Thyroid-stimulating Hormone Production Research Articles

Novel Mutation (T273R) in Thyroid Hormone Receptor β Gene Provides Further Insight into Cryptic Negative Regulation by Thyroid Hormone

Production of thyroid hormone is precisely regulated in a negative feed-back mechanism that depends critically on thyroid hormone receptor β (TRβ). This mechanism decreases production of thyrotropin- releasing hormone (TRH) and thyrotropin (TSH) in the hypothalamus and pituitary gland in response to high levels of circulating thyroid hormones (TH). Despite the wealth of accumulated knowledge, it is still not clear how exactly this negative regulation is executed. The syndrome of resistance to thyroid hormone (RTH), in which the levels of TH are not properly sensed, represents naturally occurring situations in which molecular components of this regulation are displayed and may be uncovered. TRβ, which is central to this regulation, is in the majority of RTH cases mutated in a way that preserves some functions of the receptor. Approximately 150 different mutations in TRβ have been identified to date. Here, we hypothesized that additional pathogenic mutations in TRβ are likely to exist in human population and analysed clinical cases with suspected RTH. In keeping with our prediction, analysis of 17 patients from nine families led to identification of four presumed pathogenic mutations of TRβ, including a previously unknown mutation, T273R. This suggests that threonine 273 is likely to be critical for the normal function of TRβ, possibly due to its role in helix 12 mobility and interaction with coactivators, and thus supports the concept that TRβ-dependent trans-activating function is necessary for the inhibition of TRH and TSH expression in response to elevated levels of TH.

Risk Factors Associated with Disease Recurrence among Patients with Low-Risk Papillary Thyroid Cancer Treated at the University of the Philippines-Philippine General Hospital.

BackgroundThe management of papillary thyroid carcinoma (PTC) in high-risk patients is well-standardized. However, this is not the case for low-risk patients. Filipinos show a high incidence of recurrence of thyroid cancer. Thus, the identification of risk factors for recurrence in this population could potentially identify individuals for whom radioactive iodine (RAI) therapy might be beneficial.MethodsWe reviewed the medical records of adult Filipinos with low-risk PTC who underwent near-total or total thyroidectomy at the University of the Philippines-Philippine General Hospital. Multivariate logistic regression analysis was used to determine risk factors for recurrence.ResultsRecurrence was documented in 51/145 of patients (35.17%) included in this study. Possible risk factors such as age, sex, family history, smoking history, tumor size, multifocality, prophylactic lymph node dissection, initial thyroglobulin (Tg) level, initial anti-thyroglobulin (anti-Tg) antibody concentration, suppression of thyroid stimulating hormone production, and RAI therapy were analyzed. Multivariate analysis revealed that a tumor diameter 2 to 4 cm (odds ratio [OR], 9.17; 95% confidence interval [CI], 1.62 to 51.88; P=0.012), a tumor diameter >4 cm (OR, 16.46; 95% CI, 1.14 to 237.31; P=0.04), and a family history of PTC (OR, 67.27; 95% CI, 2.03 to 2228.96; P=0.018) were significant predictors of recurrence. In addition, RAI therapy (OR, 0.026; 95% CI, 0.01 to 0.023; P≤0.005), an initial Tg level ≤2 ng/mL (OR, 0.049; 95% CI, 0.01 to 0.23; P≤0.005), and an anti-Tg antibody level ≤50 U/mL (OR, 0.087; 95% CI, 0.011 to 0.67; P=0.019) were significant protective factors.ConclusionA tumor diameter ≥2 cm and a family history of PTC are significant predictors of recurrence. RAI therapy and low initial titers of Tg and anti-Tg antibody are significant protective factors against disease recurrence among low-risk PTC patients.

