T4 Clearance Research Articles

Valifenalate-induced non-adverse thyroid changes via adaptive induction of uridine 5′-diphospho-glucuronosyltransferase (UGT) in the liver of dogs and rats but not humans

Some rat and dog toxicology studies with the fungicide valifenalate showed minimal, non-adverse thyroid changes, mostly above the maximum tolerated dose, and concomitantly with liver effects. This publication describes their mode of action (MOA), combining in vivo and new approach methodologies (NAMs), in a weight of evidence approach.Data demonstrate a MOA of liver enzyme induction via nuclear receptor CAR/PXR activation, increased thyroxine (T4) metabolism and elevated thyroid stimulating hormone (TSH) level, leading to thyroid follicular cell hypertrophy and increased thyroid weight. Non-human relevance of the MOA was demonstrated in in vitro cross species assays in rat, dog and human hepatocytes. Increased gene expression and activity of cytochrome P450s (CYPs) and uridine 5′-diphospho-glucuronosyltransferases (UGTs) were observed in rat and dog hepatocytes exposed to valifenalate, with increased T4 clearance and/or T4 glucuronidation/T4-UGT activity. Therefore, a causal relationship between increased liver enzyme induction and thyroid effects in dogs and rats is concluded. Rat hepatocytes were most sensitive, while valifenalate did not increase T4-UGT activity above 2-fold of vehicle control or T4 glucuronidation and T4 clearance in human hepatocytes. Consequently, valifenalate exposure in humans is unlikely to lead to decreased T4 levels, and subsequent thyroid and developmental neurotoxicity effects. Alternative human-relevant thyroid MOAs were excluded, i.e. inhibition of deiodinases (DIO), thyroperoxidase (TPO) or the sodium iodide symporter (NIS).Due to known species differences in thyroid homeostasis between humans and laboratory animals and, importantly, based on the presented data, this liver enzyme mediated MOA is considered not relevant for human hazard assessment.

Analysis of evolutionary and functional features of the bullfrog SULT1 family

We identified the bullfrog Rana catesbeiana sulfotransferase 1 (SULT1) family from the BLAST search tool of the public databases based on the SULT1 families of Nanorana parkeri, Xenopus laevis, and Xenopus tropicalis as queries, revealing the characteristics of the anuran SULT1 family. The results showed that the anuran SULT1 family comprises six subfamilies, four of which were related to the mammalian SULT1 subfamily. Additionally, the bullfrog has two SULT1Cc subfamily members that are consistent with the characteristics of the expanded Xenopus SULT1C subfamily. Several members of the bullfrog SULT1 family were suggested to play important roles in sulfation during metamorphosis. Among these, cDNAs encoding SULT1Cc1 and SULT1Y1 were cloned, and the sulfation activity was analyzed using recombinant proteins. The affinity for 2-naphthol and 3′-phosphoadenosine 5′-phosphosulfate (PAPS) and the enzymatic reaction rate were higher in SULT1Cc1 than in SULT1Y1. Both the enzymes showed inhibitory effect of many thyroid hormones (THs) analogs on the sulfation of 2-naphthol. The potency of sulfation activities of SULT1Cc1 and SULT1Y1 against T4 indicated their possible role in the intracellular T4 clearance during metamorphosis.

A minimal human physiologically based kinetic model of thyroid hormones and chemical disruption of plasma thyroid hormone binding proteins.

The thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3), are under homeostatic control by the hypothalamic-pituitary-thyroid axis and plasma TH binding proteins (THBPs), including thyroxine-binding globulin (TBG), transthyretin (TTR), and albumin (ALB). THBPs buffer free THs against transient perturbations and distribute THs to tissues. TH binding to THBPs can be perturbed by structurally similar endocrine-disrupting chemicals (EDCs), yet their impact on circulating THs and health risks are unclear. In the present study, we constructed a human physiologically based kinetic (PBK) model of THs and explored the potential effects of THBP-binding EDCs. The model describes the production, distribution, and metabolism of T4 and T3 in the Body Blood, Thyroid, Liver, and Rest-of-Body (RB) compartments, with explicit consideration of the reversible binding between plasma THs and THBPs. Rigorously parameterized based on literature data, the model recapitulates key quantitative TH kinetic characteristics, including free, THBP-bound, and total T4 and T3 concentrations, TH productions, distributions, metabolisms, clearance, and half-lives. Moreover, the model produces several novel findings. (1) The blood-tissue TH exchanges are fast and nearly at equilibrium especially for T4, providing intrinsic robustness against local metabolic perturbations. (2) Tissue influx is limiting for transient tissue uptake of THs when THBPs are present. (3) Continuous exposure to THBP-binding EDCs does not alter the steady-state levels of THs, while intermittent daily exposure to rapidly metabolized TBG-binding EDCs can cause much greater disruptions to plasma and tissue THs. In summary, the PBK model provides novel insights into TH kinetics and the homeostatic roles of THBPs against thyroid disrupting chemicals.

Thermoregulatory dysfunction in post‐weaning congenic corpulent rats

To assess the capacity for cold induced thermoregulation in congenic corpulent rats, groups of normally fed post‐weaning lean and obese LA/Ntul//‐cp rats 6‐9 weeks of age were subjected to a 24°C vs. 4°C environment for < 14 hours. Measures of resting VO2 (RMR) and the thermoregulatory responses to cold exposure, the thermic response to noradrenaline, thyroidal parameters, and measures of core and peripheral body temperatures following cold exposure were obtained. Body weights of obese were greater than in lean rats (p=<0.01), and 14 hours of 4°C exposure demonstrated phenotype effects on both core and rectal temperatures (lean > obese, rectal p=0.0012, core p= 0.0038). The RMR at thermal neutrality and the thermic response to 45 minutes of acute 4°C cold exposure was greater in lean than obese phenotype at all time points measured. The thermic responses to 100 vs 200 µg NE s.c. were greater in lean than in obese rats. Cold exposure induced a significant increase in circulating T3 and the T3:T4 ratios but T4 concentrations were unchanged by cold exposure challenge. Phenotype effects on outer ring T4‐5’ deiodinase activity/mg protein were present in Kidney and Liver (Lean > Obese) while Interscapular brown adipose tissue (IBAT) demonstrated both phenotype and cold induced effects (Obese > Lean). In contrast, T4‐5’deiodinase activity in gastrocnemius muscle tissue was similar in both phenotypes and lacked cold induced effects (p= n.s.). The results of this study indicate that an impaired capacity for thermoregulation occurs early in the lifespan of the corpulent rat, despite a robust increase in total IBAT 5’deiodinase activity and in circulating T3 concentrations following cold exposure in both lean and obese phenotypes. While the physiologic basis for the elevated T3 concentrations following cold exposure remains unclear and are likely secondary to the combined effects of optimizing peripheral T4 to T3 conversion rates in both phenotypes, and possible changes in kinetics of T3 receptor binding and clearance in IBAT and other tissues and which may contribute to a greater caloric efficiency and propensity for weight gain in the obese phenotype.

Weight of Evidence and Human Relevance Evaluation of the Benfluralin Mode of Action in Rats (Part II): Thyroid carcinogenesis

Benfluralin is an herbicide of the dinitroaniline class used to control grasses and weeds. In a 2 year dietary study in rats, benfluralin increased incidences of thyroid follicular adenoma and carcinoma at high dietary concentrations (≥2500 ppm). The benfluralin toxicology database suggests the mode of action (MOA) is initiated by induction of liver metabolizing enzymes, particularly thyroid hormone specific UGTs, a major pathway for T4 clearance in rats. As reported with phenobarbital, this effect triggers negative feedback regulation, increasing thyroid stimulating hormone (TSH) release into circulating blood. When sustained over time, this leads to thyroid changes such as follicular hypertrophy, hyperplasia and thyroid follicular tumors with chronic exposures. The described MOA was previously established in rat studies with various chemical activators of xenobiotic receptors in the liver. It is generally considered as non-relevant in humans, due to differences between humans and rats in T4 turnover and susceptibility to this carcinogenic MOA. A structured methodology based on the IPCS/MOA/Human Relevance framework was used in the evaluation of available benfluralin data, and the conclusion was determined that the carcinogenic potential of benfluralin in the thyroid is not relevant in humans.

