Diagnosis and management of thyroid disorders and thyroid hormone supplementation in adult horses and foals.

Equine thyroid disorders pose a diagnostic challenge in clinical practice because of the effects of nonthyroidal factors on the hypothalamic-pituitary-thyroid axis, and the horse's ability to tolerate wide fluctuations in thyroid hormone concentrations and survive without a thyroid gland. While benign thyroid tumours are common in older horses, other disorders like primary hypothyroidism or hyperthyroidism in adult horses and congenital hypothyroidism in foals are rare. There is a common misunderstanding regarding hypothyroidism in adult horses, especially when associated with the clinical profile of obesity, lethargy, and poor performance observed in dogs and humans. Low blood thyroid hormone concentrations are often detected in horses as a secondary response to metabolic and disease states, including with the nonthyroidal illness syndrome; however, it is important to note that low thyroid hormone concentrations in these cases do not necessarily indicate hypothyroidism. Assessing equine thyroid function involves measuring thyroid hormone concentrations, including total and free fractions of thyroxine (T4) and triiodothyronine (T3); however, interpreting these results can be challenging due to the pulsatile secretion of thyroid hormones and the many factors that can affect their concentrations. Dynamic testing, such as the thyrotropin-releasing hormone stimulation test, can help assess the thyroid gland response to stimulation. Although true hypothyroidism is extremely rare, thyroid hormone supplementation is commonly used in equine practice to help manage obesity and poor performance. This review focuses on thyroid gland pathophysiology in adult horses and foals, interpretation of blood thyroid hormone concentrations, and evaluation of horses with thyroid disorders. It also discusses the use of T4 supplementation in equine practice.

Models in neuroendocrinology.

The neuroendocrine systems of the hypothalamus are critical for survival and reproduction, and are highly conserved throughout vertebrate evolution. Their roles in controlling body metabolism, growth and body composition, stress, electrolyte balance and reproduction have been intensively studied, and have yielded a rich crop of original and challenging insights into neuronal function, insights that circumscribe a vision of the brain that is quite different from conventional views. Despite the diverse physiological roles of pituitary hormones, most are secreted in a pulsatile pattern, but arising through a variety of mechanisms. An important exception is vasopressin which uses bursting neural activity, but produces a graded secretion response to osmotic pressure, a sustained robust linear response constructed from noisy, nonlinear components. Neuroendocrine systems have many features such as multiple temporal scales and nonlinearity that make their underlying mechanisms hard to understand without mathematical modelling. The models presented here cover the wide range of temporal scales involved in these systems, including models of single cell electrical activity and calcium dynamics, receptor signalling, gene expression, coordinated activity of neuronal networks, whole-organism hormone dynamics and feedback loops, and the menstrual cycle. Many interesting theoretical approaches have been applied to these systems, but important problems remain, at the core the question of what is the true advantage of pulsatility.

Selenium Increases Thyroid-Stimulating Hormone-Induced Sodium/Iodide Symporter Expression Through Thioredoxin/Apurinic/Apyrimidinic Endonuclease 1-Dependent Regulation of Paired Box 8 Binding Activity.

AIMS: The sodium-iodide symporter (NIS) mediates the uptake of I(-) by the thyroid follicular cell and is essential for thyroid hormone biosynthesis. Nis expression is stimulated by thyroid-stimulating hormone (TSH) and also requires paired box 8 (Pax8) to bind to its promoter. Pax8 binding activity depends on its redox state by a mechanism involving thioredoxin/thioredoxin reductase-1 (Txn/TxnRd1) reduction of apurinic/apyrimidinic endonuclease 1 (Ape1). In this study, we investigate the role of Se in Nis expression. RESULTS: Selenium increases TSH-induced Nis expression and activity in rat thyroid cells. The stimulatory effect of Se occurs at the transcriptional level and is only observed for Nis promoters containing a Pax8 binding site in the Nis upstream enhancer, suggesting that Pax8 is involved in this effect. In fact, Se increases Pax8 expression and its DNA-binding capacity, and in Pax8-silenced rat thyroid cells, Nis is not Se responsive. By inhibiting Ape1 and TxnRd1 functions, we found that both enzymes are crucial for TSH and TSH plus Se stimulation of Pax8 activity and mediate the Nis response to Se treatment. INNOVATION: We describe that Se increases Nis expression and activity. We demonstrate that this effect is dependent on the redox functions of Ape1 and Txn/TxnRd1 through control of the DNA binding activity of Pax8. CONCLUSION: Nis expression is controlled by Txn/Ape1 through a TSH/Se-dependent mechanism. These findings open a new field of study regarding the regulation of Nis activity in thyroid cells. Antioxid. Redox Signal. 24, 855-866.

