Identification of thyroxine-sulfate (T4S) in human serum and amniotic fluid by a novel T4S radioimmunoassay

Estrogen Sulfotransferase (SULT1E1): Its Molecular Regulation, Polymorphisms, and Clinical Perspectives

Estrogen sulfotransferase (SULT1E1) is a phase II enzyme that sulfates estrogens to inactivate them and regulate their homeostasis. This enzyme is also involved in the sulfation of thyroid hormones and several marketed medicines. Though the profound action of SULT1E1 in molecular/pathological biology has been extensively studied, its genetic variants and functional studies have been comparatively rarely studied. Genetic variants of this gene are associated with some diseases, especially sex-hormone-related cancers. Comprehending the role and polymorphisms of SULT1E1 is crucial to developing and integrating its clinical relevance; therefore, this study gathered and reviewed various literature studies to outline several aspects of the function, molecular regulation, and polymorphisms of SULT1E1.

Transport, Metabolism, and Function of Thyroid Hormones in the Developing Mammalian Brain

Ever since the discovery of thyroid hormone deficiency as the primary cause of cretinism in the second half of the 19th century, the crucial role of thyroid hormone (TH) signaling in embryonic brain development has been established. However, the biological understanding of TH function in brain formation is far from complete, despite advances in treating thyroid function deficiency disorders. The pleiotropic nature of TH action makes it difficult to identify and study discrete roles of TH in various aspect of embryogenesis, including neurogenesis and brain maturation. These challenges notwithstanding, enormous progress has been achieved in understanding TH production and its regulation, their conversions and routes of entry into the developing mammalian brain. The endocrine environment has to adjust when an embryo ceases to rely solely on maternal source of hormones as its own thyroid gland develops and starts to produce endogenous TH. A number of mechanisms are in place to secure the proper delivery and action of TH with placenta, blood-brain interface, and choroid plexus as barriers of entry that need to selectively transport and modify these hormones thus controlling their active levels. Additionally, target cells also possess mechanisms to import, modify and bind TH to further fine-tune their action. A complex picture of a tightly regulated network of transport proteins, modifying enzymes, and receptors has emerged from the past studies. TH have been implicated in multiple processes related to brain formation in mammals-neuronal progenitor proliferation, neuronal migration, functional maturation, and survival-with their exact roles changing over developmental time. Given the plethora of effects thyroid hormones exert on various cell types at different developmental periods, the precise spatiotemporal regulation of their action is of crucial importance. In this review we summarize the current knowledge about TH delivery, conversions, and function in the developing mammalian brain. We also discuss their potential role in vertebrate brain evolution and offer future directions for research aimed at elucidating TH signaling in nervous system development.

A Homogeneous Time-Resolved Fluorescence Immunoassay Method for the Measurement of Compound W

OBJECTIVE

Using compound W (a 3,3'-diiodothyronine sulfate [T2S] immuno-crossreactive material)-specific polyclonal antibodies and homogeneous time-resolved fluorescence immunoassay assay techniques (AlphaLISA) to establish an indirect competitive compound W (ICW) quantitative detection method.

METHOD

Photosensitive particles (donor beads) coated with compound W or T2S and rabbit anti-W antibody were incubated with biotinylated goat anti-rabbit antibody. This constitutes a detection system with streptavidin-coated acceptor particle. We have optimized the test conditions and evaluated the detection performance.

RESULTS

The sensitivity of the method was 5 pg/mL, and the detection range was 5 to 10 000 pg/mL. The intra-assay coefficient of variation averages <10% with stable reproducibility.

CONCLUSIONS

The ICW-AlphaLISA shows good stability and high sensitivity and can measure a wide range of compound W levels in extracts of maternal serum samples. This may have clinical application to screen congenital hypothyroidism in utero.

Thyroid hormone transport and metabolism by organic anion transporter 1C1 and consequences of genetic variation