Kanechlor 500‐mediated changes in serum and hepatic thyroxine levels primarily occur in a transthyretin‐unrelated manner

The effects of Kanechlor-500 (KC500) on the levels of serum total thyroxine (T4 ) and hepatic T4 in wild-type C57BL/6 (WT) and its transthyretin (TTR)-deficient (TTR-null) mice were comparatively examined. Four days after a single intraperitoneal injection with KC500 (100mg/kg body weight), serum total T4 levels were significantly decreased in both WT and TTR-null mice. The KC500 pretreatment also promoted serum [125 I]T4 clearance in both strains of mice administrated with [125 I]T4 , and the promotion of serum [125 I]T4 clearance in WT mice occurred without inhibition of the [125 I]T4 -TTR complex formation. Furthermore, the KC500 pretreatment led to significant increases in liver weight, steady-state distribution volume of [125 I]T4 , hepatic accumulation level of [125 I]T4 , and concentration ratio of the liver to serum in both strains of mice. The present findings indicate that the KC500-mediated decrease in serum T4 level occurs in a TTR-unrelated manner and further suggest that KC500-promoted T4 accumulation in the liver occurs through the development of liver hypertrophy and the promotion of T4 transportation from serum to liver.

Thyroxine transfer from cerebrospinal fluid into choroid plexus and brain is affected by brefeldin A, low sodium, BCH, and phloretin, in ventriculo-cisternal perfused rabbits.

Background: Thyroxine (T4) hormone is synthesized by the thyroid gland and then released into the systemic circulation where it binds to a number of proteins. Dysfunction in T4 transport mechanisms has been demonstrated in multiple central nervous system (CNS) diseases including Alzheimer's disease. In the presence of different compounds that inhibit potential T4 transport mechanisms, this study investigated the transfer of T4 from cerebrospinal fluid (CSF) into Choroid Plexus (CP) and other brain tissues. The compounds used were brefeldin A, low sodium artificial CSF (aCSF), BCH, phloretin, and taurocholate (TA).Methods: Radiolabeled T4 (125I-T4) was perfused continuously into the CSF and was assessed in several brain compartments with reference molecule 14C-mannitol and blue dextran, using the in vivo ventriculo-cisternal perfusion (V-C) technique in the rabbit. The aCSF containing the drug of interest was infused after 1 h of perfusion. Drugs were applied independently to the aCSF after 1 h of control perfusion.Results: Of interest, in presence of low sodium or BCH, the percentage recovery of 125I-T4, was increased compared to controls, with concomitant increase in T4 clearance. Conversely, brefeldin A, phloretin, and TA did not exert any significant effect on the recovery and clearance of 125I-T4 assessed in aCSF. On the other hand, the uptake of 125I-T4 into CP was raised by 18 fold compared to controls in the presence of brefeldin A. In addition, low sodium, BCH, or phloretin alone, enhanced the uptake of 125I-T4 by almost 3-fold, whereas TA did not show any significant effect. Finally, the uptake and distribution of 125I-T4 into other brain regions including ependymal region (ER) and caudate putamen (CAP) were significantly higher than in controls.Conclusion: Our study suggests the involvement of different mechanisms for the transfer of 125I-T4 from CSF into CP and other brain regions. This transfer may implicate sodium-dependent mechanisms, amino acid “L” system, or organic anion transporting polypeptide (OATP).

