The Association of Thyroid Function With Maternal and Neonatal Homocysteine Concentrations.

Context: High homocysteine concentrations are associated with maternal pregnancy complications and low birth weight, jaundice, and cerebrovascular accidents in neonates. Thyroid hormone may interfere with homocysteine metabolism via stimulation of vitamin B12- and folate-dependent processes and via effects on enzymes of the remethylation pathway. Objective: Investigating the associations of maternal and neonatal thyroid function with homocysteine during pregnancy and after delivery, respectively. Design, Setting, and Participants: Within Generation R study, a population-based prospective cohort, we studied the associations of maternal and neonatal thyroid stimulating hormone (TSH) and free thyroxine (FT4) with homocysteine, folate, and vitamin B12 concentrations using multiple linear regression analyses. Main Outcome Measures: TSH, FT4, homocysteine, folate, and vitamin B12 concentrations were determined in early pregnancy (<18 weeks; N = 1094 women without folic acid supplementation) and in cord blood of 4475 neonates. Results: In neonates, there was a positive association of FT4 with homocysteine and an inverse association of TSH with homocysteine. The associations attenuated after adjustment for folate and vitamin B12 concentration (ß change: for FT4, 0.00559 ± 0.001, P < 0.0001, to 0.00310 ± 0.001, P = 0.015; and for TSH, -0.00165 ± 0.001, P = 0.005, to -0.00086 ± 0.001, P = 0.11). In mothers, there was a positive association of FT4 with homocysteine (P = 0.026) but no association of FT4 with folate or vitamin B12 (P ≥ 0.08). Conclusion: Higher thyroid function is associated with higher homocysteine concentrations in pregnant women and in neonates. These data provide new insights into the effects of thyroid hormone on folate- and vitamin B12-dependent processes during early growth and development.

Sorafenib-Induced Changes in Thyroid Hormone Levels in Patients Treated for Hepatocellular Carcinoma.

Context: The pathogenesis of tyrosine kinase inhibitor-induced thyroid hormone (TH) alterations are still a matter of debate. Objective: The objective of this study was to determine the effects of sorafenib on TH levels in patients with hepatocellular carcinoma (HCC) and to evaluate possible mechanisms. Design: We performed a prospective cohort study between 2009 and 2016. Setting: This study was conducted at a tertiary referral center. Patients: This study included 57 consecutive patients with HCC who were treated with sorafenib. Main Outcome Measure: Thyroid-stimulating hormone (TSH) and free thyroxine (FT4) levels were measured every 6 weeks, and extensive thyroid function tests (TFTs) were measured before treatment (t0), after 6 weeks (t6), and at the end of therapy. The effect of sorafenib on TH transport by monocarboxylate transporter (MCT)8 or MCT10 was tested in transfected COS1 cells. Results: Four patients (7%) developed thyroiditis. Among the other patients, 30% had elevation of TSH or FT4 above the normal range. Overall, between t0 and t6, mean TSH increased from 1.28 to 1.57 mU/L (P < 0.001) and mean FT4 from 18.4 to 21.2 pmol/L (P < 0.001). Simultaneously, the serum triiodothyronine (T3)/reverse triiodothyronine ratio and the (T3/thyroxine) ×100 ratio decreased. Sorafenib decreased cellular T3 uptake by MCT8 and to a lesser extent by MCT10. Conclusions: These in vivo data suggest that sorafenib affects TFTs on multiple levels. Our in vitro experiments suggest a possible role of sorafenib-induced inhibition of T3 transport into the cell by MCT8 and MCT10.

Selenium Status Is Positively Associated with Bone Mineral Density in Healthy Aging European Men.

