodination was studied by reaction of rat liver microsomes with unlabeled or outer ring /sup 125/I-labeled sulfate conjugate at 37 C and pH 7.2 in the presence of 5 mM dithiothreitol. Products were analyzed by HPLC or after hydrolysis by specific RIAs. T4 sulfate (T4S) was rapidly degraded by IRD to rT3S, with an apparent Km of 0.3 microM and a maximum velocity (Vmax) of 530 pmol/min X mg protein. The Vmax to Km ratio of T4S IRD was increased 200-fold compared with that of T4 IRD. However, formation of T3S by ORD of T4S could not be observed. The rT3S formed was rapidly converted by ORD to 3,3'-T2 sulfate, with an apparent Km of 0.06 microM and a Vmax of 516 pmol/min X mg protein. The enzymic mechanism of the IRD of T4S was the same as that of the deiodination of nonsulfated iodothyronines, as shown by the kinetics of stimulation by dithiothreitol or inhibition by propylthiouracil. The IRD of T4S and the ORD of rT3 were equally affected by a number of competitive inhibitors, suggesting a single enzyme for the deiodination of native and sulfated iodothyronines. In conjunction with previous findings on the deiodination of T3S, these results suggest that sulfation leads to a rapid and irreversible inactivation of thyroid hormone.

  1. Halogenated phenolic contaminants inhibit the in vitro activity of the thyroid-regulating deiodinases in human liver.

    Butt, Craig M; Wang, Dongli; Stapleton, Heather M


    Halogenated contaminants, particularly brominated flame retardants, disrupt circulating levels of thyroid hormones (THs), potentially affecting growth and development. Disruption may be mediated by impacts on deiodinase (DI) activity, which regulate the levels of active hormones available to bind to nuclear receptors. The goal of this study was to develop a mass spectrometry-based method for measuring the activity of DIs in human liver microsomes and to examine the effect of halogenated phenolic contaminants on DI activity. Thyroxine (T4) and reverse triiodothyronine (rT3) deiodination kinetics were measured by incubating pooled human liver microsomes with T4 or rT3 and monitoring the production of T3, rT3, 3,3'-diiodothyronine, and 3-monoiodothyronine by liquid chromatography tandem mass spectrometry. Using this method, we examined the effects of several halogenated contaminants, including 2,2',4,4',5-pentabromodiphenyl ether (BDE 99), several hydroxylated polybrominated diphenyl ethers (OH-BDEs), tribromophenol, tetrabromobisphenol A, and triclosan, on DI activity. The Michaelis constants (K(M)) of rT3 and T4 deiodination were determined to be 3.2 ± 0.7 and 17.3 ± 2.3μM. The V(max) was 160 ± 5.8 and 2.8 ± 0.10 pmol/ protein, respectively. All studied contaminants inhibited DI activity in a dose-response manner, with the exception of BDE 99 and two OH-BDEs. 5'-Hydroxy 2,2',4,4',5-pentabromodiphenyl ether was found to be the most potent inhibitor of DI activity, and phenolic structures containing iodine were generally more potent inhibitors of DI activity relative to brominated, chlorinated, and fluorinated analogues. This study suggests that some halogenated phenolics, including current use compounds such as plastic monomers, flame retardants, and their metabolites, may disrupt TH homeostasis through the inhibition of DI activity in vivo.

  2. Effects of substitution and high-dose thyroid hormone therapy on deiodination, sulfoconjugation, and tissue thyroid hormone levels in prolonged critically ill rabbits.