Effects of Lersivirine on Canine and Rodent Thyroid Function

Lersivirine is a nonnucleoside reverse transcriptase inhibitor (NNRTI) being developed for the treatment of HIV-1 infection. Like other NNRTIs, lersivirine is a potent enzyme inducer in rodents capable of inducing a number of hepatic enzymes including those involved in its own metabolism. Preclinically lersivirine has been associated with hepatocellular hypertrophy and thyroid gland follicular cell hypertrophy in rats, mice, and dogs. In rodents, we show that development of thyroid hypertrophy is related to the classic mechanism, namely increased thyroxine (T4) clearance secondary to induction of uridine-diphosphoglucuronosyltransferase (UDPGT) in the liver and a resulting increase in thyroid-stimulating hormone. Similarly, lersivirine-exposed dogs exhibit a significant increase in hepatic UDPGT enzyme activity along with increased T4 clearance although clear effects on serum thyroid hormone levels were less apparent. These effects on thyroid hormonal clearance in the dog suggest that thyroid gland hypertrophy in this species is due to the same mechanism shown to occur in rodents although, as expected, dogs better adapt to these effects and therefore maintain relatively normal thyroid hormonal balance. It is also notable that the minimal thyroid follicular hypertrophy that occurs in dogs does not progress as is seen in rodents. As is the case with rodents, these adaptive changes in the dog are not considered indicative of a human health risk.

RTHα, a Newly Recognized Phenotype of the Resistance to Thyroid Hormone (RTH) Syndrome in Patients With THRA Gene Mutations

RTHα, a Newly Recognized Phenotype of the Resistance to Thyroid Hormone (RTH) Syndrome in Patients With THRA Gene Mutations

Is the mechanisms of fipronil-induced thyroid disruption specific of the rat: Re-evaluation of fipronil thyroid toxicity in sheep?

In rat, fipronil treatment increases the elimination of thyroid hormones (TH). Relevance of this mechanism for the risk of fipronil for human health is subject to controversy because of the specificities of adult rat for TH plasma binding properties which often lead to the assumption that rats are more sensitive than human to thyroid disruption. This study aimed at determining if the mechanism of fipronil-induced thyroid disruption is altered in sheep a species more relevant to human from the standpoint of TH plasma binding. TSH, total triiodothyronine and free and total tetra-iodothyronine 24 h secretory profiles were not modified by fipronil treatment (5 mg/kg every 4 days per os) in rams. In euthyroid-like thyroidectomised ewes, the effect of this treatment was limited to a moderate increase in free T4 clearance. In contrast with the rat, fipronil treatment (0.5 mg/kg/day, IV for 14 days) had no effect on antipyrine clearance, a marker for hepatic cytochrome activity, in ewes. The differences between rat and sheep for the potential of fipronil as a thyroid disruptor might be related to the difference in the exposure to the toxicant, the actual exposure to the sulfone metabolite of fipronil being lower in sheep than in rat.

Predictive Modeling of a Mixture of Thyroid Hormone Disrupting Chemicals That Affect Production and Clearance of Thyroxine

Thyroid hormone (TH) disrupting compounds interfere with both thyroidal and extrathyroidal mechanisms to decrease circulating thyroxine (T(4)). This research tested the hypothesis that serum T(4) concentrations of rodents exposed to a mixture of both TH synthesis inhibitors (pesticides) and stimulators of T(4) clearance in the liver (polyhalogenated aromatic hydrocarbons, PHAHs) could be best predicted by an integrated addition model. Female Long-Evans rats, 23 days of age, were dosed with dilutions of a mixture of 18 PHAHs (2 dioxins, 4 dibenzofurans, and 12 PCBs, including dioxin-like and non-dioxin like PCBs) and a mixture of 3 pesticides (thiram, pronamide, and mancozeb) for four consecutive days. Serum was collected 24 hours after the last exposure and T(4) concentrations were measured by radioimmunoassay. Animals exposed to the highest dose of the mixture experienced a 45% decrease in serum T(4). Three additivity model predictions (dose addition, effect addition, and integrated addition) were generated based on single chemical data, and the results were compared. Effect addition overestimated the effect produced by the combination of all 21 chemicals. The results of the dose- and integrated-addition models were similar, and both provided better predictions than the effect-addition model. These results support the use of dose- and integrated additivity models in predicting the effects of complex mixtures.