Development of neuroendocrine components of the thyroid axis in the direct-developing frog Eleutherodactylus coqui: Formation of the median eminence and onset of pituitary TSH production

Direct-developing frogs lack, wholly or in part, a wide range of larval features found in metamorphosing species and form adult-specific features precociously, during embryogenesis. Most information on thyroid regulation of direct development relies on hormone manipulations; the ontogeny of many thyroid axis components has not been fully described. This analysis examines differentiation of the median eminence of the hypothalamus and production of thyroid-stimulating hormone (TSH) by the pituitary of the direct-developing frog Eleutherodactylus coqui. The median eminence is established two-thirds of the way through embryogenesis. Cells immunoreactive to human TSHβ antibodies are first detected during embryogenesis and quantitative changes in TSHβ-IR cells resemble those in metamorphosing amphibians. Formation of the median eminence of the hypothalamus and TSHβ production by the pituitary precede or coincide with morphological changes during embryogenesis that occur during metamorphosis in biphasic anurans. Thus, while the onset of neuroendocrine regulation has changed during the evolution of direct development, it is likely that these thyroid axis components still mediate the formation of adult features.

Pregnancy sickness and parent-offspring conflict over thyroid function

Pregnancy sickness is widespread in human mothers but its etiology, somewhat surprisingly, remains unclear. Human chorionic gonadotropin (hCG) has long been considered a prime hormonal suspect, but the correlation between pregnancy sickness and hCG levels is imperfect resulting in uncertainty about its causal role. As others have noted part of this uncertainty likely stems from the structural and functional diversity of hCG. One enigmatic role of hCG is its action as a thyroid stimulator during early gestation. Native hCG is weakly thyrotropic but is produced in prodigious quantities and suppresses the production of thyroid stimulating hormone (TSH) but not curiously when TSH levels are in the higher deciles. Higher levels of hCG induce higher maternal production of thyroxine (T4). hCG thus appears to augment and sometimes even supplant TSH in the regulation of thyroid hormone in early gestation. This has lead to the suggestion that hCG serves as a backup system, albeit incomplete, for the production of essential thyroid hormone during pregnancy. Another interpretation, however, is that hCG, produced by the embryo, serves as a second control circuit for the thyroid during pregnancy. If so, it serves embryonic interests that are at odds with maternal interests (maternal-embryo conflict) under conditions of iodine deficiency. Iodine is an essential micronutrient for neurodevelopment and thyroid function, and has been in short supply for most humans over most of our evolutionary history. Iodine deficiency during gestation has severe impacts on embryo neuromotor development, but also induces thyroid disease in mothers, impairing her future reproductive prospects. Under this view, embryos use hCG to push mothers to release more thyroid hormone. hCG, however, is produced outside the normal maternal thyroid control circuit and thus is not subject to a normal negative feedback. hCG also serves multiple functions simultaneously therefore its production is likely not fine-tuned for thyroid function per se. hCG levels may remain high even when thyroid hormone production is more than sufficient to meet the needs of mother and embryo. Instead, the system appears to be regulated at the back end by clearing surplus hormone using placental Type II (D2) and Type III (D3) deiodinases. As maternal thyroid hormone levels rise, placental D3 is upregulated, shunting more T4 and T3 into a deactivating pathway. The metabolites that result, particularly the inert metabolite of T4, reverse T3, are correlates of surplus thyroid hormone production and thus are strong candidates for the proximate triggers of pregnancy sickness. Nausea and vomiting of early pregnancy thus arises as a by-product of an antagonistic pleiotropy between mother and embryo over the allocation of iodine: when dietary iodine is scarce, a benefit accrues to the embryo at a cost to mother; when iodine is plentiful, pregnancy sickness ranging from frequently mild to occasionally severe, is a sequelae of undiminished embryonic demands. If pregnancy sickness serves as a marker of thyroid function, an absence of first trimester nausea and vomiting sickness may indicate a higher priority for testing of thyroid function to avert the inimical effects of hypothyroidism during gestation.