OBJECTIVE: It is still a matter of debate if subtle changes in selenium (Se) status affect thyroid function tests (TFTs) and bone mineral density (BMD). This is particularly relevant for the elderly, whose nutritional status is more vulnerable. DESIGN AND METHODS: We investigated Se status in a cohort of 387 healthy elderly men (median age 77 yrs; inter quartile range 75-80 yrs) in relation to TFTs and BMD. Se status was determined by measuring both plasma selenoprotein P (SePP) and Se. RESULTS: The overall Se status in our population was low normal with only 0.5% (2/387) of subjects meeting the criteria for Se deficiency. SePP and Se levels were not associated with thyroid stimulating hormone (TSH), free thyroxine (FT4), thyroxine (T4), triiodothyronine (T3) or reverse triiodothyronine (rT3) levels. The T3/T4 and T3/rT3 ratios, reflecting peripheral metabolism of thyroid hormone, were not associated with Se status either. SePP and Se were positively associated with total BMD and femoral trochanter BMD. Se, but not SePP, was positively associated with femoral neck and ward's BMD. Multivariate linear analyses showed that these associations remain statistically significant in a model including TSH, FT4, body mass index, physical performance score, age, smoking, diabetes mellitus and number of medication use. CONCLUSION: Our study demonstrates that Se status, within the normal European marginally supplied range, is positively associated with BMD in healthy aging men, independent of thyroid function. Thyroid function tests appear unaffected by Se status in this population.

Association of antiepileptic drug usage, trace elements and thyroid hormone status.

BACKGROUND: Levels of thyroid hormone (TH) and trace elements (copper (Cu) and selenium (Se)) are important for development and function of the brain. Anti-epileptic drugs (AEDs) can influence serum TH and trace element levels. As the relationship between AEDs, THs, and trace elements has not yet been studied directly, we explored these interactions. METHOD: In total 898 participants, from the Thyroid Origin of Psychomotor Retardation study designed to investigate thyroid parameters in subjects with intellectual disability (ID), had data available on serum Se, Cu, thyroid stimulating hormone (TSH), free thyroxine (FT4), tri-iodothyronine (T3), reverse T3, T4, and thyroxine-binding globulin (TBG); 401 subjects were on AED treatment. Differences in trace elements according to medication usage was investigated using ANOVA, and associations between trace elements and thyroid parameters were analysed using (non-) linear regression models. RESULTS: Study participants were not deficient in any of the trace elements analyzed. AED (carbamazepine, valproate and phenytoin) usage was negatively associated with serum Se and showed compound-specific associations with Cu levels. After correction for drug usage, Se was positively associated with TSH levels, negatively associated with FT4 levels, and positively with T3 levels. Cu was positively associated with T4, T3, and rT3, which was largely dependent on TBG levels. CONCLUSION: The subjects with ID did not display profound deficiencies in trace element levels. AEDs were associated with serum Se and Cu levels, while serum Se and Cu were also associated with thyroid parameters. Further studies on the underlying mechanisms and potential clinical importance are warranted.

Absence of TRH receptor 1 in male mice affects gastric ghrelin production.

TRH not only functions as a thyrotropin releasing hormone but also acts as a neuropeptide in central circuits regulating food intake and energy expenditure. As one suggested mode of action, TRH expressed in the caudal brainstem influences vagal activity by activating TRH receptor 1 (TRH-R1). In order to evaluate the impact of a diminished medullary TRH signaling on ghrelin metabolism, we analyzed metabolic changes of TRH-R1 knockout (R1ko) mice in response to 24 hours of food deprivation. Because R1ko mice are hypothyroid, we also studied eu- and hypothyroid wild-type (wt) animals and R1ko mice rendered euthyroid by thyroid hormone treatment. Independent of their thyroidal state, R1ko mice displayed a higher body weight loss than wt animals and a delayed reduction in locomotor activity upon fasting. Ghrelin transcript levels in the stomach as well as total ghrelin levels in the circulation were equally high in fasted wt and R1ko mice. In contrast, only wt mice responded to fasting with a rise in ghrelin-O-acyltransferase mRNA expression and consequently an increase in serum levels of acylated ghrelin. Together, our data suggest that an up-regulation of medullary TRH expression and subsequently enhanced activation of TRH-R1 in the vagal system represents a critical step in the stimulation of ghrelin-O-acyltransferase expression upon starvation that in turn is important for adjusting the circulating levels of acylated ghrelin to the fasting condition.