Hepatobiliary Disposition of Thyroid Hormone in Mrp2-Deficient TR− Rats: Reduced Biliary Excretion of Thyroxine Glucuronide Does Not Prevent Xenobiotic-Induced Hypothyroidism

The hepatobiliary disposition of thyroxine (T4) was evaluated in Groningen Yellow transport deficient (TR(-)) rats lacking functional multidrug resistance-associated protein 2 (Mrp2; Abcc2). Male Wistar and TR(-) rats were dosed orally (4 days) with phenobarbital (PB; 100 mg/kg) or DMP 904 (200 mg/kg), after which T4 homeostasis and hepatic cytochromes P450, UDP-glucuronosyltransferase, xenobiotic transporters, and T4 glucuronidation were determined. Serum concentrations of T4 were approximately 50% higher in control TR(-) rats than Wistars. PB and DMP 904 increased hepatic levels of P450s and T4-glucuronidation (T4-G), and these changes were associated with decreased serum T4 levels in both strains. In Wistar but not TR(-) rats, DMP 904 increased thyroid stimulating hormone levels twofold. Hepatobiliary clearance of T4 was determined after intravenous infusion of [(125)I]T4 to rats dosed with PB and DMP 904 (4 days). PB and DMP 904 increased plasma clearance and hepatic uptake of [(125)I]T4 equivalents in Wistar but not TR(-) rats. Total biliary clearance (Cl(bile)) was approximately 0.85 and 0.2 ml/h in Wistar and TR(-) rats, respectively, with virtually no T4-G excreted in bile in TR(-) rats. Biliary clearance of unconjugated T4 was also lower in control TR(-) rats than in Wistars, although DMP 904 increased its biliary clearance in both strains. These results suggest that Mrp2 is likely to be responsible for biliary excretion of T4-G and contributes in part to excretion of T4. Decreased biliary clearance of T4 and metabolites in TR(-) rats mitigated but did not prevent drug-induced changes in serum T4, suggesting that other factors contribute to changes in T4 homeostasis in these rats.

Fipronil-induced disruption of thyroid function in rats is mediated by increased total and free thyroxine clearances concomitantly to increased activity of hepatic enzymes

Fipronil is a widely used phytosanitary product and insecticide for pets. In the rat, fipronil can disrupt thyroid function by decreasing plasma concentrations of total thyroxine (T4) likely through increased T4 clearance. However, the mechanism of fipronil action on thyroid function remains unclear. The goals of the present study were to evaluate the effects of fipronil on thyroid hormone (TH) concentrations and elimination in the rat under well characterized plasma exposure to fipronil and its main metabolite fipronil sulfone. In thyroid-intact female rats, fipronil treatment (3 mg/(kg day) per os for 14 days) decreased both total and free TH plasma concentrations concomitantly to increased thyroid stimulating hormone plasma concentrations. A T4-free euthyroid-like model consisting of thyroidectomized rats treated with tri-iodothyronine (12 μg/(kg day), sc) was developed to evaluate both total and free T4 clearances. In this model, fipronil treatment induced a twofold increase in total and free T4 clearances. The same fipronil treatment increased antipyrine clearance in thyroid-intact rats suggesting an increase in the activity of cytochrome P450 enzymes. Finally, this treatment was also associated with an increase in hepatic microsomal 4-nitrophenol UDP-glucuronosyltransferase activity involved in T4 glucuronidation. Thus, fipronil-induced thyroid disruption results from an increased rate of T4 elimination likely mediated by increased hepatic enzyme activity. Plasma concentrations of fipronil sulfone were at least 20-fold higher than those of fipronil. This highlights the need to further investigate the contribution of fipronil sulfone to the fipronil-induced thyroid disruption.