Organic anion transporting polypeptide (OATP) 1C1 has been characterized as a specific thyroid hormone transporter. Based on its expression in capillaries in different brain regions, OATP1C1 is thought to play a key role in transporting thyroid hormone across the blood-brain barrier. For this reason, we studied the specificity of iodothyronine transport by OATP1C1 in detail by analysis of thyroid hormone uptake in OATP1C1-transfected COS1 cells. Furthermore, we examined whether OATP1C1 is rate limiting in subsequent thyroid hormone metabolism in cells cotransfected with deiodinases. We also studied the effect of genetic variation in the OATP1C1 gene: polymorphisms were determined in 155 blood donors and 1192 Danish twins and related to serum thyroid hormone levels. In vitro effects of the polymorphisms were analyzed in cells transfected with the variants. Cells transfected with OATP1C1 showed increased transport of T4 and T4 sulfate (T4S), little transport of rT3, and no transport of T3 or T3 sulphate, compared with mock transfected cells. Metabolism of T4, T4S, and rT3 by cotransfected deiodinases was greatly augmented in the presence of OATP1C1. The OATP1C1-intron3C>T, Pro143Thr, and C3035T polymorphisms were not consistently associated with thyroid hormone levels, nor did they affect transport function in vitro. In conclusion, OATP1C1 mediates transport of T4, T4S, and rT3 and increases the access of these substrates to the intracellular active sites of the deiodinases. No effect of genetic variation on the function of OATP1C1 was observed.

Organic anion transporter 1B1: an important factor in hepatic thyroid hormone and estrogen transport and metabolism

Sulfation is an important pathway in the metabolism of thyroid hormone and estrogens. Sulfation of estrogens is reversible by estrogen sulfatase, but sulfation of thyroid hormone accelerates its degradation by the type 1 deiodinase in liver. Organic anion transporters (OATPs) are capable of transporting iodothyronine sulfates such as T4 sulfate (T4S), T3S, and rT3S or estrogen sulfates like estrone sulfate (E1S), but the major hepatic transporter for these conjugates has not been identified. A possible candidate is OATP1B1 because model substrates for this transporter include the bilirubin mimic bromosulfophthalein (BSP) and E1S, and it is highly and specifically expressed in liver. Therefore, OATP1B1-transfected COS1 cells were studied by analysis of BSP, E1S, and iodothyronine sulfate uptake and metabolism. Two Caucasian populations (155 blood donors and 1012 participants of the Rotterdam Scan Study) were genotyped for the OATP1B1-Val174Ala polymorphism and associated with bilirubin, E1S, and T4S levels. OATP1B1-transfected cells strongly induced uptake of BSP, E1S, T4S, T3S, and rT3S compared with mock-transfected cells. Metabolism of iodothyronine sulfates by cotransfected type 1 deiodinase was greatly augmented in the presence of OATP1B1. OATP1B1-Val174 showed a 40% higher induction of transport and metabolism of these substrates than OATP1B1-Ala174. Carriers of the OATP1B1-Ala174 allele had higher serum bilirubin, E1S, and T4S levels. In conclusion, OATP1B1 is an important factor in hepatic transport and metabolism of bilirubin, E1S, and iodothyronine sulfates. OATP1B1-Ala174 displays decreased transport activity and thereby gives rise to higher bilirubin, E1S, and T4S levels in carriers of this polymorphism.

Comparative thyroidology: thyroid gland location and iodothyronine dynamics in Mozambique tilapia (Oreochromis mossambicus Peters) and common carp (Cyprinus carpio L.)

In teleosts, the thyroid gland is mostly found in the subpharyngeal region. However, in several species thyroid follicles are found in, for example, heart, head kidney and kidney. Such heterotopic thyroid follicles are active, and considered to work in concert with the subpharyngeal thyroid. In Mozambique tilapia (Oreochromis mossambicus) thyroid activity is, indeed, restricted to the subpharyngeal region; in common carp (Cyprinus carpio) the functional endocrine thyroid is associated with renal tissues. The subpharyngeal follicles of carp comprise only 10% of the total thyroid tissue, and these follicles neither accumulate iodide nor synthesize or secrete thyroid hormones to a significant degree. Although the shape and size of carp subpharyngeal and renal follicles vary, the epithelial cell height of the thyrocytes and thyroxine immunoreactivity do not differ, which suggests that the activity of the carp subpharyngeal thyroid follicles is dormant. Differences in thyroid physiology between the two fish species were further assessed at the level of peripheral thyroid hormone metabolism. Carp clears plasma of thyroid hormones faster than tilapia does. Furthermore, a significant amount of conjugated thyroid hormones was observed in the plasma of tilapia, which was preceded by the occurrence of thyroid hormone conjugates in the subpharyngeal region and coincides with the appearance of conjugates in the surrounding water. Apparently, plasma thyroid hormone conjugates in tilapia originate from the thyroid gland and function in the excretion of thyroid hormones. Our data illustrate the variability in teleostean thyroidology, an important notion for those studying thyroid physiology.