Hypothyroidism compromises hypothalamic leptin signaling in mice.

The impact of thyroid hormone (TH) on metabolism and energy expenditure is well established, but the role of TH in regulating nutritional sensing, particularly in the central nervous system, is only poorly defined. Here, we studied the consequences of hypothyroidism on leptin production as well as leptin sensing in congenital hypothyroid TRH receptor 1 knockout (Trhr1 ko) mice and euthyroid control animals. Hypothyroid mice exhibited decreased circulating leptin levels due to a decrease in fat mass and reduced leptin expression in white adipose tissue. In neurons of the hypothalamic arcuate nucleus, hypothyroid mice showed increased leptin receptor Ob-R expression and decreased suppressor of cytokine signaling 3 transcript levels. In order to monitor putative changes in central leptin sensing, we generated hypothyroid and leptin-deficient animals by crossing hypothyroid Trhr1 ko mice with the leptin-deficient ob/ob mice. Hypothyroid Trhr1/ob double knockout mice showed a blunted response to leptin treatment with respect to body weight and food intake and exhibited a decreased activation of phospho-signal transducer and activator of transcription 3 as well as a up-regulation of suppressor of cytokine signaling 3 upon leptin treatment, particularly in the arcuate nucleus. These data indicate alterations in the intracellular processing of the leptin signal under hypothyroid conditions and thereby unravel a novel mode of action by which TH affects energy metabolism.

Thyroid hormone transporters and deiodinases in the developing human hypothalamus.

OBJECTIVE: Thyroid hormone (TH) signaling in brain cells is dependent on transport of TH across the plasma membrane followed by intracellular deiodination and binding to the nuclear TH receptors. The aim of this study is to investigate the expression of the specific TH transporters monocarboxylate transporter 8 (MCT8 (SLC16A2)), MCT10, organic anion transporting polypeptide 1C1 (OATP1C1 (SLCO1C1)), and the types 2 and 3 deiodinases (D2 and D3) in the developing human hypothalamus. DESIGN: Fifteen postmortem brain samples of fetuses and young children ranging between 17 weeks of gestation and 29 months of postnatal age including one child (28 months) with central congenital hypothyroidism were studied. METHODS: Sections of the different hypothalami were stained with polyclonal rabbit antisera against MCT8, MCT10, OATP1C1, D2, and D3. RESULTS: We found MCT8 and D3 but not D2 protein expression to be present in our earliest sample of 17 weeks of gestation, indicating triiodothyronine degradation, but not production at this time of development. At term, expression of TH transporters and D2 decreased and D3 expression increased, suggesting decreased TH signaling just before birth. The child with central congenital hypothyroidism showed higher MCT8 and D2 expression compared with the other children of similar age. CONCLUSIONS: This study reports the developmental timing of expression of components crucial for central TH signaling in the human hypothalamus. In general, during fetal hypothalamic development, the coordinated expression of D2 and D3 in combination with the different TH transporters suggests that proper TH concentrations are regulated to prevent untimely maturation of brain cells.

Expression of thyroid hormone transporters in the human hypothalamus.

CONTEXT: Transport of thyroid hormone across the plasma membrane is required for proper thyroid hormone action and metabolism. Several specific thyroid hormone transporters have been identified capable of facilitating uptake and/or efflux of thyroid hormones. Monocarboxylate transporter (MCT)-8, MCT10, and organic anion transporting polypeptide 1C1 (OATP1C1) are the best-characterized specific thyroid hormone transporters to date. OBJECTIVE: Our earlier studies in the human hypothalamus have shown that MCT8 is present in neurons of the hypothalamic paraventricular nucleus (PVN) and infundibular nucleus (IFN) and in tanycytes. We hypothesized that also MCT10 and OATP1C1 are present in specific areas of the human hypothalamus. DESIGN: We studied postmortem brain samples of patients with known serum thyroid hormone levels using immunocytochemistry to investigate the distribution of MCT10 and OATP1C1 in the hypothalamus. RESULTS: We found strong neuronal MCT10 immunocytochemical staining in a number of hypothalamic nuclei, including the PVN, IFN, and supraoptic nucleus. Intense staining was also observed in neurons of the lateral hypothalamus including the perifornical area. OATP1C1 immunoreactivity was present in glial cells throughout the hypothalamus. In addition, staining was present in capillary walls and in neurons of the PVN, IFN, and supraoptic nucleus. CONCLUSION: The strong expression of MCT10 and OATP1C1 in the human hypothalamus indicates a possible role in the regulation of the hypothalamus-pituitary-thyroid axis.