    Debaveye, Yves; Ellger, Björn; Mebis, Liese; Visser, Theo J; Darras, Veerle M; Van den Berghe, Greet


    To delineate the metabolic fate of thyroid hormone in prolonged critically ill rabbits, we investigated the impact of two dose regimes of thyroid hormone on plasma 3,3'-diiodothyronine (T(2)) and T(4)S, deiodinase type 1 (D1) and D3 activity, and tissue iodothyronine levels in liver and kidney, as compared with saline and TRH. D2-expressing tissues were ignored. The regimens comprised either substitution dose or a 3- to 5- fold higher dose of T(4) and T(3), either alone or combined, targeted to achieve plasma thyroid hormone levels obtained by TRH. Compared with healthy animals, saline-treated ill rabbits revealed lower plasma T(3) (P=0.006), hepatic T(3) (P=0.02), and hepatic D1 activity (P=0.01). Substitution-dosed thyroid hormone therapy did not affect these changes except a further decline in plasma (P=0.0006) and tissue T(4) (P=0.04). High-dosed thyroid hormone therapy elevated plasma and tissue iodothyronine levels and hepatic D1 activity, as did TRH. Changes in iodothyronine tissue levels mimicked changes in plasma. Tissue T(3) and tissue T(3)/reverse T(3) ratio correlated with deiodinase activities. Neither substitution- nor high-dose treatment altered plasma T(2). Plasma T(4)S was increased only by T(4) in high dose. We conclude that in prolonged critically ill rabbits, low plasma T(3) levels were associated with low liver and kidney T(3) levels. Restoration of plasma and liver and kidney tissue iodothyronine levels was not achieved by thyroid hormone in substitution dose but instead required severalfold this dose. This indicates thyroid hormone hypermetabolism, which in this model of critical illness is not entirely explained by deiodination or by sulfoconjugation.

  3. Non-receptor-mediated actions are responsible for the lipid-lowering effects of iodothyronines in FaO rat hepatoma cells.

    Grasselli, Elena; Voci, Adriana; Canesi, Laura; Goglia, Fernando; Ravera, Silvia; Panfoli, Isabella; Gallo, Gabriella; Vergani, Laura


    Iodothyronines influence lipid metabolism and energy homeostasis. Previous studies demonstrated that 3,5-l-diiodothyronine (T(2)), as well as 3,3',5-L-triiodothyronine (T(3)), was able to both prevent and reverse hepatic steatosis in rats fed a high-fat diet, and this effect depends on a direct action of iodothyronines on the hepatocyte. However, the involvement of thyroid hormone receptors (TRs) in mediating the lipid-lowering effect of iodothyronines was not elucidated. In this study, we investigated the ability of T(2) and T(3) to reduce the lipid overloading using the rat hepatoma FaO cells defective for functional TRs. The absence of constitutive mRNA expression of both TRα1 and TRβ1 in FaO cells was verified by RT-qPCR. To mimic the fatty liver condition, FaO cells were treated with a fatty acid mixture and then exposed to pharmacological doses of T(2) or T(3) for 24 h. Lipid accumulation, mRNA expression of the peroxisome proliferator-activated receptors (PPAR-α, -γ, -δ) the acyl-CoA oxidase (AOX), and the stearoyl CoA desaturase (SCD1), as well as fuel-stimulated O(2) consumption in intact cells, were evaluated. Lipid accumulation was associated with an increase in triacylglycerol content, PPARγ mRNA expression, and a decrease in PPARδ and SCD1 mRNA expression. The addition of T(2) or T(3) to lipid-overloaded cells resulted in i) reduction in lipid content; ii) downregulation of PPARα, PPARγ, and AOX expression; iii) increase in PPARδ expression; and iv) stimulation of mitochondrial uncoupling. These data demonstrate, for the first time, that in the hepatocyte, the lipid-lowering actions of both T(2) and T(3) are not mediated by TRs.