Bexarotene-induced hypothyroidism: bexarotene stimulates the peripheral metabolism of thyroid hormones

OBJECTIVE

Therapy with the retinoid X receptor agonist bexarotene is associated with hypothyroidism caused by decreased pituitary TSH secretion. To evaluate the effects of bexarotene on peripheral thyroid hormone metabolism, we performed a study in athyreotic subjects on a fixed substitution dose with L-T4.

DESIGN

The design was an open prospective 6-wk intervention study.

METHODS

Ten athyreotic patients with pulmonary metastases of differentiated thyroid carcinoma received 6-wk redifferentiation treatment with 300 mg bexarotene/d. L-T4 doses were kept stable. Before and in the sixth week of therapy, serum levels of total T4, free T4 (FT4), T3, reverse T3 (rT3), and TSH were measured. To study nondeiodinase-mediated thyroid hormone degradation, serum levels of T4 sulfate (T4S) were measured. Recombinant human TSH was administered before and in the sixth week of bexarotene therapy.

RESULTS

Bexarotene induced profound decreases in total T4 (56% of baseline), FT4 (47%), T3 (69%), rT3 (51%), and T4S (70%) in all patients, whereas TSH levels were not affected. The T3/rT3 ratio increased by 43%, and the T4S/FT4 ratio increased by 48%. Serum TSH levels before and after recombinant human TSH were unaffected by bexarotene.

CONCLUSIONS

In the present study, we demonstrate that increased peripheral degradation of thyroid hormones by a nondeiodinase-mediated pathway contributes to bexarotene induced-hypothyroidism.

Compound W, a 3,3'-diiodothyronine sulfate cross-reactive substance in serum from pregnant women--a potential marker for fetal thyroid function

Compound W, a 3,3'-diiodothyronine sulfate (T2S) cross-reactive material in maternal serum, was found to be useful as a marker for fetal hypothyroidism. In the present report, we explored its biochemical properties and studied its concentrations in cord and in maternal serum obtained from various gestational periods and at term from different continents. Mean W concentrations, expressed as nmol/L T2S-equivalent, in maternal serum during gestation showed a moderate increase at 20-26 wk (1.57 nmol/L) and an accelerated increase to 34-40 wk (3.59 nmol/L). The mean serum level was relatively low in nonpregnant women (0.17 nmol/L). Compound W levels in cord and maternal serum at term were not significantly different among samples obtained from Taiwan compared with samples from the United States. The mean cord serum "corrected" (by hot acid digestion) concentrations of W were significantly higher than maternal serum concentrations at birth and were also higher in venous than in paired arterial samples, suggesting that the placenta may play a role in its production. We compared a total of 45 iodothyronine analogs by antibody, gel filtration, and HPLC chromatographic studies and found only one compound, N,N-dimethyl-T2S, that has close similarities to Compound W. Further studies are needed.

Human fetal and cord serum thyroid hormones: developmental trends and interrelationships

Thyroid hormone is essential for fetal and neonatal development in particular of the brain, but little is known about regulation of fetal thyroid hormone levels throughout human gestation. The purpose of this study was to clarify developmental trends and interrelationships among T(4), free T(4) (FT4), thyroxine-binding globulin (TBG), TSH, T(3), rT(3), and T(4) sulfate (T4S) levels in cord and fetal blood sera (n = 639, 15-42 wk gestation) and correlate infant levels (23-42 wk gestation) to maternal values (n = 428, 16-45 yr) and those of nonpregnant women (n = 233, 16-46 yr). In cord and fetal serum, T(4), T(3), and TBG levels increase with gestation until term; TSH, FT4, T4S, and rT(3) levels increase and peak in the late second/early third trimester and then decline to term; T(4)/TBG ratios increase until late second trimester and plateau to term. Term cord sera TSH, TBG, and all iodothyronine levels, except T(3), are higher than nonpregnant women. In the third trimester, cord serum FT4, TSH, rT(3), and T4S levels are also higher than corresponding maternal levels, but T(4), T(3), and TBG levels are lower than maternal values. The late second/early third trimester is a critical transition period in fetal thyroid hormone metabolism, which may be interrupted by preterm birth and contribute to postnatal thyroid dysfunction.

Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases

The goal of this review is to place the exciting advances that have occurred in our understanding of the molecular biology of the types 1, 2, and 3 (D1, D2, and D3, respectively) iodothyronine deiodinases into a biochemical and physiological context. We review new data regarding the mechanism of selenoprotein synthesis, the molecular and cellular biological properties of the individual deiodinases, including gene structure, mRNA and protein characteristics, tissue distribution, subcellular localization and topology, enzymatic properties, structure-activity relationships, and regulation of synthesis, inactivation, and degradation. These provide the background for a discussion of their role in thyroid physiology in humans and other vertebrates, including evidence that D2 plays a significant role in human plasma T(3) production. We discuss the pathological role of D3 overexpression causing "consumptive hypothyroidism" as well as our current understanding of the pathophysiology of iodothyronine deiodination during illness and amiodarone therapy. Finally, we review the new insights from analysis of mice with targeted disruption of the Dio2 gene and overexpression of D2 in the myocardium.