Sorafenib induced thyroiditis in two patients with hepatocellular carcinoma.

BACKGROUND: Sorafenib is a multi-targeted tyrosine kinase inhibitor licensed for the treatment of hepatocellular carcinoma and renal cell carcinoma. Thyroid function test abnormalities have been reported for different tyrosine kinase inhibitors, but only limited data on thyroid function test abnormalities related to sorafenib are available, demonstrating the occurrence of hypothyroidism in patients treated with sorafenib. SUMMARY: We describe two patients who developed temporary hyperthyroidism during the course of sorafenib treatment, which was followed by overt and subclinical hypothyroidism, respectively. Thyroid ultrasonography showed an atrophic thyroid gland in patient 1 , and signs of thyroiditis in patient 2 . Detailed reassessment of thyroid volumes on routinely performed computerized tomography scans showed a gradual decrease in thyroid volume during sorafenib treatment in one patient, suggesting progressive thyroid destruction. CONCLUSION: This case report describes in detail and for the first time two cases of sorafenib-induced thyroiditis. We assume that this sorafenib-induced destructive thyroiditis is an important cause of sorafenib-induced hypothyroidism.

Changes in the central component of the hypothalamus-pituitary-thyroid axis in a rabbit model of prolonged critical illness.

INTRODUCTION: Prolonged critically ill patients reveal low circulating thyroid hormone levels without a rise in thyroid stimulating hormone (TSH). This condition is labeled "low 3,5,3'-tri-iodothyronine (T3) syndrome" or "nonthyroidal illness syndrome (NTI)" or "euthyroid sick syndrome". Despite the low circulating and peripheral tissue thyroid hormone levels, thyrotropin releasing hormone (TRH) expression in the hypothalamus is reduced and it remains unclear which mechanism is responsible. We set out to study whether increased hypothalamic T3 availability could reflect local thyrotoxicosis and explain feedback inhibition-induced suppression of the TRH gene in the context of the low T3 syndrome in prolonged critical illness. METHODS: Healthy rabbits were compared with prolonged critically ill, parenterally fed animals. We visualized TRH mRNA in the hypothalamus by in situ-hybridization and measured mRNA levels for the type II iodothyronine diodinase (D2), the thyroid hormone transporters monocarboxylate transporter (MCT) 8, MCT10 and organic anion co-transporting polypeptide 1C1 (OATP1C1) and the thyroid hormone receptors alpha (TRalpha) and beta (TRbeta) in the hypothalamus. We also measured the activity of the D2 and type III iodothyronine deiodinase (D3) enzymes. RESULTS: In the hypothalamus of prolonged critically ill rabbits with low circulating T3 and TSH, we observed decreased TRH mRNA, increased D2 mRNA and increased MCT10 and OATP1C1 mRNA while MCT8 gene expression was unaltered as compared with healthy controls. This coincided with low hypothalamic thyroxine (T4) and low-normal T3 concentrations, without a change at the thyroid hormone receptor level. CONCLUSIONS: Although expression of D2 and of the thyroid hormone transporters MCT10 and OATP1C1 were increased in the hypothalamus of prolonged critical ill animals, hypothalamic T4 and T3 content or thyroid hormone receptor expression were not elevated. Hence, decreased TRH gene expression, and hereby low TSH and T3 during prolonged critical illness, is not exclusively brought about by hypothalamic thyrotoxicosis, and infer other TRH suppressing factors to play a role.

