T3 Sulfate Research Articles

Investigation of hormone levels in postpartum psychosis

Objective: The etiology of postpartum psychosis (PP) remains unclear. In this study, we examined thyroid-stimulating hormone, free T4, free T3, cortisol, prolactin, follicle-stimulating hormone, luteinizing hormone (LH), and dehydroepiandrosterone sulfate (DHEAS) levels in PP. Method: The study included 23 patients who were hospitalized with the diagnosis of PP within the first 4 weeks after delivery and 30 age-matched healthy controls. Organic etiological factors were excluded. Blood samples were obtained from all participants at the same time of the day. In PP patients, blood samples were obtained within the first 24 h after hospitalization. Results: Mean ages of PP and control groups were 26.2 ± 5.5 and 27.6 ± 5.1 years, respectively. The frequency of vaginal delivery was significantly higher in the PP compared with the control group (p = 0.011). The fT3 levels were significantly lower in the PP compared with the control group, while the fT3 levels were within normal physiological limits (p = 0.034) and no significant differences were found with regard to other hormones. To examine the effect of breastfeeding on the results, the control group was further divided into two subgroups: breastfeeding (BFC) and non-breastfeeding (NBFC). The fT4 levels (within normal physiological limits) and prolactin levels were significantly higher in the PP compared with NBFC (p = 0.013 vs p = 0.007). LH levels were 3.11 ± 3.47 mIU/mL in the PP group, 1.48 ± 2.45 mIU/mL in BFC and 4.56 ± 3.69 mIU/mL in NBFC, but for the LH levels, the only significant difference was between the control groups (p = 0.027). Conclusion: The results of thyroid function tests in our study suggest a condition that develops impaired thyroid functions secondary to acute psychotic episode rather than an underlying thyroid disease. Comprehensive prospective studies, including follow-up data, may better explain the relationships between thyroid function and PP. In our study, there was no evidence for the possible role of DHEAS, prolactin, and LH hormones. However, changes in the hormone profile according to breastfeeding status suggest that the effects of breastfeeding on hormones may also be important.

Sustained Release T3 Therapy: Animal Models and Translational Applications.

The standard of care to treat hypothyroidism is daily administration of levo-thyroxine (LT4). This is based on the understanding that deiodinases can restore production of T3 and compensate for the small amounts of T3 that are normally produced by the thyroid gland. However, pre-clinical and clinical evidence indicating that deiodinases fall short of restoring T3 production is accumulating, opening the possibility that liothyronine (LT3) might have a role in the treatment of some hypothyroid patients. LT3 tablets taken orally result in a substantial peak of circulating T3 that is dissipated during the next several hours, which is markedly distinct from the relative stability of T3 levels in normal individuals. Thus, the effort to developing new delivery strategies for LT3, including slow release tablets, liquid formulations, use of T3-related/hybrid molecules such as T3 sulfate, poly-zinc-T3 and glucagon-T3, nanoparticles containing T3, subcutaneous implant of T3-containing matrices, and stem cells for de novo development of the thyroid gland. This article reviews these strategies, their applicability in animal models and translatability to humans.

Polychlorinated biphenyl exposure and deiodinase activity in young infants

BackgroundSeveral studies have shown effects of polychlorinated biphenyls (PCBs) on serum thyroid hormone levels in pregnant woman and their infants, while other studies did not find such effects. How PCBs might affect thyroid hormone metabolism, is still unclear. Potential mechanisms are direct influence on the thyroid gland, binding to thyroid binding proteins, increased excretion or metabolism of thyroid hormones by deiodinases or sulfatases. It is also not well known whether the effect on thyroid hormone levels is caused by PCBs themselves, or by their hydroxylated metabolites (OH-PCBs). ObjectiveTo determine the effects of perinatal exposure to PCBs and OH-PCBs on thyroid hormone levels in cord blood and in serum of newborn infants. MethodsIn a Dutch cohort of 100 mother-infant pairs, exposed to background PCB levels, correlations were assessed between 10 PCBs and 6 OH-PCBs in maternal blood during pregnancy and serum thyroxine (T4), T4 sulfate (T4S), triiodothyronine (T3), reverse T3 (rT3), thyroid-stimulating hormone (TSH) and thyroxine-binding globulin (TBG) levels in cord blood and in serum of three- and 18-month-old infants. We corrected for age of the mother, gestational age, gender and type of feeding. ResultsAfter correction, prenatal levels of three of 10 measured PCBs showed a positive correlation with cord serum T3, and four PCBs showed a negative correlation with cord serum rT3. After correction, two PCBs and the sum of the 10 measured PCBs were positively correlated with the cord serum T3/rT3 ratio, an indicator of deiodinase 3 activity. No correlations were found between PCBs and T4, TSH and TBG in cord blood. 4-OH-PCB-107 was correlated with T4 at 3months and T4, T4S and T3 at 18months. ConclusionOur results suggest that PCBs have a negative effect on deiodinase type 3 activity, as reflected by a positive correlation with the T3/rT3 ratio. We identified a potential mechanism by which PCBs may affect thyroid hormone metabolism during human development.