3,3'-Diiodothyronine sulfate excretion in maternal urine reflects fetal thyroid function in sheep

We have shown that there is significant fetal-to-maternal transfer of sulfated metabolites of thyroid hormone after fetal infusion of a pharmacologic amount of 3,3',5-triiodothyronine (T(3)) or sulfated T(3) in late pregnancy in sheep (Am J Physiol 277:E915, 1999). The transferred iodothyronine sulfoconjugate, i.e. 3,3'-diiodothyronine sulfate (T(2)S), of fetal origin appears in maternal sheep urine. The present study was carried out to assess the contribution of T(2)S of fetal origin to the urinary pool in ewes. Eighteen date-bred ewes (mean gestational age of 115 d) and their twin fetuses were divided into four groups. In group I (control, n = 5), both ewes (M) and their fetuses (F) were sham operated for thyroidectomy (Tx). In group II, the ewes (MTx, n = 4) and, in group III, the fetuses (FTx, n = 4) were subjected to Tx. In group IV (MTx.FTx, n = 5), both the ewe and fetus had Tx. After 10-12 d, fetal and/or maternal hypothyroidism were confirmed by serum thyroxine (<15 nmol/L) measurements. In addition, we infused radioactive T(3) without disturbing the T(3) pool in three singleton near-term fetuses and assessed the amount of radioactive iodothyronine that appeared in maternal urine (MU). After infusing [(125)I-3'],3,5-T(3) via fetal vein to the near-term normal fetuses, radioactive T(2)S was identified as the major metabolite in MU by HPLC and T(2)S-specific antibody. MU T(2)S excretion (pmol/mmol creatinine) was significantly reduced by FTx and MTx.FTx but not by MTx. In addition, positive correlations (p < 0.01) were found between MU T(2)S excretion and fetal serum thyroxine and T(3) concentrations but not with maternal serum thyroxine or T(3) levels. T(2)S of fetal origin contributes significantly to the MU pool.

Iodothyronine sulfotransferase activity in rat uterus during gestation

In developing mammals, we and others demonstrated that sulfation is an important pathway in the metabolism of thyroid hormone, and there is significant fetal-maternal transfer of sulfated iodothyronine. In the present study, we characterized a novel iodothyronine sulfotransferase (IST) in pregnant rat uterus. (125)I-labeled 3,3'-diiodothyronine (T(2)), T(3), rT(3), and T(4) were used as substrates with unlabeled 3'-phosphoadenosine-5'-phosphosulfate (PAPS) as the sulfate donor. Sulfated iodothyronine products were separated by Sephadex LH-20 column and further identified on reverse phase HPLC. We measured IST activity in pregnant rat uterus by incubating 1 microM substrate, 50 microM PAPS, and 50 microg cytosol protein, pH 7.2, 30 min at 37 degrees C. The results show that the substrate preference of the uterine IST activity is: T(2 )> rT(3 )> T(3)> T(4); the pH optimum is 6.0 for T(2). The K(m) and V:(max) (for gestational day 21 uterus) for T(2) are 0.62 microM and 3466 pmol/mg protein/h, respectively; for PAPS the values are 2.6 microM and 1523 pmol/mg protein/h, respectively. During pregnancy, the total activities exhibit a U-shaped curve with minimum activity at day 13 of gestation; while a thermostable activity increases significantly near term. In summary, there is significant uterine IST that varies during pregnancy. The role of this uterine sulfotransferase activities in regulating the bioavailability of thyroid hormone in the developing fetus remains to be elucidated.