Clocks for all seasons: unwinding the roles and mechanisms of circadian and interval timers in the hypothalamus and pituitary.

Adaptation to the environment is essential for survival, in all wild animal species seasonal variation in temperature and food availability needs to be anticipated. This has led to the evolution of deep-rooted physiological cycles, driven by internal clocks, which can track seasonal time with remarkable precision. Evidence has now accumulated that a seasonal change in thyroid hormone (TH) availability within the brain is a crucial element. This is mediated by local control of TH-metabolising enzymes within specialised ependymal cells lining the third ventricle of the hypothalamus. Within these cells, deiodinase type 2 enzyme is activated in response to summer day lengths, converting metabolically inactive thyroxine (T4) to tri-iodothyronine (T3). The availability of TH in the hypothalamus appears to be an important factor in driving the physiological changes that occur with season. Remarkably, in both birds and mammals, the pars tuberalis (PT) of the pituitary gland plays an essential role. A specialised endocrine thyrotroph cell (TSH-expressing) is regulated by the changing day-length signal, leading to activation of TSH by long days. This acts on adjacent TSH-receptors expressed in the hypothalamic ependymal cells, causing local regulation of deiodinase enzymes and conversion of TH to the metabolically active T3. In mammals, the PT is regulated by the nocturnal melatonin signal. Summer-like melatonin signals activate a PT-expressed clock-regulated transcription regulator (EYA3), which in turn drives the expression of the TSHß sub-unit, leading to a sustained increase in TSH expression. In this manner, a local pituitary timer, driven by melatonin, initiates a cascade of molecular events, led by EYA3, which translates to seasonal changes of neuroendocrine activity in the hypothalamus. There are remarkable parallels between this PT circuit and the photoperiodic timing system used in plants, and while plants use different molecular signals (constans vs EYA3) it appears that widely divergent organisms probably obey a common set of design principles.

Thyroid hormone and seasonal regulation of reproduction.

Organisms living outside the tropics use changes in photoperiod to adapt to seasonal changes in the environment. Several models have contributed to an understanding of this mechanism at the molecular and endocrine levels. Subtropical birds are excellent models for the study of these mechanisms because of their rapid and dramatic response to changes in photoperiod. Studies of birds have demonstrated that light is perceived by a deep brain photoreceptor and long day-induced thyrotropin (TSH) from the pars tuberalis (PT) of the pituitary gland causes local thyroid hormone activation within the mediobasal hypothalamus (MBH). The locally generated bioactive thyroid hormone, T3, regulates seasonal gonadotropin-releasing hormone (GnRH) secretion, and hence gonadotropin secretion. In mammals, the eyes are the only photoreceptor involved in photoperiodic time perception and nocturnal melatonin secretion provides an endocrine signal of photoperiod to the PT to regulate TSH. Here, I review the current understanding of the hypothalamic mechanisms controlling seasonal reproduction in mammals and birds.

Molecular pathways involved in seasonal body weight and reproductive responses governed by melatonin.

Seasonal mammals typically of temperate or boreal habitats use the predictable annual cycle of daylength to initiate a suite of physiological and behavioural changes in anticipation of adverse environmental winter conditions, unfavourable for survival and reproduction. Daylength is encoded as the duration of production of the pineal hormone melatonin, but how the melatonin signal is decoded has been elusive. From the studies carried out in birds and mammals together with the advent of technologies such as microarray analysis of gene expression, progress has been achieved to demystify how seasonal physiology is regulated in response to the duration of melatonin signalling. The critical tissue for the action of melatonin is the pars tuberalis (PT) where melatonin receptors are located. At the molecular level, regulation of cyclic adenosine monophosphate (cAMP) signalling in this tissue is likely to be a key event for melatonin action, either an acute inhibitory action or sensitization of this pathway by prolonged stimulation of melatonin receptors reflecting durational melatonin presence. Melatonin action at the PT has been shown to have both positive and negative effects on gene transcription, incorporating components of the circadian clock as part of the mechanism of decoding the melatonin signal and regulating thyrotrophin-stimulating hormone (TSH) expression, a key output hormone of the PT. Microarray analysis of gene expression of PT tissue exposed to long and short photoperiods has identified important new genes that may be regulated by melatonin and contributing to the seasonal regulation of TSH production by this tissue. In the brain, tanycytes lining the third ventricle of the hypothalamus and regulation of thyroid hormone synthesis by PT-derived TSH in these cells are now established as an important component of the pathway leading to seasonal changes in physiology. Beyond the tanycyte, identified changes in gene expression for neuropeptides, receptors and other signalling molecules pinpoint some of the areas of the brain, the hypothalamus in particular, that are likely to be involved in the regulation of seasonal physiology.