Steady-State Serum T3 Concentrations for 48 Hours Following the Oral Administration of a Single Dose of 3,5,3'-Triiodothyronine Sulfate (T3S)

Sulfate conjugation of thyroid hormones is an alternate metabolic pathway that facilitates the biliary and urinary excretion of iodothyronines and enhances their deiodination rate, leading to the generation of inactive metabolites. A desulfating pathway reverses this process, and thyromimetic effects have been observed following the parenteral administration of 3,5,3'-triiodothyronine (T3) sulfate (T3S) in rats. The present study investigated whether T3S is absorbed after oral administration in humans and if it represents a source of T3. Twenty-eight hypothyroid patients (7 men and 21 women; mean age, 44 ± 11 years) who had a thyroidectomy for thyroid carcinoma were enrolled. Replacement thyroid hormone therapy was withdrawn (42 days for thyroxine, 14 days for T3) prior to 131I remnant ablation. A single oral dose of 20, 40, 80 (4 patients/group), or 160 μg (16 patients/group) of T3S was administered 3 days before the planned administration of 131I. Blood samples for serum T3S and total T3 (TT3) concentrations were obtained at various times up to 48 hours after T3S administration. At all T3S doses, serum T3S concentrations increased, reaching a peak at 2 to 4 hours and progressively returning to basal levels within 8 to 24 hours. The T3S maximum concentration (Cmax) and area under the 0- to 48-hour concentration-time curve (AUC0-48h) were directly and significantly related to the administered dose. An increase in serum TT3 concentration was observed (significant after 1 hour), and the concentration increased further at 2 and 4 hours and then remained steady up to 48 hours after T3S administration. There was a significant direct correlation between the TT3 AUC0-48h and the administered dose of T3S. No changes in serum free thyroxine (T4) concentrations during the entire study period were observed, whereas serum thyroid-stimulating hormone levels increased slightly at 48 hours, but this was not related to the dose of T3S. No adverse events were reported. (1) T3S is absorbed following oral administration in hypothyroid humans; (2) after a single oral dose, T3S is converted to T3 in a dose-dependent manner, resulting in steady-state serum T3 concentrations for 48 hours; (3) T3S may represent a new agent in combination with T4 in the therapy of hypothyroidism, if similar conversion of T3S to T3 can be demonstrated in euthyroid patients who are already taking T4.

Inhibition of Thyroid Hormone Sulfotransferase Activity by Brominated Flame Retardants and Halogenated Phenolics

Many halogenated organic contaminants (HOCs) are considered endocrine disruptors and affect the hypothalamic-pituitary-thyroid axis, often by interfering with circulating levels of thyroid hormones (THs). We investigated one potential mechanism for TH disruption, inhibition of sulfotransferase activity. One of the primary roles of TH sulfation is to support the regulation of biologically active T3 through the formation of inactive THs. We investigated TH sulfotransferase inhibition by 14 hydroxylated polybrominated diphenyl ethers (OH BDEs), BDE 47, triclosan, and fluorinated, chlorinated, brominated, and iodinated analogues of 2,4,6-trihalogenated phenol and bisphenol A (BPA). A new mass spectrometry-based method was also developed to measure the formation rates of 3,3'-T2 sulfate (3,3'-T2S). Using pooled human liver cytosol, we investigated the influence of these HOCs on the sulfation of 3,3'-T2, a major substrate for TH sulfation. For the formation of 3,3'-T2S, the Michaelis constant (Km) was 1070 ± 120 nM and the Vmax was 153 ± 6.6 pmol min(-1) (mg of protein)(-1). All chemicals investigated inhibited sulfotransferase activity with the exception of BDE 47. The 2,4,6-trihalogenated phenols were the most potent inhibitors followed by the OH BDEs and then halogenated BPAs. The IC50 values for the OH BDEs were primarily in the low nanomolar range, which may be environmentally relevant. In silico molecular modeling techniques were also used to simulate the binding of OH BDE to SULT1A1. This study suggests that some HOCs, including antimicrobial chemicals and metabolites of flame retardants, may interfere with TH regulation through inhibition of sulfotransferase activity.