<i>In Vivo</i> Evaluation of 1-Benzyl-4-aminoindole-Based Thyroid Hormone Receptor β Agonists: Importance of Liver Selectivity in Drug Discovery

We recently reported that the novel thyroid hormone receptor β (TRβ) selective agonists SKL-12846 and SKL-13784 reduce blood cholesterol levels without affecting thyroid-stimulating hormone (TSH) in cholesterol-fed rats. Our aim in this study was to elucidate what sets apart these SKL-compounds as TRβ agonists with no effect on TSH. To this end, we determined SKL-compounds pharmacokinetics and tissue distribution in normal rats and compared them to those of GC-1, a liver-selective TRβ agonist with concomitant effect on TSH. The present study explains why SKL-12846 and SKL-13784 have beneficial effects on lowering lipids without affecting heart rate and TSH production at the therapeutic dose in cholesterol-fed rats. In addition, we found that SKL-13784 shows no sign of escape phenomenon in fructose-fed rats. These results demonstrate the advantages of extremely high liver specificity to TRβ agonists. However, SKL-13784 has been found significantly to reduce endogenous T4 levels at doses lower than its lipid-lowering dose, which may raise concerns over this compound's ability to alter thyroid hormone metabolism in the liver. While the mechanism by which SKL-13784 reduces endogenous T4 levels is still unclear, our results would help design better liver-selective TRβ modulators.

Solution of a Mathematical Model Describing the Change of Hormone Level in Thyroid Using the Laplace Transform

In the present paper, a mathematical model describing the thyroid-pituitary homeostatic mechanism is analyzed for its physiological and clinical significance. The influence of different parameters on the stability behavior of the system is discussed. We have assumed in the present paper that the rate of thyrotropin production is reduced by an amount which is proportional to the blood concentration of thyroxine and also the rate of loss of thyrotropin is proportional to the existing thyrotropin concentration. The stability behavior of the system is analyzed and the possibility of occurrence of periodic solutions is looked into. As the pituitary gland can produce no output in the presence of thyroxine concentration greater than a certain value, we have also included a degenerate form of the equation for thyrotropin production in the present paper. The solutions of the system of governing equations are obtained by applying the Laplace transform. Also, the nature of the solution is interpreted graphically using the Maple12 technique for both stable and unstable behavior.

TSH restores a summer phenotype in photoinhibited mammals via the RF‐amides RFRP3 and kisspeptin

In mammals, melatonin is the pivotal messenger synchronizing biological functions, notably reproductive activity, with annual daylength changes. Recently, two major findings clarified melatonin's mode of action. First, melatonin controls the production of thyroid stimulating hormone (TSH) by the pars tuberalis of the adenohypophysis. This TSH regulates local thyroid hormone availability in the mediobasal hypothalamus. Second, the RF-amides kisspeptin and RFRP-3, recently discovered regulators of the gonadotropic axis, are involved in the melatonin control of reproduction. This study aims to establish a mechanistic link between the melatonin-driven TSH and the RF-amide control of reproduction. We treated short-day-adapted male Djungarian and Syrian hamsters with a chronic central infusion of TSH. In both hamster species, the central administration of 5 mIU/d TSH for 4 to 6 wk restored the summer phenotype of both testicular activity and kisspeptin and RFRP expression. Vehicle treated hamsters remain sexually inactive. Furthermore, the TSH treatment increased the body weight of lean short-day-adapted Djungarian hamsters and reduced hypothalamic somatostatin expression to the summer phenotype. In summary, our study demonstrates the pivotal role of melatonin-driven TSH for the seasonal regulation of reproduction and body weight, and uncovers the neuropeptides relaying this signal within the hypothalamus.