[Thyroid effect on brain activity in adolescents during heart rhythm biofeedback session].

Types of neurophysiologic and thyroid condition in 15-17-year old adolescents were studied for the purpose of heart rhythm biofeedback session effect by heart rhythm variability parameters. Changes of heart rhythm vegetative regulation activity modulate functional capacities of central vegetative regulation structures. The biofeedback training with heart rhythm variability parameters increases brain bioelectrical activity in different frequency ranges. The thyroid system modulates functional activity of vegetative regulation central structures uppermost at sympathotonic and thyreotropin increasing leads to increase of rhythm maker structure reactivity in brain.

Seasonal biology: avian photoreception goes deep.

The avian hypothalamus senses light directly, allowing endocrine physiology to synchronise to seasonal day-length changes. New data implicate the photopigment VA-opsin in this deep brain photoreception.

Ancestral TSH mechanism signals summer in a photoperiodic mammal.

In mammals, day-length-sensitive (photoperiodic) seasonal breeding cycles depend on the pineal hormone melatonin, which modulates secretion of reproductive hormones by the anterior pituitary gland [1]. It is thought that melatonin acts in the hypothalamus to control reproduction through the release of neurosecretory signals into the pituitary portal blood supply, where they act on pituitary endocrine cells [2]. Contrastingly, we show here that during the reproductive response of Soay sheep exposed to summer day lengths, the reverse applies: Melatonin acts directly on anterior-pituitary cells, and these then relay the photoperiodic message back into the hypothalamus to control neuroendocrine output. The switch to long days causes melatonin-responsive cells in the pars tuberalis (PT) of the anterior pituitary to increase production of thyrotrophin (TSH). This acts locally on TSH-receptor-expressing cells in the adjacent mediobasal hypothalamus, leading to increased expression of type II thyroid hormone deiodinase (DIO2). DIO2 initiates the summer response by increasing hypothalamic tri-iodothyronine (T3) levels. These data and recent findings in quail [3] indicate that the TSH-expressing cells of the PT play an ancestral role in seasonal reproductive control in vertebrates. In mammals this provides the missing link between the pineal melatonin signal and thyroid-dependent seasonal biology.

Cell-based imaging of sodium iodide symporter activity with the yellow fluorescent protein variant YFP-H148Q/I152L.

The sodium iodide symporter (NIS) mediates iodide (I(-)) transport in the thyroid gland and other tissues and is of increasing importance as a therapeutic target and nuclear imaging reporter. NIS activity in vitro is currently measured with radiotracers and electrophysiological techniques. We report on the development of a novel live cell imaging assay of NIS activity using the I(-)-sensitive and genetically encodable yellow fluorescent protein (YFP) variant YFP-H148Q/I152L. In FRTL-5 thyrocytes stably expressing YFP-H148Q/I152L, I(-) induced a rapid and reversible decrease in cellular fluorescence characterized by 1) high affinity for extracellular I(-) (35 muM), 2) inhibition by the NIS inhibitor perchlorate, 3) extracellular Na(+) dependence, and 4) TSH dependence, suggesting that fluorescence changes are due to I(-) influx via NIS. Individual cells within a population of FRTL-5 cells exhibited a 3.5-fold variation in the rate of NIS-mediated I(-) influx, illustrating the utility of YFP-H148Q/I152L to detect cell-to-cell difference in NIS activity. I(-) also caused a perchlorate-sensitive decrease in YFP-H148Q/I152L fluorescence in COS-7 cells expressing NIS but not in cells lacking NIS. These results demonstrate that YFP-H148Q/I152L is a sensitive biosensor of NIS-mediated I(-) uptake in thyroid cells and in nonthyroidal cells following gene transfer and suggest that fluorescence detection of cellular I(-) may be a useful tool by which to study the pathophysiology and pharmacology of NIS.