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.

Bexarotene-Induced Hypothyroidism: Bexarotene Stimulates the Peripheral Metabolism of Thyroid Hormones

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. The design was an open prospective 6-wk intervention study. 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. 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. In the present study, we demonstrate that increased peripheral degradation of thyroid hormones by a nondeiodinase-mediated pathway contributes to bexarotene induced-hypothyroidism.

Euthyroid Sick Syndrome in Meningococcal Sepsis: The Impact of Peripheral Thyroid Hormone Metabolism and Binding Proteins

The objective of this study was to elucidate the influence of disease severity, deiodination, sulfation, thyroid hormone binding, and dopamine use on thyroid function in euthyroid sick syndrome. The study was performed at a university-affiliated pediatric intensive care unit (PICU). This was an observational cohort study. Sixty-nine children with meningococcal sepsis were studied. Differences in thyroid function among nonsurvivors, shock survivors, and sepsis survivors on PICU admission were the main outcome measures. The main study group consisted of 45 non-dopamine-treated children. All children had decreased total T3 (TT3)/rT3 ratios without elevated TSH. T4 sulfate levels were decreased in 88%. Nonsurvivors had paradoxically higher TT3/rT3 ratios than shock survivors (0.71 vs. 0.30); this ratio also correlated with shorter duration of disease (r = -0.43). TT4 and T4-binding globulin (TBG) levels declined with increasing disease severity. TBG levels correlated inversely with elastase levels (r = -0.46). Only TSH levels were significantly lower in 24 dopamine-treated children compared with non-dopamine-treated children (0.65 vs. 0.84), whereas other thyroid hormones did not significantly differ. Both higher TT3/rT3 ratios and lower TT4 levels were predictive for mortality, but this disappeared when IL-6 was entered into the regression model. All children with meningococcal sepsis showed signs of euthyroid sick syndrome. Alterations in peripheral thyroid hormone metabolism related inversely to the duration of disease and seemed to be enacted by profound induction of type 3 deiodinase rather than by down-regulation of type 1. Lower TT4 levels were related to increased turnover of TBG by elastase. Dopamine was found to suppress only TSH secretion, not other thyroid hormone levels, on PICU admission. Both the TT3/rT3 ratio and TT4 levels were predictive for mortality, but were not superior to IL-6.

Alternate Pathways of Thyroid Hormone Metabolism

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 sulfated iodothyronines can be hydrolyzed to their precursors in gastrointestinal tract and various tissues. Thus, these conjugates can serve as a reservoir for biologically active iodothyronines (e.g., T(4), T(3), or T(2)). The acetic acid derivatives of T(4), tetrac and triac, are minor products in normal thyroid physiology. However, triac has a different pattern of receptor affinity than T(3), binding preferentially to the beta receptor. This makes it useful in the treatment of the syndrome of resistance to thyroid hormone action, where the typical mutation affects only the beta receptor. Thus, adequate binding to certain mutated beta receptors can be achieved without excessive stimulation of alpha receptors, which predominate in the heart. Ether link cleavage of TH is also a minor pathway in normal subjects. However, this pathway may become important during infections, when augmented TH breakdown by ether-link cleavage (ELC) may assist in bactericidal activity. There is a recent claim that decarboxylated derivates of thyronines, that is, monoiodothyronamine (T(1)am) and thyronamine (T(0)am), may be biologically important and have actions different from those of TH. Further information on these interesting derivatives is awaited.