Thyroid hormones – obesity and metabolic syndrome

Recently there has been increased interest in the association between thyroid function and obesity. Based on the notion that triiodothyronine (T3) controls metabolic and energy homeostasis and influences body weight, thermogenesis, lipolysis and metabolism of cholesterol, and that thyroid-stimulating hormone (TSH) via receptors in fat tissue, induces differentiation of preadipocytes in to adipocites and expansion of adipose tissue (adipogenesis), thyroid function has been extensively investigated in obese adults. An elevated level of TSH with normal peripheral thyroid hormone concentration suggesting sub-clinical hypothyroidism has been consistently found in obese subjects. Several mechanisms leading to hyperthyrotropinemia have hypothesis including sub clinical hypothyroidism caused by iodine deficiency, autoimmune thyroiditis or mutations in TSH-R gene. TSH production is also regulated by neurotransmitters and hormones that influence body weight such as neuropeptide Y and alfa-melanocyte-stimulating hormone related peptide, that innervate hypophysiotropic TRH neurons. These neurotransmitters and hormones are also influenced by leptin. Thyrotropin also directly induces the synthesis and release of adipokines. Some of them control appetite by acting on the brain. Metabolic syndrome (MS) is clustering obesity, hypertension, dyslipidemia and insulin resistance. MS is a status where most features of hypothyroidism can be seen. TSH increase was shown to be associated with increased cholesterol and triglycerides and with decreased HDL-C. Thyroid hormones are important determinants of glucose homeostasis. Increased thyroid hormone levels impair the ability of insulin to suppress hepatic glucose production and increase glucose uptake in muscles. An association between TSH and fasting insulin and insulin sensitivity has been reported in adults with obesity. The increased TSH and peripheral hormone levels, which are usually in the upper normal range in obese subjects may be adaptation process to increase energy expenditure in order to reduce further weight gain. The changes of thyroid hormones concentration may be regarded as a consequence rather than a cause of obesity. MS is cluster of metabolic abnormalities with insulin resistance as a major component. The patients with MS have significantly increased thyroid volume and nodule prevalence and insulin resistance is an independent risk factor for nodule formation. The prevalence of insulin resistance would be an important risk factor for developing thyroid cancer and some other non-thyroid cancers.

Transdifferentiation of pituitary thyrotrophs to lactothyrotrophs in primary hypothyroidism: case report

Primary hypothyroidism causes adenohypophysial hyperplasia via stimulation by hypothalamic thyrotropin-releasing hormone (TRH). The effect was long thought to simply result in thyroid-stimulating hormone (TSH) and prolactin (PRL) cell hyperplasia, an increase in TSH and PRL blood levels with resultant pituitary enlargement, often mimicking adenoma. Recently, it was shown that transformation of growth hormone (GH) cells into TSH cells takes place in both clinical and experimental primary hypothyroidism. Such shifts from one cell to another with a concomitant change in hormone production are termed "transdifferentiation" and involve the gradual acquisition of morphologic features of thyrotrophs ("somatothyrotrophs"). We recently encountered a unique case of pituitary hyperplasia in a 40-year-old female with primary hypothyroidism wherein increased TSH production was by way of PRL cell recruitment. The resultant "lactothyrotrophs" maintained TSH cell morphology (cellular elongation and prominence of PAS-positive lysosomes) but expressed immunoreactivity for both hormones. No co-expression of GH was noted nor was thyroidectomy cells seen. This form of transdifferentiation has not previously been described.

Consultation with the Specialist: Thyroid Nodules

Based on strong research evidence, thyroid nodules in children and teenagers are more likely to be malignant than in adults. Based on strong research evidence, a history of ionizing radiation to the head or neck is an independent risk factor for the development of thyroid malignancies. There is strong research evidence, including a recent meta-analysis, supporting the use of fine-needle aspiration biopsy in the evaluation of all pediatric and adolescent patients presenting with a thyroid nodule. The surgical management and postoperative care of pediatric and adolescent patients who have well-differentiated thyroid carcinomas remains controversial, because the rarity of the disease limits the ability to conduct randomized, prospective research studies. Numerous studies have demonstrated that, despite presenting with more advanced disease, pediatric and adolescent patients with thyroid carcinoma have a higher survival rate than adults. The American Thyroid Association has issued strong evidence-based recommendations for the management of medullary thyroid carcinoma, including RET mutation testing and early prophylactic total thyroidectomy in children with high-risk mutations.

