Physiological regulation of thyrotropin

Long COVID and pituitary dysfunctions: a bidirectional relationship?

Long COVID is a novel emerging syndrome known to affect multiple health areas in patients previously infected by SARS-CoV-2 markedly impairing their quality of life. The pathophysiology of Long COVID is still largely poorly understood and multiple mechanisms were proposed to underlie its occurrence, including alterations in the hormonal hypothalamic-pituitary axes. Aim of this review is to present and discuss the potential negative implications of these hormonal dysfunctions in promoting and influencing the Long COVID syndrome. To date, the hypothalamic-pituitary-adrenal axis is the mostly investigated and several studies have reported a prolonged impairment leading to mild and subclinical forms of central adrenal insufficiency. Few data are also available regarding central hypogonadism, central hypothyroidism and growth hormone (GH) deficiency. A high prevalence of central hypogonadism in COVID-19 survivors several months after recovery was consistently reported in different cohorts. Conversely, very few data are available on the hypothalamic-pituitary-thyroid axis function that was mainly shown to be preserved in COVID-19 survivors. Finally, a potential impairment of the hypothalamic-GH axis in Long COVID has also been reported. These data altogether may suggest a novel possible pituitary-centred pathophysiological view of Long COVID syndrome which if confirmed by large clinical studies may have relevant implication for the diagnostic and therapeutic approach at least in a subset of patients with the syndrome.

Effects of Antihypertensive Drugs on Thyroid Function in Type 2 Diabetes Patients With Euthyroidism

Objective: There is little literature about whether antihypertensive drugs would affect thyroid function in patients with euthyroid type 2 diabetes, which was significant in maintaining a proper balance of thyroid function. A retrospective cohort study was conducted to evaluate the influence of antihypertensive drugs on thyroid function in patients with type 2 diabetes with euthyroidism. Design and Methods: The study involved dividing 698 patients with antihypertensive monotherapy into five groups according to the antihypertensive drugs they were treated with. Antihypertensive drugs included in this study were β-blockers, angiotensin-converting enzyme inhibitors (ACEI), angiotensin receptor blockers (ARB), and calcium channel blockers (CCB). The clinical data and thyroid function level between or within groups were compared. Multiple logistic regression analysis was conducted to evaluate the association of antihypertensive drugs with thyroid function level. Results: Selective β₁- adrenergic receptor blockers treatment was related to thyroid-stimulating hormone (TSH), increasing in patients with diabetes and euthyroidism as shown by multiple logistic regression analysis. The association existed after adjustment for confounding factors. No significant influence on thyroid function was found among other antihypertensive drugs. Conclusion: These data show the TSH-lifting effect of selective β₁-adrenergic receptor blockers in patients with type 2 diabetes with euthyroidism.

Thyroid Hormone and Thyrotropin Concentrations and Responses to Thyrotropin-Stimulating Hormone in Horses with PPID Compared with Age-Matched Normal Horses

Glucocorticoids are known to exert inhibitory action on the hypothalamic-pituitary-thyroid axis. With recent evidence that free plasma cortisol and urinary excretion of cortisol metabolites may be increased in horses with pituitary pars intermedia dysfunction (PPID), it is important to further examine thyroid function in horses with PPID. To test the hypothesis that serum thyrotropin (TSH) concentrations are decreased in horses with PPID, baseline serum thyroid hormone and TSH concentrations, and responses to TSH-releasing hormone (TRH), were compared between 12 horses diagnosed as having PPID and 14 age-matched normal horses. Horses with PPID had resting serum concentrations of free thyroxine by equilibrium dialysis (fT4D) and TSH that were significantly lower than serum concentrations of fT4D and TSH in age-matched normal horses. Serum concentrations of total T4 and total and free triiodothyronine (T3) were also lower in horses with PPID compared with normal horses, but the differences did not reach statistical significance. Thyroid hormone and TSH responses to TRH administration were not different between horses with PPID and normal horses. In conclusion, serum fT4D concentrations are decreased in horses with PPID without an appropriate increase in serum TSH concentrations. Normal serum thyroid hormone and TSH concentration responses to exogenous TRH administration support the theory that increased glucocorticoid activity in horses with PPID may exert prolonged tonic suppression, but not complete inhibition, of TRH and subsequent TSH release, similar to what has been observed in other species.

Discovery and Development of Small Molecule Allosteric Modulators of Glycoprotein Hormone Receptors

Glycoprotein hormones, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and thyroid-stimulating hormone (TSH) are heterodimeric proteins with a common α-subunit and hormone-specific β-subunit. These hormones are dominant regulators of reproduction and metabolic processes. Receptors for the glycoprotein hormones belong to the family of G protein-coupled receptors. FSH receptor (FSHR) and LH receptor are primarily expressed in somatic cells in ovary and testis to promote egg and sperm production in women and men, respectively. TSH receptor is expressed in thyroid cells and regulates the secretion of T3 and T4. Glycoprotein hormones bind to the large extracellular domain of the receptor and cause a conformational change in the receptor that leads to activation of more than one intracellular signaling pathway. Several small molecules have been described to activate/inhibit glycoprotein hormone receptors through allosteric sites of the receptor. Small molecule allosteric modulators have the potential to be administered orally to patients, thus improving the convenience of treatment. It has been a challenge to develop a small molecule allosteric agonist for glycoprotein hormones that can mimic the agonistic effects of the large natural ligand to activate similar signaling pathways. However, in the past few years, there have been several promising reports describing distinct chemical series with improved potency in preclinical models. In parallel, proposal of new structural model for FSHR and in silico docking studies of small molecule ligands to glycoprotein hormone receptors provide a giant leap on the understanding of the mechanism of action of the natural ligands and new chemical entities on the receptors. This review will focus on the current status of small molecule allosteric modulators of glycoprotein hormone receptors, their effects on common signaling pathways in cells, their utility for clinical application as demonstrated in preclinical models, and use of these molecules as novel tools to dissect the molecular signaling pathways of these receptors.