Fetal-to-maternal transfer of 3,3',5-triiodothyronine sulfate and its metabolite in sheep

Earlier studies have shown that sulfoconjugation is a major pathway of thyroid hormone metabolism in fetal mammals. To assess the placental transfer of sulfoconjugates in the pregnant sheep model, we measured 3,3',5-triiodothyronine (T(3)) sulfate (T(3)S), 3, 3'-diiodothyronine sulfate (T(2)S), and T(3) concentrations in fetal serum and in maternal serum and urine after T(3)S infusion to the fetus (n = 5) or the ewe (n = 6). Maternal infusion of T(3)S did not increase fetal serum T(2)S, T(3)S, or T(3) concentrations. In contrast, fetal infusion of T(3)S produced significant increases in maternal serum T(2)S and T(3)S but not T(3) concentrations. Fetal T(3)S infusion also increased maternal urine excretion of T(3)S. However, the 4-h cumulative maternal urinary excretion of T(2)S and T(3)S after fetal T(3)S infusion was less than the excretion observed after fetal infusion of equimolar amounts of T(3) in our previous study. It is concluded that fetal serum T(2)S and T(3)S can be transferred to maternal compartments. However, compared with T(3), these sulfoconjugates may be less readily transferred.

Regulation of thyroid hormone metabolism during fetal development

Compared with adults, plasma T3 concentrations in the human fetus are decreased, whereas levels of rT3 and the different iodothyronine sulfates, T4S, T3S, rT3S and 3,3'-T2S, are increased. The low T3 and high rT3 concentrations reflect the preponderance of inner ring versus outer ring deiodinase activity due to high type III iodothyronine deiodinase (D3) expression in fetal tissues, such as liver and brain, the placenta, and perhaps also the uterus, in combination with still incomplete expression of hepatic type I iodothyronine deiodinase (D1) expression. In contrast to humans, D3 is hardly expressed in the fetal rat liver. However, high D3 expression is observed in the embryonic chicken liver which decreases dramatically towards the end of incubation, resulting in a marked increase in plasma T3. Thyroid hormone is essential for the development of the brain, in which local conversion of the prohormone T4 to the active hormone T3 by the type II iodothyronine deiodinase (D2) plays a very important role. In contrast to the rat, however, little is known about the ontogeny of D2 in different human brain areas. The cause of the high concentrations of sulfated iodothyronines in fetal plasma is unknown. In adults, the liver is an important site for the clearance of these conjugates, where they are rapidly degraded by D1. Although fetal human liver expresses significant D1 activity, clearance of iodothyronine sulfates may be defective due to the lack of transporters mediating their hepatic uptake. However, production of iodothyronine sulfates may also be increased in the human fetus, although the responsible sulfotransferases and their location remain to be identified. Sulfation may be a reversible pathway of thyroid hormone inactivation, depending on the recovery of free hormone by sulfatases. However, little is known at present about the characteristics and regulation of these enzymes in fetal human tissues. Further studies are required to increase our understanding of the tissue-specific and stage-dependent regulation of thyroid hormone bioactivity during human development.

Characterization of human iodothyronine sulfotransferases

Sulfation is an important pathway of thyroid hormone metabolism that facilitates the degradation of the hormone by the type I iodothyronine deiodinase, but little is known about which human sulfotransferase isoenzymes are involved. We have investigated the sulfation of the prohormone T4, the active hormone T3, and the metabolites rT3 and 3,3'-diiodothyronine (3,3'-T2) by human liver and kidney cytosol as well as by recombinant human SULT1A1 and SULT1A3, previously known as phenol-preferring and monoamine-preferring phenol sulfotransferase, respectively. In all cases, the substrate preference was 3,3'-T2 >> rT3 > T3 > T4. The apparent Km values of 3,3'-T2 and T3 [at 50 micromol/L 3'-phosphoadenosine-5'-phosphosulfate (PAPS)] were 1.02 and 54.9 micromol/L for liver cytosol, 0.64 and 27.8 micromol/L for kidney cytosol, 0.14 and 29.1 micromol/L for SULT1A1, and 33 and 112 micromol/L for SULT1A3, respectively. The apparent Km of PAPS (at 0.1 micromol/L 3,3'-T2) was 6.0 micromol/L for liver cytosol, 9.0 micromol/L for kidney cytosol, 0.65 micromol/L for SULT1A1, and 2.7 micromol/L for SULT1A3. The sulfation of 3,3'-T2 was inhibited by the other iodothyronines in a concentration-dependent manner. The inhibition profiles of the 3,3'-T2 sulfotransferase activities of liver and kidney cytosol obtained by addition of 10 micromol/L of the various analogs were better correlated with the inhibition profile of SULT1A1 than with that of SULT1A3. These results indicate similar substrate specificities for iodothyronine sulfation by native human liver and kidney sulfotransferases and recombinant SULT1A1 and SULT1A3. Of the latter, SULT1A1 clearly shows the highest affinity for both iodothyronines and PAPS, but it remains to be established whether it is the prominent isoenzyme for sulfation of thyroid hormone in human liver and kidney.