Preliminary evidence that a functional polymorphism in type 1 deiodinase is associated with enhanced potentiation of the antidepressant effect of sertraline by triiodothyronine.

BACKGROUND: Triiodothyronine (T3) is used to potentiate the clinical effect of antidepressant drugs. Inter-individual differences in efficacy may be related to genetically-based variability in thyroid function. METHODS: DNA was obtained from 64 patients treated with sertraline plus T3 (SERT-T3, N=35) or plus placebo (SERT-PLB, N=29), for 8 weeks. Antidepressant efficacy was rated with the 21 item Hamilton Rating Scale for Depression (HRSD-21). Functional polymorphisms in type 1 (DIO1-C785T, DIO1-A1814G) and type 2 deiodinase (DIO2-Thr92Ala and DIO2-ORFa-Gly3Asp) were genotyped. RESULTS: DIO1-C785T was associated with efficacy of T3 but not placebo supplementation, as indicated by the interaction of treatment, DIO1-C758T genotype and time (p=0.04) and a stronger effect of SERT-T3 among DIO1-758T allele carriers (p=0.01). HRSD-21 scores of DIO1-758T allele carriers declined by 68.7+26.6% (mean+SD) over 8 weeks compared to 42.9+37.8% among non-carriers (p=0.02). DISCUSSION: DIO1 plays a key-role in T4 to T3 conversion and in clearance of the inactive metabolite, rT3. Previous data associate the DIO1-785T allele with lower DIO1 activity. This is consistent with our observation that responders to T3 supplementation had lower baseline serum T3 levels than non-responders. Depressed patients, who have a genetically determined lower T4 to T3 conversion, may be more likely to benefit from T3 supplementation.

Thyroid hormone signaling in the hypothalamus.

PURPOSE OF REVIEW: Proper thyroid hormone signaling is essential for brain development and adult brain function. Signaling can be disrupted at many levels due to altered thyroid hormone secretion, conversion or thyroid hormone receptor binding. RECENT FINDINGS: Mutated genes involved in thyroid hormone signaling in patients and animal models have increased the understanding of the (patho-)physiological consequences of altered thyroid hormone signaling. Neuroanatomical studies have provided more insight in the underlying neuroanatomical pathways. SUMMARY: A number of thyroid hormone signaling pathways in the hypothalamus have been proposed, which may be involved in the adaptation of the thyroid axis, not only to hypo- and hyperthyroidism, but also to inflammation, critical illness and fasting. Studies in knockout and transgenic mouse models have shown that the individual characteristics of mutations in thyroid hormone receptors can cause striking differences in the observed phenotypes.

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

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

A phytoestrogen-rich diet increases energy expenditure and decreases adiposity in mice.

BACKGROUND: Obesity is an increasingly prevalent health problem, and natural effective therapeutic approaches are required to prevent its occurrence. Phytoestrogens are plant-derived compounds with estrogenic activities; they can bind to both estrogen receptors alpha and beta and mimic the action of estrogens on target organs. OBJECTIVES: The purpose of this study was to examine the influence of soy-derived phytoestrogens on energy balance and metabolism. METHODS: Male outbred mice (CD-1) were allowed ad libitum access to either a high soy-containing diet or a soy-free diet from conception to adulthood. We measured circulating serum isoflavone levels using reverse-phase solid-phase extraction for subsequent liquid chromatography electrospray tandem mass spectrometry analysis. Adult animals were analyzed for body composition by dual-energy X-ray absorptiometry, locomotor activity by running-wheel experiments, respiratory exchange rate by indirect calorimetry, and food intake using metabolic cages. Quantitative reverse transcriptase-polymerase chain reaction was performed to determine the expression of hypothalamic neuropeptide genes. RESULTS: We found that adult mice fed a soy-rich diet had reduced body weight, adiposity, and resistance to cold. This lean phenotype was associated with an increase in lipid oxidation due to a preferential use of lipids as fuel source and an increase in locomotor activity. The modulation of energy balance was associated with a central effect of phytoestrogens on the expression of hypothalamic neuropeptides, including agouti-related protein. CONCLUSION: The data suggest that dietary soy could have beneficial effects on obesity, but they also emphasize the importance of monitoring the phytoestrogen content of diets as a parameter of variability in animal experiments.