Elders Are at Risk for Thyroid Dysfunction

Doug Brunk is with the San Diego bureau of Elsevier Global Medical News. SALT LAKE CITY — It's hard to know whom among the elderly to screen for thyroid dysfunction because, when it comes to adults without thyroid-disease symptoms, current recommendations from various medical societies differ widely. For example, the American Thyroid Association recommends screening every 5 years for all women and men older than the age of 35 years. The American College of Physicians' guideline—although officially inactive because it's more than 5 years old—recommends screening only women who are older than 50 and who show incidental symptoms of thyroid disease. And the U.S. Preventive Services Task Force says there is insufficient evidence for or against screening anyone. Nevertheless, Dr. Naushira Pandya, CMD, said that the consensus among geriatricians is to screen both men and women aged 65 years and older at least once, and thereafter if they develop symptoms of hypothyroidism or hyperthyroidism. Many diseases and drugs common in elderly people can affect the thyroid, she said. Speaking at the AMDA Annual Symposium, Dr. Pandya offered a list of conditions that warrant testing for thyroid dysfunction. These include previous thyroid disease or surgery; the presence of a goiter or nodule; type 1 diabetes; previous postpartum thyroid dysfunction; Down or Turner's syndrome; chronic kidney disease; previous irradiation of the head and neck; radical pharyngeal surgery; history of polycystic ovary syndrome; pituitary surgery or irradiation; and severe head injury. The list of drugs that affect thyroid function is also lengthy. Lithium, iodine (including that in x-ray contrast media), interleukin-2, and interferon-α may cause hypothyroidism, said Dr. Pandya, of the department of internal medicine at Nova Southeastern University College of Osteopathic Medicine, Fort Lauderdale, Fla. Glucocorticoids, iodine, propylthiouracil, β-blockers, and amiodarone may impair the conversion of T4 to T3, whereas dopamine, dobutamine, glucocorticoids, phenytoin, bromocriptine, and octreotide may suppress thyroid-stimulating hormone. Carbamazepine, phenytoin, rifampin, and phenobarbital may increase clearance of T4, whereas salsalate, salicylates, nonsteroidal anti-inflammatory drugs, furosemide, and heparin may reduce binding of T4 to thyroid-binding globulin. Dr. Pandya also noted that cholestyramine, aluminum hydroxide, ferrous sulfate, sucralfate, and cation exchange resins influence the absorption of thyroxine. Compared with their younger counterparts, older people are more likely to have overt and subclinical hypothyroidism as well as subclinical hyperthyroidism and thyroid nodules, said Dr. Pandya. The prevalence of overt hyperthyroidism and thyroid cancer is believed to be the same in young and old people. Thyroid cancer tends to be more aggressive in older adults. Signs and symptoms of hypothyroidism in older patients mimic certain processes of normal aging, including anorexia, cognitive decline, cold intolerance, constipation, dry skin, fatigue, hearing loss, hoarseness, paresthesia, slowed reflexes, and weakness. “A high index of suspicion is important,” said Dr. Pandya. Symptoms of hyperthyroidism may be absent or subtle. Cardiac complications such as atrial arrhythmias, heart failure, and angina are the most common indicators of a thyroid problem. A decrease in physical activity with fatigue, weakness, lethargy, agitation, confusion, and dementia are also common. Weight loss with anorexia may be present, and myopathy is predominant. Dr. Pandya said that she had no relevant conflicts of interest to disclose.

Congenital Hypothyroidism Following Maternal Exposure throughout Pregnancy to Imatinib.

Imatinib mesylate has revolutionized treatment of Chronic Myeloid Leukemia. Yet for women of child bearing age problems may arise during pregnancy. While continuation of the drug is in the interest of maternal health, the safety of this approach for the fetus has been questioned. Therapy is therefore usually immediately interrupted on recognition of pregnancy. Only three successful pregnancies have been reported to date in which imatinib was continued. A 34-year-old woman of Chinese descent in complete cytogenetic and major molecular remission (i.e. 3-log reduction of BCR-ABL transcripts) of her CML in chronic phase on 400mg imatinib daily continued treatment during conception and throughout pregnancy. After an uneventful full term pregnancy she gave birth to an apparently healthy son. The mother retained her major molecular remission. Routine screening for congenital hypothyroidism of her child however repeatedly revealed an elevated TSH, rising from 11 mU/l at the age of 1 week to15 mU/l at 4 weeks, associated with a relatively low free T4 of 24.8 and 17.4 pmol/l respectively. Thyroglobulin and thyroxine binding globulin concentrations were normal for age. The mother was euthyroid and had no detectable anti-thyroid antibodies. A mild primary hypothyroidism was suspected. An ultrasound showed no developmental defect of his thyroid gland and a 123I scan with a perchlorate discharge test ruled out an intra-thyroidal iodide organification defect. There was no family history of congenital hypothyroidism. The child was put on levothyroxine and is currently doing well. Hypothyroidism has been noticed as a potential side effect of imatinib therapy in thyroidectomized patients receiving levothyroxine and has been ascribed to enhanced non-deiodination clearance of T4 (de Groot et al Clin Pharmacol Ther 2005; 78:433–8). An induction of uridine diphosphate-glucuronosyl transferases by imatinib was hypothesized as mechanism for an altered levothyroxine metabolism in these patients. Congenital hypothyroidism has to the best of our knowledge not been reported as a potential consequence of imatinib therapy.