Ontogeny of iodothyronine deiodinases in human liver

The role of the deiodinases D1, D2, and D3 in the tissue-specific and time-dependent regulation of thyroid hormone bioactivity during fetal development has been investigated in animals but little is known about the ontogeny of these enzymes in humans. We analyzed D1, D2, and D3 activities in liver microsomes from 10 fetuses of 15-20 weeks gestation and from 8 apparently healthy adult tissue transplant donors, and in liver homogenates from 2 fetuses (20 weeks gestation), 5 preterm infants (27-32 weeks gestation), and 13 term infants who survived up to 39 weeks postnatally. D1 activity was determined using 1 microM [3',5'-125I]rT3 as substrate and 10 mM dithiothreitol (DTT) as cofactor, D2 activity using 1 nM [3',5'-125I]T4 and 25 mM DTT in the presence of 1 mM 6-propyl-2-thiouracil (to block D1 activity) and 1 microM T3 (to block D3 activity), and D3 activity using 10 nM [3,5-125I]T3 and 50 mM DTT, by quantitation of the release of 125I. The assays were validated by high performance liquid chromatography of the products, and kinetic analysis [Michaelis-Menten constant (Km) of rT3 for D1: 0.5 microM; Km of T3 for D3: 2 nM]. In liver homogenates, D1 activity was not correlated with age, whereas D3 activity showed a strong negative correlation with age (r -0.84), with high D3 activities in preterm infants and (except in 1 infant of 35 weeks) absent D3 activity in full-term infants. In microsomes, D1 activities amounted to 4.3-60 pmol/min/mg protein in fetal livers and to 170-313 pmol/min/mg protein in adult livers, whereas microsomal D3 activities were 0.15-1.45 pmol/min/mg protein in fetuses and <0.1 pmol/min/mg protein in all but one adult. In the latter sample, D3 activity amounted to 0.36 pmol/min/mg protein. D2 activity was negligible in both fetal and adult livers. These findings indicate high D1 and D3 activities in fetal human liver, and high D1 and mostly absent D3 activities in adult human liver. Therefore, the low serum T3 levels in the human fetus appear to be caused by high hepatic (and placental) D3 activity rather than caused by low hepatic D1 activity. The occasional expression of D3 in adult human liver is intriguing and deserves further investigation.

3,5,3'-Triiodothyronine (T3) clearance and T3-glucuronide (T3G) appearance kinetics in plasma of freshwater-reared male tilapia, Oreochromis mossambicus

Distribution and metabolism of the thyroid hormone 3,5, 3'-l-triiodothyronine (T3) were studied in several ways to gain insights into these processes in the warm water fish tilapia Oreochromis mossambicus. Trace doses of 125I-labeled T3 (T*3)1 were injected intraarterially, extraarterially, or intraperitoneally in freshwater-reared male tilapia to explore plasma clearance kinetic responses to these different input modalities. Multicompartmental analysis of the plasma clearance data indicated a kinetic distribution of T*3 much like that reported for the rat and human, with about 2% of total body T*3 in plasma, 5% in rapidly exchanging tissues such as kidney and liver, and 93% in slowly exchanging tissues such as muscle. However, plasma clearance rates (PCR, 5.37 mL/h . 100 g body wt) and plasma appearance rates (PAR3 = PCR x [T3] plasma = 36.3 ng/h . 100 g body wt) were quite different than these indices in rat and human and 5 to 50 times larger than values reported for rainbow trout. On a whole-body basis, normalized for body weight, the tilapia we studied produced and accumulated much more T3 than rat, human, or rainbow trout. Enzymatic and chromatographic analyses of the plasma clearance data samples indicated substantial production of labeled glucuronide, but not sulfate, conjugates of iodothyronines (TiG) of unknown origin appearing in plasma. The TiG appeared beginning a few hours postinjection, peaked at 6 hours, and yielded a predicted steady-state TiG level of 8.3% of the T3 level in plasma. In contrast, in published studies, no conjugates were detected in rainbow trout plasma from 2 to 24 h after iv injection of T*3, T*4, or reverse-T*3, although conjugates of all were present in bile. To our knowledge, although T3 and T4 sulfate conjugates are present in the sera of several mammals, this is the first quantification of iodothyronine glucuronides reported in blood of any species under normal conditions. This might have physiological significance for the tilapia, with T3G providing a reversible storage form of T3 in blood, as has been suggested for sulfate conjugates of T3 and T4 in blood of several mammals.