Hypothalamic thyroid hormone catabolism acts as a gatekeeper for the seasonal control of body weight and reproduction.

Seasonal adaptations in physiology exhibited by many animals involve an interface between biological timing and specific neuroendocrine systems, but the molecular basis of this interface is unknown. In this study of Siberian hamsters, we show that the availability of thyroid hormone within the hypothalamus is a key determinant of seasonal transitions. The expression of the gene encoding type III deiodinase (Dio3) and Dio3 activity in vivo (catabolism of T(4) and T(3)) is dynamically and temporally regulated by photoperiod, consistent with the loss of hypothalamic T(3) concentrations under short photoperiods. Chronic replacement of T(3) in the hypothalamus of male hamsters exposed to short photoperiods, thus bypassing synthetic or catabolic deiodinase enzymes located in cells of the ependyma of the third ventricle, prevented the onset of short-day physiology: hamsters maintained a long-day body weight phenotype and failed to undergo testicular and epididymal regression. However, pelage moult to a winter coat was not affected. Type II deiodinase gene expression was not regulated by photoperiod in these hamsters. Collectively, these data point to a pivotal role for hypothalamic DIO3 and T(3) catabolism in seasonal cycles of body weight and reproduction in mammals.

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.

Abnormal thyroid hormone metabolism in mice lacking the monocarboxylate transporter 8.

In humans, inactivating mutations in the gene of the thyroid hormone transporter monocarboxylate transporter 8 (MCT8; SLC16A2) lead to severe forms of psychomotor retardation combined with imbalanced thyroid hormone serum levels. The MCT8-null mice described here, however, developed without overt deficits but also exhibited distorted 3,5,3'-triiodothyronine (T3) and thyroxine (T4) serum levels, resulting in increased hepatic activity of type 1 deiodinase (D1). In the mutants' brains, entry of T4 was not affected, but uptake of T3 was diminished. Moreover, the T4 and T3 content in the brain of MCT8-null mice was decreased, the activity of D2 was increased, and D3 activity was decreased, indicating the hypothyroid state of this tissue. In the CNS, analysis of T3 target genes revealed that in the mutants, the neuronal T3 uptake was impaired in an area-specific manner, with strongly elevated thyrotropin-releasing hormone transcript levels in the hypothalamic paraventricular nucleus and slightly decreased RC3 mRNA expression in striatal neurons; however, cerebellar Purkinje cells appeared unaffected, since they did not exhibit dendritic outgrowth defects and responded normally to T3 treatment in vitro. In conclusion, the circulating thyroid hormone levels of MCT8-null mice closely resemble those of humans with MCT8 mutations, yet in the mice, CNS development is only partially affected.

Neuroanatomical pathways for thyroid hormone feedback in the human hypothalamus.

CONTEXT: Recent findings point to an increasing number of hypothalamic proteins involved in the central regulation of thyroid hormone feedback. The functional neuroanatomy of these proteins in the human hypothalamus is largely unknown at present. OBJECTIVE: The aim of this study was to report the distribution of type II and type III deiodinase (D2 and D3) as well as the recently identified T(3) transporter, monocarboxylate transporter 8 (MCT8), in the human hypothalamus. DESIGN: The study included enzyme activity assays, immunocytochemical studies, and mRNA in situ hybridizations in postmortem human hypothalamus (n = 9). RESULTS: D2 immunoreactivity is prominent in glial cells of the infundibular nucleus/median eminence, blood vessels, and cells lining the third ventricle. By contrast, both D3 and MCT8 are expressed by neurons of the paraventricular (PVN), supraoptic, and infundibular nucleus (IFN). In support of these immunocytochemical data, D2 and D3 enzyme activities are detectable in the mediobasal human hypothalamus. Combined D2, D3, MCT8, and thyroid hormone receptor immunohistochemistry and TRH mRNA in situ hybridization clearly showed that D3, MCT8, and thyroid hormone receptor isoforms are all expressed in TRH neurons of the PVN, whereas D2 is not. CONCLUSIONS AND IMPLICATIONS: Based on these findings, we propose three possible routes for thyroid hormone feedback on TRH neurons in the human PVN: 1) local thyroid hormone uptake from the vascular compartment within the PVN, 2) thyroid hormone uptake from the cerebrospinal fluid in the third ventricle followed by transport to TRH neurons in the PVN or IFN neurons projecting to TRH neurons in the PVN, and 3) thyroid hormone sensing in the IFN of the mediobasal hypothalamus by neurons projecting to TRH neurons in the PVN.