Type 3 Deiodinase Deficiency Results in Functional Abnormalities at Multiple Levels of the Thyroid Axis

The type 3 deiodinase (D3) is a selenoenzyme that inactivates thyroid hormones and is highly expressed during development and in the adult central nervous system. We have recently observed that mice lacking D3 activity (D3KO mice) develop perinatal thyrotoxicosis followed in adulthood by a pattern of hormonal levels that is suggestive of central hypothyroidism. In this report we describe the results of additional studies designed to investigate the regulation of the thyroid axis in this unique animal model. Our results demonstrate that the thyroid and pituitary glands of D3KO mice do not respond appropriately to TSH and TRH stimulation, respectively. Furthermore, after induction of severe hypothyroidism by antithyroid treatment, the rise in serum TSH in D3KO mice is only 15% of that observed in wild-type mice. In addition, D3KO animals rendered severely hypothyroid fail to show the expected increase in prepro-TRH mRNA in the paraventricular nucleus of the hypothalamus. Finally, treatment with T(3) results in a serum T(3) level in D3KO mice that is much higher than that in wild-type mice. This is accompanied by significant weight loss and lethality in mutant animals. In conclusion, the absence of D3 activity results in impaired clearance of T(3) and significant defects in the mechanisms regulating the thyroid axis at all levels: hypothalamus, pituitary, and thyroid.

Developmental Control of Iodothyronine Deiodinases by Cortisol in the Ovine Fetus and Placenta Near Term

Preterm infants have low serum T4 and T3 levels, which may partly explain the immaturity of their tissues. Deiodinase enzymes are important in determining the bioavailability of thyroid hormones: deiodinases D1 and D2 convert T4 to T3, whereas deiodinase D3 inactivates T3 and produces rT3 from T4. In human and ovine fetuses, plasma T3 rises near term in association with the prepartum cortisol surge. This study investigated the developmental effects of cortisol and T3 on tissue deiodinases and plasma thyroid hormones in fetal sheep during late gestation. Plasma cortisol and T3 concentrations in utero were manipulated by exogenous hormone infusion and fetal adrenalectomy. Between 130 and 144 d of gestation (term 145+/-2 d), maturational increments in plasma cortisol and T3, and D1 (hepatic, renal, perirenal adipose tissue) and D3 (cerebral), and decrements in renal and placental D3 activities were abolished by fetal adrenalectomy. Between 125 and 130 d, iv cortisol infusion raised hepatic, renal, and perirenal adipose tissue D1 and reduced renal and placental D3 activities. Infusion with T3 alone increased hepatic D1 and decreased renal D3 activities. Therefore, in the sheep fetus, the prepartum cortisol surge induces tissue-specific changes in deiodinase activity that, by promoting production and suppressing clearance of T3, may be responsible for the rise in plasma T3 concentration near term. Some of the maturational effects of cortisol on deiodinase activity may be mediated by T3.

Mechanisms of Disease: psychomotor retardation and high T3 levels caused by mutations in monocarboxylate transporter 8