  4. Do thyroid hormones function in insects?

    Davey, K G


    Earlier work demonstrated that phenoxy-phenyl compounds such as fenoxycarb and thyroxine mimicked the effects of JH III in causing a reduction in volume of the follicle cells of Locusta migratoria. While these compounds were only moderately effective, a derivative of thyroxine, 3,3',5-triiodothyronine (T3) was as effective as JH III, and T3 has been shown to bind to the same membrane receptor and activate the same pathway as JH III. The current paper shows that other thyroxine derivatives vary in activity. 3,3', 5'-Triiodothyronine (reverse T3) is inactive. 3,5-Diiodothyronine (T2) is more active than JH III, while its relatives (iodines at 3', 5' or at 3,3') are inactive. When follicles are exposed in vitro to rhodamine conjugated T3, the fluorescent compound can be seen to enter the cells and accumulate there: this process is inhibited by cycloheximide or by a temperature of 0 degrees C. The accumulation is antagonised by JH III but not JH I (which does not bind to the JH III membrane receptor) and by an antiserum raised against the putative membrane receptor protein. The action of T3, but not T2, is inhibited by 6-n-propyl-2-thiouracil or by aurothioglucose, both known to inhibit deiodinases. The activity of T3, but not of T2, increases with time of exposure to the follicle cells. These facts suggest that T3 enters the cells by receptor mediated endocytosis and is converted to a more active compound. Immunoreactivity to T3, but not thyroxine, can be detected in the haemolymph of locusts, and the titre varies slightly with the gonotrophic cycle. The food shows immunoreactivity for both thyroxine and T3. These findings suggest that thyroid hormones are ingested by locusts and have the potential to be used as hormonal signals in the control of egg production.

  5. Responses of skeletal muscle lipid metabolism in rat gastrocnemius to hypothyroidism and iodothyronine administration: a putative role for FAT/CD36.

    Lombardi, Assunta; De Matteis, Rita; Moreno, Maria; Napolitano, Laura; Busiello, Rosa Anna; Senese, Rosalba; de Lange, Pieter; Lanni, Antonia; Goglia, Fernando


    Iodothyronines such as triiodothyronine (T(3)) and 3,5-diiodothyronine (T(2)) influence energy expenditure and lipid metabolism. Skeletal muscle contributes significantly to energy homeostasis, and the above iodothyronines are known to act on this tissue. However, little is known about the cellular/molecular events underlying the effects of T(3) and T(2) on skeletal muscle lipid handling. Since FAT/CD36 is involved in the utilization of free fatty acids by skeletal muscle, specifically in their import into that tissue and presumably their oxidation at the mitochondrial level, we hypothesized that related changes in lipid handling and in FAT/CD36 expression and subcellular redistribution would occur due to hypothyroidism and to T(3) or T(2) administration to hypothyroid rats. In gastrocnemius muscles isolated from hypothyroid rats, FAT/CD36 was upregulated (mRNA levels and total tissue, sarcolemmal, and mitochondrial protein levels). Administration of either T(3) or T(2) to hypothyroid rats resulted in 1) little or no change in FAT/CD36 mRNA level, 2) a decreased total FAT/CD36 protein level, and 3) further increases in FAT/CD36 protein level in sarcolemma and mitochondria. Thus, the main effect of each iodothyronine seemed to be exerted at the level of FAT/CD36 cellular distribution. The effect of further increases in FAT/CD36 protein level in sarcolemma and mitochondria was already evident at 1 h after iodothyronine administration. Each iodothyronine increased the mitochondrial fatty acid oxidation rate. However, the mechanisms underlying their rapid effects seem to differ; T(2) and T(3) each induce FAT/CD36 translocation to mitochondria, but only T(2) induces increases in carnitine palmitoyl transferase system activity and in the mitochondrial substrate oxidation rate.