Multihormonal resistance to parathyroid hormone, thyroid stimulating hormone, and other hormonal and neurosensory stimuli in patients with pseudohypoparathyroidism.

In patients with pseudohypoparathyroidism, hormonal resistance first affects parathyroid hormone (PTH), which leads to calcipenia, a decrease in renal vitamin D activation, and a tendency to bone receptor remodeling. However, because G proteins are ubiquitously distributed, multiple hormonal resistance occurs in pseudohypoparathyroidism type Ia and type Ic, impairing responses to other calciotropic hormones (PTHrP, calcitonin), TSH, and also pituitary and hypothalamic hormones, and to neurosensory stimuli. The diversity of multihormonal resistance contributes to the various phenotypes of the disease. Some clinical discomfort and medical consequences of the disease can be treated or prevented with hormone supplementation or modulation.

Tissue-specific thyroid hormone deprivation and excess in monocarboxylate transporter (mct) 8-deficient mice.

Mutations of the X-linked thyroid hormone (TH) transporter (monocarboxylate transporter, MCT8) produce in humans unusual abnormalities of thyroid function characterized by high serum T3 and low T4 and rT3. The mechanism of these changes remains obscure and raises questions regarding the regulation of intracellular availability and metabolism of TH. To study the pathophysiology of MCT8 deficiency, we generated Mct8 knockout mice. Male mice deficient in Mct8 (Mct8(-/y)) replicate the thyroid abnormalities observed in affected men. TH deprivation and replacement with L-T3 showed that suppression of TSH required higher serum levels T3 in Mct8(-/y) than wild-type (WT) littermates, indicating hypothalamus and/or thyrotroph resistance to T3. Furthermore, T4 is required to maintain the high serum T3 level because the latter was not different between the two genotypes during administration of T3. Mct8(-/y) mice have 2.3-fold higher T3 content in liver associated with 6.1- and 3.1-fold increase in deiodinase 1 mRNA and enzymatic activity, respectively. The relative T3 excess in liver of Mct8(-/y) mice produced a decrease in serum cholesterol (79 +/- 18 vs. 137 +/- 38 mg/dl in WT) and an increase in alkaline phosphatase (107 +/- 23 vs. 58 +/- 3 U/liter in WT) levels. In contrast, T3 content in cerebrum was 1.8-fold lower in Mct8(-/y) mice, associated with a 1.6- and 10.6-fold increase in D2 mRNA and enzymatic activity, respectively, as previously observed in TH-deprived WT mice. We conclude that cell-specific differences in intracellular TH content due to differences in contribution of the various TH transporters are responsible for the unusual clinical presentation of this defect, in contrast to TH deficiency.

Calreticulin enhances the transcriptional activity of thyroid transcription factor-1 by binding to its homeodomain.

Transcription factors are often regulated by associated protein cofactors that are able to modify their activity by several different mechanisms. In this study we show that calreticulin, a Ca2+-binding protein with chaperone activity, binds to thyroid transcription factor-1 (TTF-1), a homeodomain-containing protein implicated in the differentiation of lung and thyroid. The interaction between calreticulin and TTF-1 appears to have functional significance because it results in increased transcriptional stimulation of TTF-1-dependent promoters. Calreticulin binds to the TTF-1 homeodomain and promotes its folding, suggesting that the mechanism involved in stimulation of transcriptional activity is an increase of the steady-state concentration of active TTF-1 protein in the cell. We also demonstrate that calreticulin mRNA levels in thyroid cells are under strict control by the thyroid-stimulating hormone, thus implicating calreticulin in the modulation of thyroid gene expression by thyroid-stimulating hormone.

The TSH-dependent variation of the essential elements iodine, selenium and zinc within human thyroid tissues.