Urinary compound W in pregnant women is a potential marker for fetal thyroid function

OBJECTIVE

Previously we reported 3,3'-diiodothyronine sulfate-like material (compound W) in maternal serum, and studies suggest that compound W is derived from thyroid hormones of fetal origin. In this study we characterized gestational changes of urinary compound W concentrations to correlate with changes in serum concentrations.

STUDY DESIGN

Urinary samples were collected from 94 women at various gestational ages ranging from 3 to 40 weeks. Urinary compound W was first identified biochemically. The concentrations of compound W (adjusted for creatinine levels) were assessed by a 3,3'-diiodothyronine sulfate radioimmunoassay in ethanol extracts of urine samples.

RESULTS

Compound W increased to 88 +/- 1.4 pmol (of 3,3'-diiodothyronine sulfate equivalent)/mmol creatinine in urinary samples obtained from 26 women in the first trimester of pregnancy compared with 40 +/- 6.9 pmol/mmol creatinine in 10 nonpregnant women. Excretion of compound W increased further during the second and third trimesters: 171 +/- 17 (n = 18) and 434 +/- 26 (n = 50) respectively. In contrast, urinary 3,3',5-triiodothyronine sulfate concentrations measured by radioimmunoassay were similar during pregnancy to values in nonpregnant women.

CONCLUSIONS

Urinary compound W concentrations increase with the progression of normal pregnancy and correlate with the increase in serum levels. Random spot urine compound W concentrations, adjusted for creatinine levels, may be used in place of serum levels in conditions in which obtaining serum samples may be technically difficult, especially during population screening.

Characterization of thyroid hormone sulfotransferases

Sulfation is an intriguing pathway of thyroid hormone metabolism since it facilitates the degradation of the hormone by the type I deiodinase (D1). This study reports the preliminary characterization of iodothyronine sulfotransferase activities of rat and human liver cytosol and recombinant rSULT1C1 and hSULT1A1 isoenzymes. All these enzyme preparations catalyzed the sulfation of--in decreasing order of efficiency--3,3'-diiodothyronine (3,3'-T2) > 3,3',5-triiodothyronine (T3) approximately 3,3',5'-triiodothyronine (rT3) > thyroxine (T4). 3,3'-T2 sulfotransferase activity was found to be higher in male than in female rat liver, which has also been shown by others for the expression of rSULT1A1 and rSULT1C1. No sulfation of iodothyronines was observed with rSULT1A1. Different phenol derivatives were found to be potent inhibitors of the sulfation of 3,3'-T2 by native and recombinant sulfotransferases, with pentachlorophenol and 2,4,6-tribromophenol being the most potent. The inhibitions exerted by the different phenols on 3,3'-T2 sulfation by rSULT1C1 correlated better with the effects observed in male than with those in female liver. A strong correlation was also observed between the inhibition profiles of human liver cytosol and hSUL1T1A1. These results suggest that: (1) rSULT1C1 is an important isoenzyme for the sulfation of thyroid hormone in male rat liver; (2) another isoenzyme with similar properties, perhaps rSULT1B1, is responsible for thyroid hormone sulfation in female rat liver and may also contribute to this process in male rat liver; and (3) hSULT1A1 is an important isoenzyme for thyroid hormone sulfation in human liver.

Increased urinary excretion of sulfated 3,3',5-triiodothyronine in patients with nodular goiters receiving suppressive thyroxine therapy

Increased serum 3,3',5-triiodothyronine sulfate (T3S) levels have been detected in various pathophysiologic states. However, little is known about T3S concentrations in other biological fluids. By employing a highly sensitive, specific, and reproducible radioimmunoassay (RIA), we measured T3S in the serum and urine of 20 premenopausal women with benign nodular goiters before and after administration of thyroxine for 6 months (T4; 3.2 micrograms/kg/day). Serum T3 concentrations did not change significantly after treatment (2.0 vs. 1.7 nmol/L; p > 0.05). However, the mean serum T4 and free T4 concentrations were significantly higher after treatment (138 vs. 88 nmol/L and 28 vs. 17 pmol/L; p < 0.01, respectively). Serum thyroid stimulating hormone (TSH) levels were significantly reduced after T4 treatment (0.13 vs. 0.66 mU/L, p < 0.01) and the serum levels of T3S were significantly increased after treatment (82 vs. 45 pmol/L; p < 0.01). A good correlation was observed between increased serum T3S and T4 concentrations (r = 0.66; p < 0.001). The sulfoconjugate of T3 was significantly increased in creatinine-corrected urine after treatment (606 vs. 253 pmol/umol Cr.; p < 0.01). There was a significant correlation between increased creatinine-corrected urine T3S and increased serum free T4 (r = 0.65; p < 0.001). In summary, significant increases in serum and urine T3S levels were noted in T4-treated patients with subnormal serum TSH and borderline elevated T4. We thus conclude that the sulfation pathway may play a role in the homeostasis of thyroid hormone metabolism in T4-treated subjects with relative hyperthyroxinemia. In addition, the creatinine-corrected urine concentrations of T3S may serve as an index for the evaluation of T4-treated patients with elevated levels of T4.