Encephalopathy and sinustachycardia in childhood a possible differential diagnosis

We report on a 7-year-old girl with generalized seizures, somnolence, fever, and respiratory distress. The increase of sinus tachycardia with good hydration, sufficient analgesia, and hyperthermia in our patient led to the determination of thyroid hormones, and therefore finally to the diagnosis of a thyrotoxic crisis in Graves' disease. Thyrotoxic crisis is a very rare but severe disease with an incidence of 0.1/100,000-3/100,000 among children. Important differential diagnoses of hyperthyroidism are Hashimoto thyroiditis, paraneoplastic thyroid-stimulating hormone production, thyroid autonomy, as well as central hyperthyroidism. Although symptoms disappear by thyrostatic therapy (thiamazole, carbimazole, dexamethasone), a euthyroid status could only be achieved by thyroidectomy. Thyrotoxic crisis should be considered a differential diagnosis in case of resistant unexplained sinus tachycardia, seizures, and encephalopathy. Immediate and adequate therapy contributes significantly to a reduction in the high morbidity and mortality rates. A combination of several treatment approaches for hyperthyroidism can lead to a successful outcome.

Evidence for RGS4 Modulation of Melatonin and Thyrotrophin Signalling Pathways in the Pars Tuberalis

In mammals, the pineal hormone melatonin is secreted nocturnally and acts in the pars tuberalis (PT) of the anterior pituitary to control seasonal neuroendocrine function. Melatonin signals through the type 1 Gi-protein coupled melatonin receptor (MT1), inhibiting adenylate cyclase (AC) activity and thereby reducing intracellular concentrations of the second messenger, cAMP. Because melatonin action ceases by the end of the night, this allows a daily rise in cAMP levels, which plays a key part in the photoperiodic response mechanism in the PT. In addition, melatonin receptor desensitisation and sensitisation of AC by melatonin itself appear to fine-tune this process. Opposing the actions of melatonin, thyroid-stimulating hormone (TSH), produced by PT cells, signals through its cognate Gs-protein coupled receptor (TSH-R), leading to increased cAMP production. This effect may contribute to increased TSH production by the PT during spring and summer, and is of considerable interest because TSH plays a pivotal role in seasonal neuroendocrine function. Because cAMP stands at the crossroads between melatonin and TSH signalling pathways, any protein modulating cAMP production has the potential to impact on photoperiodic readout. In the present study, we show that the regulator of G-protein signalling RGS4 is a melatonin-responsive gene, whose expression in the PT increases some 2.5-fold after melatonin treatment. Correspondingly, RGS4 expression is acutely sensitive to changing day length. In sheep acclimated to short days (SP, 8 h light/day), RGS4 expression increases sharply following dark onset, peaking in the middle of the night before declining to basal levels by dawn. Extending the day length to 16 h (LP) by an acute 8-h delay in lights off causes a corresponding delay in the evening rise of RGS4 expression, and the return to basal levels is delayed some 4 h into the next morning. To test the hypothesis that RGS4 expression modulates interactions between melatonin- and TSH-dependent cAMP signalling pathways, we used transient transfections of MT1, TSH-R and RGS4 in COS7 cells along with a cAMP-response element luciferase reporter (CRE-luc). RGS4 attenuated MT1-mediated inhibition of TSH-stimulated CRE-luc activation. We propose that RGS4 contributes to photoperiodic sensitivity in the morning induction of cAMP-dependent gene expression in the PT.

Painless Thyroiditis Complicated by Acromegaly

The serum thyroid stimulating hormone (TSH) level is decreased in acromegalic patients. Although this phenomenon is thought to be caused by the enhanced secretion of somatostatin which suppresses TSH production, it has not yet been proven. We describe a 60-year-old woman with acromegaly who showed a low concentration of TSH. We diagnosed her as painless thyroiditis based on an increased level of thyroglobulin, depressed radioactive iodine uptake (RAIU), normal vascularity and mild swelling of the thyroid, and normal T3, T4, free T3 and free T4 levels. To our knowledge, this is the second reported case of acromegaly complicated by painless thyroiditis. The differential diagnosis between central hypothyroidism and painless thyroiditis is so important. Since it is difficult to diagnose precisely based on only the data of a low level of TSH and normal levels of thyroid hormones, we consider that measurement of thyroglobulin and RAIU is necessary when the complication of painless thyroiditis is suspected.