The actions and the metabolism of thyroid hormone are intracellular events that require the transport of iodothyronines across the plasma membrane. It is increasingly clear that this process does not occur by simple diffusion, but is facilitated by transport proteins. Only recently have iodothyronine transporters been identified at the molecular level, of which organic anion transporting polypeptide 1C1 and monocarboxylate transporter 8 (MCT8) deserve special mention, because of their high activity and specificity for iodothyronines. Organic anion transporting polypeptide 1C1 is almost exclusively expressed in brain capillaries, and may be crucial for the transport of the prohormone T4 across the blood-brain barrier. MCT8 is also expressed in the brain--in particular, in neurons--but also in other tissues. MCT8 seems to be especially important for the uptake of active hormone T3 into neurons, which is essential for optimal brain development. T3 is produced from T4 by type 2 deiodinase in neighboring astrocytes. Neurons express type 3 deiodinase, the enzyme that terminates T3 activity. The SLC16A2 (formerly MCT8) gene is located on chromosome Xq13.2 and has recently been associated with a syndrome combining severe, X-linked, psychomotor retardation and high serum T3 levels. In over 20 families, where affected males have developed this syndrome, several mutations in MCT8 have been identified. The disease mechanism is thought to involve a defect in the neuronal entry of T3 and, therefore, in the action and metabolism of T3 in these cells. This defect results in impaired neurological development and a decrease in T3 clearance.

Children with Differentiated Thyroid Cancer Achieve Adequate Hyperthyrotropinemia within 14 Days of Levothyroxine Withdrawal

The preparation for radioiodine administration recommended by the current pediatric literature is a 6-wk withdrawal that typically includes the transient administration of T3. Compared with adults, T4 clearance rates and serum TSH to free T4 ratios are higher in children, implying that pediatric patients can achieve adequate hyperthyrotropinemia with shorter levothyroxine withdrawals. The objective of this study was to determine whether children with differentiated thyroid cancer achieve adequate hyperthyrotropinemia using an abbreviated levothyroxine withdrawal protocol. The study design was a retrospective analysis of 15 consecutive levothyroxine withdrawals performed without T3 at Children's Hospital Boston. Eleven children with differentiated thyroid cancer were included. The average age at the time of withdrawal was 12.5 +/- 0.8 yr. Serum TSH concentrations obtained after the discontinuation of levothyroxine were analyzed to determine the time interval required to achieve a serum TSH level greater than 25 microU/ml for each patient. Adequate hyperthyrotropinemia was documented in all children tested by d 14. The mean interval required to achieve a serum TSH level above 25 microU/ml from a suppressed serum TSH was 12.3 +/- 0.7 d. Shorter withdrawals minimize hypothyroid morbidity and the theoretical risk of decreased 131micro)I residence time from excessive hyperthyrotropinemia. These benefits are amplified in children due to their high incidence of distant metastases. We propose an abbreviated 2-wk withdrawal protocol to facilitate the adjunctive therapy and surveillance of children with follicular cell-derived cancers.

Involvement of multispecific organic anion transporter, Oatp14 (Slc21a14), in the transport of thyroxine across the blood-brain barrier.

The present study was aimed at investigating the involvement of mouse organic anion transporting polypeptide 14 (mOatp14) in the uptake of T4 across the blood-brain barrier. Functional expression of mOatp14 in HEK293 cells revealed that T4 and rT3 are high affinity substrates of mOatp14 (Michaelis constant, 0.34 and 0.46 microm, respectively), and the specific uptake of T3 was 4-fold less than that of T4 and rT3. Taurocholate, probenecid, and estrone-3-sulfate were moderate inhibitors for mOatp14, whereas digoxin (substrate of Oatp2), benzylpenicillin (substrate of Oat3), and large neutral amino acids had no effect. mOatp14 is widely expressed throughout the brain, except for the cerebellum. The expression of mOatp14 in the isolated brain capillaries and the choroid plexus was shown by Western blot. The uptake clearance of T4 by the cerebral cortex determined using the in situ brain perfusion technique in mice was 580 microl/min.g tissue, 3-fold greater than that by the cerebellum, and a saturable component (Michaelis constant, 1.0 microm) accounts for the major fraction of the total uptake. Taurocholate inhibited the uptake of T4 by the cerebral cortex completely, but the inhibition by estrone-3-sulfate was partial (50%). These results suggest that transporters play a predominant role in the delivery of T4 to the brain, and mOatp14 accounts for estrone-3-sulfate inhibitable fraction, at least partly. The absence of inhibition by digoxin, benzylpenicillin, leucine, and 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid for the uptake of T4 by the cerebral cortex suggests the presence of other unknown transporter for T4 uptake by the brain. Immunohistochemical staining revealed basolateral localization of mOatp14 in the choroid plexus in which it may also play a role in T4 uptake.