Altered Thyroid and Adrenal Function in Children Born at Term and Preterm, Small for Gestational Age

Intrauterine growth retardation may permanently influence the endocrine system by affecting its programming during development. The aim of this study was to evaluate thyroid and adrenal function together with insulin sensitivity in a group of children born small for gestational age (SGA). Forty SGA children (mean age, 6.7 +/- 1.7 yr) and 35 children born appropriate for gestational age (mean age, 6.5 +/- 2.2 yr) were selected for the study. TSH, free T4, free T3 (fT3), rT3, antithyroid antibodies, cortisol, and dehydroepiandrosterone sulfate (DHEAS) were assessed. Insulin sensitivity was evaluated with the quantitative insulin sensitivity check index (QUICKI). A thyroid ultrasound was also performed in the SGA children. We found that TSH was significantly higher in SGA than in children born appropriate for gestational age [2.9 +/- 1.1 vs. 1.7 +/- 0.7 microU/ml (mIU/liter); P < 0.001]; furthermore, eight SGA children (20%), seven born preterm and one at term, had TSH levels above the upper limit of normality. fT3 was also higher in SGA children (4.2 +/- 0.4 vs. 3.6 +/- 0.6 pg/ml; 6.4 +/- 0.6 vs. 5.5 +/- 0.9 pmol/liter; P < 0.0001), whereas no difference was found for free T4, rT3, and the fT3/rT3 ratio. Urinary iodine was normal, and antithyroid antibodies were absent. Thyroid ultrasound showed a normal echographic pattern with a normal volume in SGA children. Serum cortisol was similar in both groups, whereas DHEAS was significantly lower in SGA subjects (43 +/- 18 vs. 65 +/- 50 microg/dl; 1.1 +/- 0.4 vs. 1.7 +/- 1.3 micromol/liter; P < 0.05). There was no difference in insulin sensitivity between the two groups. Birth length and birth weight were the main determinants of TSH and DHEAS serum levels, respectively. In conclusion, functional thyroid and adrenal changes have been found in children who suffered from intrauterine growth retardation. A larger survey with an appropriate follow-up is, however, required to confirm these findings and to assess their natural evolution.

The hypothalamic-pituitary-thyroid axis in preterm infants; changes in the first 24 hours of postnatal life.

The purpose of this study was to measure serum T4, free T4, TSH, T3, rT3, T4 sulfate, and thyroxine binding globulin at four time points within the first 24 h of life (cord and 1, 7, and 24 h) in infants between 24 and 34 wk gestation. The infants were subdivided into gestational age groups: 24-27 wk (n = 22); 28-30 wk (n = 26); and 31-34 wk (n = 24). The TSH surge in the first hour of postnatal life was markedly attenuated in infants of 24-27 wk gestation [8 compared with 20 (28-30 wk) and 23 mU/liter (31-34 wk)]. T4 levels in the most immature group declined over the first 24 h, whereas levels increased in the more mature groups [mean cord and 24-h levels: 65 and 59 (NS) vs. 70 and 84 (P < 0.002) vs. 98 and 125 (NS) nmol/liter]. Free T4 and T3 showed only small, transient increases in the most immature group and progressively larger and sustained increases in the other gestational groups. rT3 and T4 sulfate levels in cord serum were higher in the most immature infants, and in all groups levels decreased initially and then variably increased. The features of a severely attenuated or failed hypothalamic-pituitary-thyroid response to delivery critically define this 24- to 27-wk group as distinct from more mature preterm infants.

Concentrations of Seven Iodothyronine Metabolites in Brain Regions and the Liver of the Adult Rat

The concentrations of the iodothyronine metabolites T4, T3, 3,5-diiodothyronine (3,5-T2), 3,3′-diiodothyronine (3,3′-T2), reverse T3 (rT3), 3,3′-T2 sulfate (3,3′T2S), and T3 sulfate (T3S) were measured in 12 regions of the brain, the pituitary gland, and liver in adult male rats. Quantification of iodothyronine was performed by RIA following a newly developed method of purification and separation by HPLC. 3,5-T2, 3,3′-T2, rT3 and T2S were detectable in the low femtomolar range (20–200 fmol/g) in most areas of the rat brain. T3S was detectable only in the hypothalamus. The concentrations of T3 and T4 were approximately 20- to 60-fold higher, ranging between 1 and 6 pmol/g. There was a significant negative correlation between the activities of inner-ring deiodinase and T3 concentrations across brain areas. In the liver, 3,5-T2, rT3, and T3S were measurable in the low femtomolar range, whereas 3,3′-T2 and 3,3′T2S were not detectable. 3,5-T2 and 3,3′-T2 were not detectable in mitochondrial fractions of the brain regions. Tissue concentrations of 3,5-T2 exhibited a circadian variation closely parallel to those of T3 in the brain regions and liver. T3 was not a substrate for outer-ring deiodination under different experimental conditions; thus, it remains unclear which substrate(s) and enzyme(s) are involved in the production of 3,5-T2. These results indicate that five iodothyronine metabolites other than T3 and T4 are detectable in the low femtomolar range in the rat brain and/or liver. The physiological implications of this finding are discussed.