Involvement of thyrotropin in photoperiodic signal transduction in mice

Local thyroid hormone catabolism within the mediobasal hypothalamus (MBH) by thyroid hormone-activating (DIO2) and -inactivating (DIO3) enzymes regulates seasonal reproduction in birds and mammals. Recent functional genomics analysis in birds has shown that long days induce thyroid-stimulating hormone production in the pars tuberalis (PT) of the pituitary gland, which triggers DIO2 expression in the ependymal cells (EC) of the MBH. In mammals, nocturnal melatonin secretion provides an endocrine signal of the photoperiod to the PT that contains melatonin receptors in high density, but the interface between the melatonin signal perceived in the PT and the thyroid hormone levels in the MBH remains unclear. Here we provide evidence in mice that TSH participates in this photoperiodic signal transduction. Although most mouse strains are considered to be nonseasonal, a robust photoperiodic response comprising induced expression of TSHB (TSH beta subunit), CGA (TSH alpha subunit), and DIO2, and reduced expression of DIO3, was observed in melatonin-proficient CBA/N mice. These responses could not be elicited in melatonin-deficient C57BL/6J, but treatment of C57BL/6J mice with exogenous melatonin elicited similar effects on the expression of the above-mentioned genes as observed in CBA/N after transfer to short-day conditions. The EC was found to express TSH receptor (TSHR), and ICV injection of TSH induced DIO2 expression. Finally, we show that melatonin administration did not affect the expression of TSHB, DIO2, and DIO3 in TSHR-null mice. Taken together, our findings suggest that melatonin-dependent regulation of thyroid hormone levels in the MBH appears to involve TSH in mammals.

Index of Suspicion

Index of Suspicion

A Biologically Based Dose-Response Model for Dietary Iodide and the Hypothalamic-Pituitary-Thyroid Axis in the Adult Rat: Evaluation of Iodide Deficiency

A biologically based dose-response (BBDR) model was developed for dietary iodide and the hypothalamic-pituitary-thyroid (HPT) axis in adult rats. This BBDR-HPT axis model includes submodels for dietary iodide, thyroid-stimulating hormone (TSH), and the thyroid hormones, T(4) and T(3). The submodels are linked together via key biological processes, including (1) the influence of T(4) on TSH production (the HPT axis negative feedback loop), (2) stimulation of thyroidal T(4) and T(3) production by TSH, (3) TSH upregulation of the thyroid sodium (Na(+))/iodide symporter, and (4) recycling of iodide from metabolism of thyroid hormones. The BBDR-HPT axis model was calibrated to predict steady-state concentrations of iodide, T(4), T(3), and TSH for the euthyroid rat whose dietary intake of iodide was 20 mug/day. Then the BBDR-HPT axis model was used to predict perturbations in the HPT axis caused by insufficient dietary iodide intake, and simulation results were compared to experimental findings. The BBDR-HPT axis model was successful in simulating perturbations in serum T(4), TSH, and thyroid iodide stores for low-iodide diets of 0.33-1.14 mug/day. Model predictions of serum T(3) concentrations were inconsistent with observations in some cases. BBDR-HPT axis model simulations show a steep dose-response relationship between dietary intake of iodide and serum T(4) and TSH when dietary iodide intake becomes insufficient (less than 2 mug/day) to sustain the HPT axis. This BBDR-HPT axis model can be linked with physiologically based pharmacokinetic models for thyroid-active chemicals to evaluate and predict dose-dependent HPT axis alterations based on hypothesized modes of action. To support continued development of this model, future studies should include time course data after perturbation of the HPT axis to capture changes in endogenous iodide, serum TSH, T(4), and T(3).