Identification of 3,3'-T2S as a fetal thyroid hormone derivative in maternal urine in sheep

We measured 3,3'-diiodothyronine sulfate (T2S) in serum and urine (n = 5-6) obtained from euthyroid fetal (94-145 days of gestation, term = 150 days), newborn, and adult sheep and in serum and urine samples from ovine fetuses 13 days after total thyroidectomy conducted between 110 and 113 gestation days (n = 5). Sham-operated twin fetuses served as controls (n = 5). Mean serum T2S concentrations increased progressively from 94 days (74 ng/dl) to 130 days (420 ng/dl), decreasing thereafter to 145 days (197 ng/dl). T2S concentrations in fetal urine peaked at 110 days (117 ng/dl). In hypothyroid fetuses, mean serum and urine T2S were 60 and 53% of control values. To assess the possibility that the T2S in maternal serum/urine is derived from fetal serum 3,5,3'-triiodothyronine (T3), we measured T3, T3 sulfate (T3S), and T2S in fetal serum and in maternal serum and urine after bolus infusion of T3 to the fetus (n = 4). Additionally, T3, T3S, and T2S concentrations were measured in maternal serum and urine after T3 infusion to the maternal ewes (n = 4). Fetal T3 infusion rapidly increased fetal serum T3S and T2S. Maternal serum and urine T3S and T2S concentrations increased, whereas T3 concentrations remained unchanged. Maternal T3 infusion increased in serum and urine T3S and T2S levels, but the levels, relative to T3, were less than values measured after fetal T3 infusion. We conclude that T2S is a normal thyroid hormone metabolite in the ovine fetus and suggest that a major pathway of fetal T2S production is T3 to T3S to T2S.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of sulfation in thyroid hormone metabolism

The type I iodothyronine deiodinase (ID-I) in liver and kidney converts the prohormone thyroxine (T4) by outer ring deiodination (ORD) to bioactive 3,3',5-triiodothyronine (T3) or by inner ring deiodination (IRD) to inactive 3,3',5-triiodothronine (rT3), while it also catalyzes the IRD of T3 and the ORD of rT3, with the latter as the preferred substrate. Sulfation of the phenolic hydroxyl group blocks the ORD of T4, while it strongly stimulates the IRD of both T4 and T3, indicating that sulfation is an important step in the irreversible inactivation of thyroid hormone. This review summarizes recent studies concerning this interaction between sulfation and deiodination of iodothyronines, the characterization of iodothyronine sulfotransferase activities, the measurement of iodothyronine sulfates in humans and animals, and the possible physiological importance of iodothyronine sulfation.

Sulfate conjugates of iodothyronines in developing sheep: effect of fetal hypothyroidism

We recently showed that thyroxine sulfate (T4S) and 3,3',5-triiodothyronine sulfate (T3S) were major thyroid hormone metabolites in ovine fetuses and neonates. To further characterize the sulfation pathway in ovine fetuses, we measured 3,3',5'-triiodothyronine (rT3S) in serum and other body fluids in samples obtained from fetal (n = 23, 94-145 days of gestational age, term = 150 days), newborn (n = 6), and adult (n = 6) sheep. In addition, T3S, T4S, and rT3S levels were measured in tissue fluids and serum samples obtained from ovine fetuses 13 days after total thyroidectomy (Tx) conducted at gestational age of 110-113 days (n = 5). Sham-operated twin fetuses served as controls (n = 5). The relative order of mean rT3S concentration for various tissue fluids in fetuses were meconium > bile > serum > allantoic fluid > urine or amniotic fluid. Peak mean tissue fluid levels generally occurred at 110-130 days gestation. In hypothyroid fetuses, significant decreases in the mean serum concentrations of T4S and rT3S, but not T3S, were noted. The mean rT3S level also was decreased significantly in allantoic fluid, bile, and meconium, whereas T4S and T3S levels were reduced only in bile of the Tx fetuses. These data demonstrate that sulfation is a major pathway in thyroid hormone metabolism in both euthyroid and hypothyroid ovine fetuses.