Effect of T3 Treatment and Food Ration on Hepatic Deiodination and Conjugation of Thyroid Hormones in Rainbow Trout, Oncorhynchus mykiss

We studied the 7-day effects of 3,5,3′-triiodothyronine (T3) hyperthyroidism (induced by 12 ppm T3 in food) and food ration (0, 0.5, or 2% body weight/day) on in vitro hepatic glucuronidation, sulfation, and deiodination of thyroxine (T4), T3, and 3,3′,5′-triiodothyronine (rT3). T3 treatment doubled plasma T3 with no change in plasma T4, depressed hepatic low-Km (1 nM) outer-ring deiodination (ORD) of T4, induced low-Km (1 nM) inner-ring deiodination (IRD) of both T4 and T3 but did not alter high-Km (1 μM) rT3ORD, glucuronidation, or sulfation of T4, T3, or rT3. Plasma T4 levels were greater for 0 and 2% rations than for a 0.5% ration. Fasting decreased low-Km T4ORD activity and increased high-Km rT3ORD activity but did not alter T4IRD or T3IRD activities. T4, T3, and rT3 glucuronidation were greater for 0 and 0.5% rations than for a 2% ration. T3 glucuronidation was greater for a 0.5% ration than for a 0% ration. T3 and rT3 sulfation were greater for a 2% ration than for a 0 or a 0.5% ration; ration did not change T4 sulfation. We conclude that (i) modest experimental T3 hyperthyroidism induces T3 autoregulation by adjusting hepatic low-Km ORD and IRD activities but not high-Km rT3ORD or conjugation activities; (ii) in contrast, ration level changes both deiodination and conjugation pathways, suggesting that the response to ration does not solely reflect altered T3 production; (iii) deiodination and conjugation appear complementary in regulating thyroidal status in response to ration; and (iv) high-Km rT3ORD in trout differs from rat type I deiodination in that it does not respond to T3 hyperthyroidism and it increases, rather than decreases, its activity during fasting.

Effects of Pentachlorophenol and Hydroxylated Polychlorinated Biphenyls on Thyroid Hormone Conjugation in a Rat and a Human Hepatoma Cell Line

It was previously demonstrated in our laboratory that hydroxylated metabolites of polychlorinated biphenyls (PCB-OHs) inhibit the sulfation of iodothyronines in rat liver cytosol. In this study, the inhibition of 3,3′-diiodothyronine (T2) sulfation by pentachlorophenol (PCP) and PCB-OHs was investigated in hepatoma cell lines in relation to the cellular uptake of these compounds, providing a more appropriate model of the in vivo situation. The human HepG2 hepatoma cell line was shown to conjugate T2 almost exclusively by sulfation, glucuronidation being negligible. The rat FaO hepatoma cell line, on the other hand, produced 37% T2 sulfate and 63% T2 glucuronide. It was demonstrated that PCP inhibited T2 sulfation in both cell lines, although it was 10 3 times less potent in cells than in rat liver cytosol. Remarkably, 10 μ mPCP inhibited the sulfation and glucuronidation of T2 by FaO cells to the same extent. Micromolar concentrations of 4-hydroxy-3,3′,4′,5-tetrachlorobiphenyl or 4-hydroxy-2′,3,3′,4′,5-pentachlorobiphenyl hardly affected T2 conjugation in FaO cells, but both PCB-OHs reduced T2 sulfate formation in HepG2 cells. Inhibition of T2 sulfation was stronger using medium without foetal calf serum (FCS) than medium with 5% FCS. This was due to a lower uptake of inhibitor by the cells in the presence of serum, as demonstrated using radiolabelled PCP. In conclusion, this study confirms the inhibition of T2 sulfation by PCP and PCB-OHs previously observed in rat liver cytosol in a rat and a human hepatoma cell line. Thus, it seems reasonable to assume that iodothyronine sulfation is also inhibited by PCB metabolites and PCP in vivo.