Diurnal Change of Thyroid‐Stimulating Hormone mRNA Expression in the Rat Pars Tuberalis

Thyroid-stimulating hormone (TSH)-producing cells (TSH cells), which account for a large fraction of the cells in the rat pars tuberalis (PT), have been found to express MT1 melatonin receptor and mammalian clock genes at high densities. Although these findings suggest that TSH production in the rat PT is regulated by melatonin and/or the biological clock, there have been no studies focusing on the diurnal change and regulation mechanism of TSH production in the rat PT. Therefore, in the present study, we examined diurnal changes of in TSH beta and alpha-glycoprotein subunit (alpha GSU) mRNA expression and TSH immunoreactivity (-ir) in the rat PT, and also examined the relationship between melatonin and TSH production in vivo. Both TSH beta mRNA expression and alpha GSU mRNA expression in the PT showed diurnal variations: the expression levels were lowest at the light phase [Zeitgeber time (ZT)4] and high at the dark phase (ZT12 and ZT20). TSH-ir in the PT showed the lowest level at ZT4, as was found for mRNA expression. Interestingly, TSH-ir, which was confined to the Golgi apparatus at ZT4, spread to the cytoplasm, and most of the TSH cells in the PT were uniformly immunostained in the cytoplasm at ZT20. Despite the fact that chronic administration of melatonin suppressed TSH beta and alpha GSU mRNA expression, TSH-ir in the PT was significantly enhanced. These findings results clearly show that there are diurnal changes in TSH expression and accumulation in rat PT-TSH cells and suggest that these fluctuations are regulated by melatonin.

Interactions of Vitamin A and Iodine Deficiencies: Effects on the Pituitary-Thyroid Axis

Vitamin A (VA) deficiency (VAD) and the iodine deficiency disorders (IDD) affect > 30% of the global population and these deficiencies often coexist in vulnerable groups. VAD has multiple effects on the pituitary-thyroid axis; VA status modulates thyroid gland metabolism, peripheral metabolism of thyroid hormone, and production of thyrotropin (TSH) by the pituitary. Findings from Africa children indicate that VAD in severely-IDD-affected children increases TSH stimulation and thyroid size, and reduces risk for hypothyroidism. In children with VAD, the higher TSH concentrations in the face of higher circulating total thyroxine suggest central resistance to normal TSH suppression by thyroid hormone. In IDD- and VAD-affected children receiving iodized salt, concurrent VA supplementation improves iodine efficacy. Recent VA and iodine depletion studies in rats indicate moderate VAD alone has no measurable effect on the pituitary-thyroid axis; however, concurrent iodine deficiency (ID) and VAD produce more severe primary hypothyroidism than ID alone. Repletion studies in VA- and iodine-deficient animals suggest: 1) primary hypothyroidism in animals with concurrent moderate VAD and ID does not reduce the efficacy of high doses of oral VA; 2) VAD does not reduce the efficacy of dietary iodine to correct pituitary-thyroid axis dysfunction due to iodine deficiency; and 3) given alone, without iodine repletion, high-dose VA supplementation in combined VAD and ID may reduce thyroid hyperstimulation and reduce risk for goiter.

The NAS Perchlorate Review: Adverse Effects?