  6. A radioimmunoassay for measurement of thyroxine sulfate

    Chopra, I.J.; Santini, F.; Hurd, R.E.; Chua Teco, G.N. (Univ. of California Center for the Health Sciences, Los Angeles (United States))


    A highly sensitive, specific, and reproducible RIA has been developed to measure T[sub 4] sulfate (T[sub 4]S) in ethanol extracts of serum. rT[sub 3] sulfate (rT[sub 3]S) cross-reacted 7.1%, and T[sub 3]S cross-reacted 0.59% in the RIA; T[sub 4], T[sub 3], rT[sub 3] and 3,3[prime]-diiodothyronine cross-reacted 0.004% or less. The recovery of nonradioactive T[sub 4]S added to serum averaged 95%. The detection threshold of the RIA was 18 pmol/L. The coefficient of variation averaged 6.9% within an assay and 12% between assays. T[sub 4]S was bound by T[sub 4]-binding globulin and albumin in serum. The free fraction of T[sub 4]S in four normal sera averaged 0.06% compared to a value of 0.03% for T[sub 4] (P < 0.001). The serum concentration of T[sub 4]S was (mean [+-] SE) 19 [+-] 1.2 pmol/L in normal subjects, 33 [+-] 10 in hyperthyroid patients with Graves disease, 42 [+-] 15 in hypothyroid patients, 34 [+-] 6.9 in patients with systematic nonthyroidal illnesses, 21 [+-] 4.3 in pregnant women at 15-40 weeks gestation, and 245 [+-] 26 in cord blood sera of newborns; the value in the newborn was significantly different from normal (P < 0.001). Administration of sodium ipodate (Oragrafin; 3 g, orally) to hyperthyroid patients was associated with a transient increase in serum T[sub 4]S. The T[sub 4]S content of the thyroid gland was less than 1/4000th that of T[sub 4]. We conclude that (1) T[sub 4]S is a normal component of human serum, and its levels are markedly increased in newborn serum and amniotic fluid; and (2) the sulfation pathway plays an important role in the metabolism of T[sub 4] in man. 28 refs., 4 figs., 2 tabs.

  7. Divergent deiodination of thyroid hormones in the separated parts of the fetal and maternal placenta in pigs.

    Krysin, E; Brzezińska-Slebodzińska, E; Slebodziński, A B


    Previous work from this laboratory has shown that the thyroid gland of the fetal pig begins to function at about day 46-47 (0.40-0.415 fraction of gestational age). Sera from fetuses contain lower thyroxine (T4), 3,3',5-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) concentrations than maternal sera, except for about 2 weeks before term. The fetal T4 metabolism is dominated by the 5'-monodeiodinating activity (5'-MD). In the present study we measured the iodothyronines content, and the outer (5'-MD) and inner (5-MD) monodeiodinases activity, in homogenates of the placenta. The pig placenta, which is of the epitheliochorial type, was separated into the fetal and the maternal part. The concentrations of T4, T3 and rT3 were lower, and the deiodinating activity of 5'-MD and 5-MD higher, in the fetal than in the maternal placenta. The fetal placenta not only deiodinated more actively T4 to T3 and T4 to rT3, but degraded T3 to 3,3'-diiodothyronine (3,3'-T2) more actively than rT3 to 3,3'-T2. Such divergent deiodinating activity of T4 to T3, T3 to 3,3'-T2 and rT3 to 3,3'-T2 might favor establishing a relatively high and constant rT3 concentrations in fetal and maternal placentas, and a lower T3 in the fetal placenta. The inner ring deiodinating activity (excluding a day before parturition) was always more active in the fetal placenta, while the outer ring deiodinations varied in this respect, depending on the gestation stage. These results support the hypothesis that in the fetal pig, enzymatic deiodination of thyroid hormones forms a barrier which reduces transplacental passage of the hormones and that the fetal part of the placenta is the primary factor in the mechanism regulating the hormonal transfer. In spite of the presence of the barrier, there is an adequate maternal supply of thyroid hormones to the fetus in early gestation, which suggests that the enzymatic mechanism is influenced in some way by the thyroid status of the fetus.

  8. Increased plasma clearance rate of thyroxine despite decreased 5'-monodeiodination: study with a peroxisome proliferator in the rat

    Kaiser, C.A.; Seydoux, J.; Giacobino, J.P.; Girardier, L.; Burger, A.G.