Serum iodothyronines in the human fetus and the newborn: evidence for an important role of placenta in fetal thyroid hormone homeostasis.

The pattern of circulating iodothyronines in the fetus differs from that in the adult, being characterized by low levels of serum T3. In this study, concentrations of various iodothyronines were measured in sera from neonates of various postconceptional age (PA). Results obtained in cord sera at birth (PA, 24-40 weeks), reflecting the fetal pattern, were compared with those found during extrauterine life in newborns of 5 days or more of postnatal life (PA, 27-46 weeks). The main findings are: Starting at 30 weeks of PA, serum levels increase linearly during extrauterine life; and at 40 weeks, they are more than 200% of those measured in cord sera from newborns of equivalent PA. Serum reverse T3 (rT3) levels during fetal life are higher than those measured during extrauterine life; but they significantly decrease, starting at 30 weeks of PA. Serum T3 sulfate (T3S) does not significantly differ between the two groups, showing the highest values at 28-30 weeks of PA, and significantly decreasing at 30-40 weeks. T3S levels are directly correlated with rT3, both in fetal and extrauterine life, whereas a significant negative correlation between T3S and T3 is found only during extrauterine life. 1) changes in serum concentrations of iodothyronines in umbilical cord and during postnatal life indicate that maturation of extrathyroidal type I-iodothyronine monodeiodinase (MD) accelerates, starting at 30 weeks of PA; 2) high levels of type III-MD activity in fetal tissues prevent the rise of serum T3, whereas they maintain high levels of rT3 during intrauterine life; 3) an important mechanism leading to the transition from the fetal to the postnatal thyroid hormone balance is a sudden decrease in type III-MD activity; iv) because placenta contains a high amount of type III-MD, it is conceivable that placenta contributes to maintain low T3 and high rT3 serum concentrations during fetal life and that its removal at birth is responsible for most changes in iodothyronine metabolism occurring afterwards.

Inhibition of thyroid hormone sulfation by hydroxylated metabolites of polychlorinated biphenyls

In this study we investigated the possible inhibitory effects of hydroxylated metabolites of polychlorinated biphenyls (OH-PCBs) on iodothyronine sulfotransferase activity. The results indicate that OH-PCBs are potent inhibitors of this activity in vitro, with IC 50 concentrations in the low micro molar range. Inhibition of sulfotransferase activity towards 3,3′-diiodothyronine (T2) was similar to that towards 3,3′,5-triiodothyronine (T3) in this in vitro assay, therefore, T2 can be used as the model substrate for the active hormone T3. An important structural requirement for T2 sulfotransferase inhibition is a hydroxyl group on the para or meta position of the OH-PCBs. Since T3 is the active hormone, playing a very important role in somatic and brain development and since hydroxylated PCBs can accumulate in fetuses, inhibition of T3 sulfation could be a possible mechanism for the developmental neurotoxicity of PCBs.

Expression of rat liver cell membrane transporters for thyroid hormone in Xenopus laevis oocytes.

The present study was conducted to explore the possible use of Xenopus laevis oocytes for the expression cloning of cell membrane transporters for iodothyronines. Injection of stage V-VI X. laevis oocytes with 23 ng Wistar rat liver polyadenylated RNA (mRNA) resulted after 3-4 days in a highly significant increase in [125I]T3 (5 nM) uptake from 6.4 +/- 0.8 fmol/oocyte x h in water-injected oocytes to 9.2 +/- 0.65 fmol/oocyte x h (mean +/- SEM; n = 19). In contrast, [125I]T4 (4 nM) uptake was not significantly stimulated by injection of total liver mRNA. T3 uptake induced by liver mRNA was significantly inhibited by replacement of Na+ in the incubation medium by choline+ or by simultaneous incubation with 1 microM unlabeled T3. In contrast, T3 uptake by water-injected oocytes was not Na+ dependent. Fractionation of liver mRNA on a 6-20% sucrose gradient showed that maximal stimulation of T3 uptake was obtained with mRNA of 0.8-2.1 kilobases (kb). In contrast to unfractionated mRNA, the 0.7- to 2.1-kb fraction also significantly stimulated transport of T4, and it was found to induce uptake of T3 sulfate (T3S). Because T3S is a good substrate for type I deiodinase (D1), 2.3 ng rat D1 complementary RNA (cRNA) were injected either alone or together with 23 ng of the 0.8- to 2.1-kb fraction of rat liver mRNA. Compared with water-injected oocytes, injection of D1 cRNA alone did not stimulate uptake of [125I]T3S (1.25 nM). T3S uptake in liver mRNA and D1 cRNA-injected oocytes was similar to that in oocytes injected with mRNA alone, showing that transport of T3S is independent of the metabolic capacity of the oocyte. Furthermore, coinjection of liver mRNA and D1 cRNA strongly increased the production of 125I-, showing that the T3S taken up by the oocyte is indeed transported to the cell interior. In conclusion, injection of rat liver mRNA into X. laevis oocytes resulted in a stimulation of saturable, Na+-dependent T4, T3 and T3S transport, indicating that rat liver contains mRNA(s) coding for plasma membrane transporters for these iodothyronine derivatives.