Independence of hyperleptinemia-induced fat disappearance from thyroid hormone.

Sustained hyperleptinemia induced in normal rats causes the rapid disappearance of body fat. This is attributed to a marked increase in uncoupled fatty acid oxidation in the white adipocytes, which also occurs in hyperthyroidism. Because hyperleptinemic rats have normal plasma T3 or T4 levels, we tested the possibility of "localized hyperthyroidism" due to increased conversion of T4 to T3 in the adipose tissue. We therefore induced sustained hyperleptinemia in normal rats by intravenous injection of recombinant adenovirus containing the leptin cDNA (AdCMV-leptin) and measured the mRNA and the activity of enzymes involved in T4 metabolism in the disappearing fat. The epididymal fat pad remnants exhibited a decrease in mRNA of deiodinase 1 and a doubling of deiodinase 2 mRNA (p<0.05), but their enzyme activities did not differ from normoleptinemic controls. To determine if thyroid hormone was required for the fat-wasting action of hyperleptinemia, we infused AdCMV-leptin into rats made athyroid by total thyroidectomy or by methimazole therapy. The fat loss in hyperleptinemic athyroid rats was as great as in euthyroid controls. We conclude that the fat-wasting effect of sustained hyperleptinemia does not involve "local hyperthyroidism" in white adipose tissue and does not require thyroid hormone.

An ascidian homolog of vertebrate iodothyronine deiodinases.

In all classes of vertebrates, the deiodination of the prohormone T(4) to T(3) represents an essential activation step in thyroid hormone action. The possible presence of iodothyronine deiodinase activity in protochordates has been demonstrated in vivo. Recent molecular cloning of the genomes and transcripts of several ascidian species allows further investigation into thyroid-related processes in ascidians. A cDNA clone from Halocynthia roretzi (hrDx) was found to have significant homology (30% amino acid identity) with the iodothyronine deiodinase gene sequences from vertebrates, including the presence of an in-frame UGA codon that might encode a selenocysteine (SeC) in the active site. Because it was not certain that the 3' untranslated region (UTR) contained a SeC insertion sequence (SECIS) element essential for SeC incorporation, a chimeric expression vector of the hrDx coding sequence and the rat deiodinase SECIS element was produced, as well as an expression vector containing the intact hrDx cDNA. COS, CHO, and HEK cells were transfected with these vectors, and deiodinase activity was measured in cell homogenates. Outer-ring deiodinase activity was detected using both T(4) and reverse T(3) as substrates, and activity was enhanced by the presence of the reductive cofactor dithiothreitol. The enzyme activity was optimal during incubation between 20 and 30 C (pH 6-7) and was strongly inhibited by gold-thioglucose. The Halocynthia deiodinase appears to be a high Michaelis-Menten constant (K(m)) enzyme (K(m) reverse T(3), 2 microM; and K(m) T(4), 4 microM). Deiodinase activity was completely lost upon the substitution of the SeC residue in the putative catalytic center by either cysteine or alanine. Transfection of the full-length hrDx cDNA produced deiodinase activity confirming the presence of a SECIS element in the 3'UTR, as revealed by the SECISearch program. In conclusion, our results show, for the first time, the existence of an ascidian iodothyronine outer-ring deiodinase. This raises the hypothesis that, in protochordates, the prohormone T(4) is activated by enzymatic outer-ring deiodination to T(3).