    In euthyroid rats a 17-day treatment with nafenopin, a hypolipidemic agent and peroxisome proliferator, decreased serum total and free T4 concentrations to 32 +/- 5% and 62 +/- 8% (mean +/- SEM; n = 10), respectively, with no change in serum T3 and TSH concentrations. In methimazole-treated rats infused with 3 nmol T4/day/100 g BW, the nafenopin inhibitory effect was not significantly different from that in euthyroid rats. Nafenopin treatment had the following effects on peripheral T4 and T3 metabolism in euthyroid rats. The plasma clearance rate of T4 (PCR), which was measured by Alzet minipump infusion of tracer, was increased 2-fold (1.58 +/- 0.09 vs. 0.82 +/- 0.06 ml/h.100 g BW; P less than 0.001; n = 5), while the PCR of T3 was decreased (37.5 +/- 1.3 vs. 53.8 +/- 1.8; P less than 0.001; n = 5). The fecal clearance rate of radioactivity derived from T4 was increased 2-fold (1.93 +/- 0.10 vs. 0.77 +/- 0.07 ml/h.100 g BW), whereas the urinary clearance rate was not significantly modified. The 5'-deiodinase (5'D) activity, measured by deiodination of labeled rT3, was strongly inhibited in liver and kidney, not modified in brown fat and anterior pituitary, and increased in cerebral cortex. In methimazole-treated rats substituted with isopropyl-diiodothyronine only hepatic 5'D activity was decreased. It is concluded that the decrease in serum total and free T4, without alteration in serum T3 and TSH concentrations, resulting from nafenopin treatment is mainly due to changes in peripheral T4 and T3 metabolism, since it is also observed in T4-substituted animals. The increased PCR of T4 cannot be explained by an increase in deiodination activity, since the major 5'D pathways are inhibited after nafenopin treatment, and the urinary clearance rate is not modified. It can partly be explained by an increase in the fecal clearance rate of T4, which could be due to an increase in glucoronoconjugation.

  9. Alternate pathways of thyroid hormone metabolism.

    Wu, Sing-Yung; Green, William L; Huang, Wen-Sheng; Hays, Marguerite T; Chopra, Inder J


    The major thyroid hormone (TH) secreted by the thyroid gland is thyroxine (T(4)). Triiodothyronine (T(3)), formed chiefly by deiodination of T(4), is the active hormone at the nuclear receptor, and it is generally accepted that deiodination is the major pathway regulating T(3) bioavailability in mammalian tissues. The alternate pathways, sulfation and glucuronidation of the phenolic hydroxyl group of iodothyronines, the oxidative deamination and decarboxylation of the alanine side chain to form iodothyroacetic acids, and ether link cleavage provide additional mechanisms for regulating the supply of active hormone. Sulfation may play a general role in regulation of iodothyronine metabolism, since sulfation of T(4) and T(3) markedly accelerates deiodination to the inactive metabolites, reverse triiodothyronine (rT(3)) and T(2). Sulfoconjugation is prominent during intrauterine development, particularly in the precocial species in the last trimester including humans and sheep, where it may serve both to regulate the supply of T(3), via sulfation followed by deiodination, and to facilitate maternal-fetal exchange of sulfated iodothyronines (e.g., 3,3'-diiodothyronine sulfate [T(2)S]). The resulting low serum T(3) may be important for normal fetal development in the late gestation. The possibility that T(2)S or its derivative, transferred from the fetus and appearing in maternal serum or urine, can serve as a marker of fetal thyroid function is being studied. Glucuronidation of TH often precedes biliary-fecal excretion of hormone. In rats, stimulation of glucuronidation by various drugs and toxins may lead to lower T(4) and T(3) levels, provocation of thyrotropin (TSH) secretion, and goiter. In man, drug induced stimulation of glucuronidation is limited to T(4), and does not usually compromise normal thyroid function. However, in hypothyroid subjects, higher doses of TH may be required to maintain euthyroidism when these drugs are given. In addition, glucuronidates and