Characterization of a propylthiouracil-insensitive type I iodothyronine deiodinase.

Mammalian type I iodothyronine deiodinase (D1) activates and inactivates thyroid hormone by outer ring deiodination (ORD) and inner ring deiodination (IRD), respectively, and is potently inhibited by propylthiouracil (PTU). Here we describe the cloning and characterization of a complementary DNA encoding a PTU-insensitive D1 from teleost fish (Oreochromis niloticus, tilapia). This complementary DNA codes for a protein of 248 amino acids, including a putative selenocysteine (Sec) residue, encoded by a TGA triplet, at position 126. The 3' untranslated region contains two putative Sec insertion sequence (SECIS) elements. Recombinant enzyme expressed in COS-1 cells catalyzes both ORD of T4 and rT3 and IRD of T3 and T3 sulfate with the same substrate specificity as native tilapia D1 (tD1), i.e. rT3 >> T4 > T3 sulfate > T3. Native and recombinant tD1 show equally low sensitivities to inhibition by PTU, iodoacetate, and gold thioglucose compared with the potent inhibitions observed with mammalian D1s. Because the residue 2 positions downstream from Sec is Pro in tD1 and in all (PTU-insensitive) type II and type III iodothyronine deiodinases but Ser in all PTU-sensitive D1s, we prepared the Pro128Ser mutant of tD1. The mutant enzyme showed strongly decreased ORD and somewhat increased IRD activity, but was still insensitive to PTU. These results provide new information about the structure-activity relationship of D1 concerning two characteristic properties, i.e. catalysis of both ORD and IRD, and inhibition by PTU.

Demonstration of thyromimetic effects of 3,5,3'-triiodothyronine sulfate (T3S) in euthyroid rats.

We have previously demonstrated that T3 sulfate (T3S) exhibits thyromimetic effects in hypothyroid rats and that, on a molar basis, its activity approximates 20% that of T3. Since T3S is avidly deiodinated by 5'-deiodinase, type I (5'-DI) and 5'-DI activity is markedly reduced in hypothyroidism, it seemed possible that T3S is active only in hypothyroidism and not in the euthyroid state wherein normal tissue 5'-DI activity will rapidly degrade T3S and little T3S will be left for metabolism (desulfation) to biologically active T3. This study was undertaken to test this possibility. We studied the effect of T3S (4.6, 14, or 42 nmol/day for 7 days, ip) and T3 (1.0, 3.0 or 9.0 nmol/day for 7 days, ip) in groups of male Sprague-Dawley rats (5-6/group); the control group was treated with saline ip. Treatment with both T3 and T3S caused a significant (p < 0.05) increase in hepatic and renal 5'-DI. Similarly, both treatments caused a significant reduction in serum total T4 and TSH levels. In these effects, T3S was approximately one-fifth as potent as T3 on a molar basis. Interestingly, changes in body weight during treatment with T3 and T3S suggested that at doses that caused a comparable tissue or pituitary effect, T3S treatment permitted a significantly greater weight gain than treatment with T3. We conclude that T3S exhibits thyromimetic effects in euthyroid rats in a manner comparable to that in hypothyroid rats. The biological effects of T3S may be due to T3 generated in tissues by desulfation of T3S.