Ginsberg and Rice (2005) argued that the reference dose for perchlorate of 0.0007 mg/kg per day recommended by the National Academy of Sciences (NAS) Committee to Assess the Health Implications of Perchlorate Ingestion is not adequately protective. As members of the committee, we disagree. Ginsberg and Rice (2005) based their conclusion on three points. The first involves the designation of the point of departure as a NOEL (no observed effect level) versus a LOAEL (lowest observed adverse effect level). The committee chose as its point of departure a dose of perchlorate (0.007 mg/kg per day) that, when given for 14 days to seven normal subjects, did not cause a statistically significant decrease in the group mean thyroid iodide uptake (Greer et al. 2002). Accordingly, the committee considered it a NOEL. Ginsberg and Rice (2005) focused on the fact that only seven subjects were given that dose; they seem to say that attention should be paid only to the results in those subjects in whom there was a decrease in thyroid iodide uptake and that the results in those in whom there was no decrease or an increase should be ignored. They considered the dose to be a LOAEL because of the decrease in uptake in those few subjects. It is important to note that a statistically significant decrease of, for example, 5% or even 10% would not be biologically important and, more important, would not be sustained. For example, in another study (Braverman et al. 2004), administration of 0.04 mg/kg per day to normal subjects for 6 months had no effect on thyroid iodide uptake when measured at 3 and 6 months, and no effect on serum thyroid hormone or thyrotropin concentrations measured monthly. [Inspection of Figure 5A in Greer et al. (2002) suggests that this dose would inhibit thyroid iodide uptake by about 25% if measured at 2 weeks.] The second issue involves database uncertainty. In clinical studies, perchlorate has been administered prospectively to 68 normal subjects for 2 weeks to 6 months. In one study (Brabant et al. 1992), a dose of 9.2 mg/kg per day for 4 weeks had no effect on thyroid function. In occupational studies, doses as high as 0.5 mg/kg per day had no effect on thyroid hormone or thyrotropin production in workers. In epidemiologic studies, there were no abnormalities in growth or thyroid function in children exposed life-long to 100–120 μg perchlorate per liter of drinking water, or in pregnant women and newborn infants similarly exposed. Given the choice of a nonadverse effect (inhibition of iodide uptake by the thyroid) as the point of departure and the multiple studies in which doses of perchlorate much higher than 0.007 mg/kg per day had no effect on any aspect of thyroid function, the committee did not apply a database uncertainty factor. Finally, Ginsberg and Rice (2005) argued that inhibition of thyroid iodide uptake is adverse. That conclusion assumes that any acute inhibition would be sustained, so thyroid hormone production would decrease. That is not the case. There is remarkable compensation for even substantial reductions in thyroid iodide uptake—and thyroid hormone production. As noted above, subjects given 0.04 mg/kg per day for 6 months and 9.2 mg/kg per day for 4 weeks—doses that certainly would inhibit thyroid iodide uptake for a few weeks—had no decrease in serum thyroid hormone or increase in serum thyrotropin concentrations (the hallmark of even mild hypothyroidism). Short-term inhibition of thyroid iodide uptake is not an adverse effect; it has no adverse consequences because there is rapid compensation mediated by several independent processes. One of these processes is up-regulation of the thyroid sodium-iodide transport system, as a result of intrathyroidal iodide deficiency. The second, should there be even a very small decrease in thyroid hormone production, is an increase in thyrotropin secretion, resulting in overall stimulation of the thyroid gland. Analyses of the effects of any substance on thyroid function must take these compensatory processes into account, particularly the fact that the effect of any substance that inhibits thyroid function will diminish with time. Only if all of these mechanisms fail will there be hypothyroidism, the first adverse effect in the continuum of effects resulting from perchlorate ingestion. If there is no inhibition of iodide uptake to begin with, there will be no other changes in thyroid function at any time. We believe that the committee’s recommended reference dose of 0.0007 mg/kg per day provides a wide margin of safety for all subjects of all ages.

Serum thyroid‐stimulating hormone is elevated in men with Gleason 8 prostate cancer

To measure the levels of serum thyroid- stimulating hormone (TSH) in men with prostate cancer, as those with a Gleason score of > or = 8 are at high risk of skeletal metastases (and should be considered for bone scintigraphy at diagnosis), and because the structural integrity of the skeleton depends on constant remodelling controlled by many local and systemic factors, including TSH, an important regulator of this process. We evaluated 51 men referred for treatment of localized prostate cancer and 10 with biopsy-confirmed benign prostatic hypertrophy. Serum TSH was determined with a chemoluminescent immunoassay and a commercially available instrument (Immulite, Diagnostic Products Corporation, Los Angeles). There was significant variation in TSH levels with Gleason score (P = 0.004); men with Gleason 8 tumours had the highest serum TSH levels. Because serum TSH levels increase with age, we used a multivariate analysis of variance with both age and Gleason score as covariates. The effect of Gleason score on TSH level was significant (P = 0.036) and independent of the effect of age (P = 0.392). We propose that the high serum TSH levels in men with Gleason 8 prostate cancer is a result of the elaboration of TSH by cancer cells. Bone mineral density in the face of normal levels of thyroid hormone depends on an intact response to TSH, which ordinarily suppresses both osteoblast and osteoclast differentiation, thereby exerting control over bone remodelling. However, with abnormally high TSH levels this process may become deranged, promoting the development of bone metastases. If TSH production by prostate cancer cells could be suppressed, the incidence of bone metastases might be reduced.