Instrumental Neutron Activation Analysis was used in order to measure iodine, selenium and zinc concentration in thyroid samples. A pair of samples of normal and nodular tissue were collected from the thyroid gland from 72 patients selected on the basis of pathological criteria (44 cases of multinodular goiter, 12 of chronic lymphocytic thyroiditis (CLT), 6 of thyroid adenoma (TA) and 12 of thyroid cancer (TC)). The check for tissue homogeneity and sampling error was performed by means of the coefficient of variation (CV%) of the elements in replicate samples of normal and altered tissues. High CV% values (> 15%) for iodine reflected a functional variability in thyroid follicles, while low CV% values (< 10%) for selenium and zinc indicated that the composition of selected tissues was rather homogeneous. The variation of the element's concentration was compared in normal and altered tissues. The mean element concentrations had values close to those already reported in the literature; furthermore, our patients had marginal iodine and selenium deficiency. Both normal and nodular tissues in CLT showed statistically significant lower zinc values as compared with the other thyroid diseases. To evaluate the thyroid function, thyroid stimulating hormone (TSH) and thyroxine (T4) levels were measured in the serum of patients. Two arbitrary serum-TSH threshold levels (TSH < 1.0 and > 4.0 mU/L) were introduced in order to classify, respectively, hyperthyroidism and hypothyroidism, as well as euthyroid conditions (1.0 < TSH < 4.0 mU/L), and each patient was assigned to one of these groups. The influence of TSH in the variation of the concentration of iodine, selenium and zinc in normal and altered human thyroid tissues was significant.

Hypothalamic thyrotropin-releasing hormone and thyrotropin biological activity.

Hypothalamic thyrotropin-releasing hormone (TRH) is the main positive regulator of thyrotropin (TSH) secretion. TRH action and the negative feedback of thyroid hormone are integrated in order to guarantee appropriate thyroid stimulation. TRH action affects various steps of the biosynthetic process within thyrotrophs, with major effects on the posttranslational maturation of TSH oligosaccharide chains, and is necessary for the secretion of the glycoprotein hormone with full biological activity. Since the first description in 1979 of some patients with central hypothyroidism of hypothalamic origin associated with the secretion of TSH molecules with conserved immunoreactivity but decreased bioactivity, a large body of evidence has accumulated in more recent years showing that changes of the oligosaccharide chains have a great impact on the biological properties of circulating TSH and occur in various in vivo situations. These findings have lead to the new concept of a qualitative regulation of TSH secretion. This can be achieved mainly through the transcriptional and posttranscriptional regulation of the complex enzymatic machinery devoted to the processing of the three oligosaccharide chains linked to specific asparagine residues of TSH heterodimer. Data obtained in several physiological and pathological conditions, which are characterized by an increased or diminished TRH action, indicate that both qualitative and quantitative regulations cooperate within thyrotrophs in order to adjust thyroid-stimulating activity to the temporary needs.

Interferon gamma and iodide increase the inducibility of the 72 kD heat shock protein in cultured human thyroid epithelial cells.

The 72 kD heat shock protein (hsp 72) has been postulated to play a role in the development of autoimmune disease and has been shown to be overexpressed in the Graves' disease thyroid gland. The expression and modulation of the 72 kD heat shock protein were therefore studied in cultured human thyroid epithelial cells (TEC). TEC from normal thyroid tissue as well as from non-toxic goitres, thyroid adenomas and Graves' disease thyroids were analysed by Western blotting. Potential modulatory effects of heat shock treatment, TSH, IFN gamma, different serum supplements, sodium iodide and sodium selenite were investigated. Hsp 72 was detectable in cells from all tissue types under basal culture conditions and, at increased concentrations, after heat shock treatment. Quantitative evaluation of Western blotting results by densitometer scanning revealed that hsp 72 concentrations were identical in TEC from normal and diseased thyroids. IFN gamma increased the expression of hsp 72 under basal culture conditions as well as after heat shock treatment. TSH had no effect. Sodium iodide did not affect hsp 72 under basal culture conditions, but augmented the susceptibility of TEC to the effect of heat shock treatment. In contrast, sodium selenite had no effect on the expression of hsp 72. These results demonstrate that local as well as environmental factors may facilitate the induction of hsp 72 in TEC. This may be an important factor for the initiation and progression of thyroid autoimmunity. The stimulatory effect of iodide on hsp induction may also provide an explanation for the frequent occurrence of thyroid autoimmune diseases after iodine exposure.