The effect of metformin on the hypothalamic-pituitary-thyroid axis in women with polycystic ovary syndrome and subclinical hypothyroidism

The effect of metformin treatment on the hypothalamic-pituitary-thyroid axis is relatively weakly understood. This study included 24 prediabetic patients with polycystic ovary syndrome and untreated subclinical hypothyroidism, 12 of whom had already been treated with bromocriptine (5.0-7.5 mg daily). The included patients received metformin (2.55 g daily) for 6 months. Glucose homeostasis markers, serum prolactin, and thyroid function tests were determined before, after 3 months, and at the end of the treatment. Beyond improving glucose homeostasis, metformin administered for 6 months reduced serum levels of thyrotropin but did not affect serum levels of total and free thyroid hormones. Thyrotropin-lowering effect of this agent was stronger in patients not treated with bromocriptine than in patients receiving this drug, and weakly correlated with an improvement in insulin sensitivity. The obtained results indicate that metformin treatment may have an impact on thyrotrope function in hypothyroid patients, probably by enhancing the effect of thyroid hormone action in the pituitary.

Cyclic mRNA expression of thyrotropin subunits and deiodinases in red drum, Sciaenops ocellatus

The role of thyrotropin (thyroid-stimulating hormone, TSH) in driving peripheral thyroid function in non-mammalian species is still poorly understood. Thyroxine (T₄), the principal hormone released from the thyroid gland in response to TSH stimulation, circulates with a robust daily rhythm in the teleost fish the red drum. Previous research suggests that the red drum T₄ cycle is circadian in nature, driven by TSH secretion in the early photophase and inhibited by T₄ feedback in the early scotophase. To determine whether TSH is produced in a pattern consistent with feedback inhibition by this T₄ cycle, we used quantitative real time PCR (qPCR) to quantify the daily cycle of expression of the pituitary TSH subunits GSUα, and TSHβ. We found that TSH expression cycled inversely to, and 6-12 h out of phase with, the T₄ cycle, consistent with the hypothesis that TSH secretion drives the T₄ cycle. To examine the potential role of deiodinases in negative feedback regulation of this TSH cycle, we also utilized qPCR to assess the pituitary expression patterns of the TH activating enzyme outer-ring deiodinase (Dio2) and the TH deactivating enzyme inner-ring deiodinase (Dio3). Dio2 was not expressed with an obvious daily cycle, whereas Dio3 expression mirrored the expression of TSH. These results are consistent with circulating T₄ providing the negative feedback signal controlling both TSH production and Dio3 expression in the pituitary, and suggest that TH inactivation by inner ring deiodination is an important component of TSH negative feedback control.

Somatostatin inhibition of GnRH neuronal activity and the morphological relationship between GnRH and somatostatin neurons in rats

In rodents, GnRH neurons are diffusely distributed from the medial septum through to the medial preoptic area and control gonadal functions through the pituitary. The activity of GnRH neurons is regulated by a variety of bioactive substances, including the inhibitory peptide somatostatin. In the present study, we focused on somatostatin because intracerebroventricular injection of somatostatin inhibits the LH surge in rats and reduces LH secretion in ewes. Somatostatin also decreases GnRH release from rat hypothalamic slices. In mice, somatostatin is also thought to suppress GnRH neuronal activity through contact on the soma of GnRH neurons. However, similar data are missing in rats. Moreover, rat GnRH neurons receive only a few synaptic inputs. In this study, we assessed the morphological relationship between GnRH and somatostatin neurons. Confocal microscopy on the sections from the medial septum through medial preoptic area revealed about 35 close contacts per rat between the GnRH and somatostatin neuronal fibers in the organum vasculosum of the lamina terminalis region. No contact of somatostatin fibers on the GnRH neuronal somata was observed. Multicell RT-PCR for somatostatin receptor mRNA in rat GnRH neurons was also performed, which revealed moderate expression of somatostatin receptor subtypes 1-5. In addition, patch clamp experiments were carried out in acute slice preparations. Somatostatin suppressed neuronal firing in cells recorded in a cell-attached configuration and also induced whole-cell outward currents in GnRH neurons. These findings suggest that somatostatin directly inhibits the activity of rat GnRH neurons through volume transmission in the organum vasculosum of the lamina terminalis region.

[Therapeutic possibilities in patients with selective pituitary resistance to thyroid hormones]

Selective pituitary resistance to thyroid hormones (SPRTH) is a non-neoplastic form of inappropriate secretion of thyrotropin (TSH). The etiology of this hormonal resistance is linked to inactivating mutations in the thyroid hormone receptor beta (TR-beta) gene. These mutations affect critical portions of the receptor's triiodothyronine (T3)-binding domain. Clinically, SPRTH is characterized by hyperthyroidism with goiter and absence of pituitary mass in the morphologic study. Laboratory data show an elevation of free T3 and free thyroxine concentrations without suppression of TSH, with normal molar subunit alpha/TSH ratio. At this time, there is no specific therapy for SPRHT. Beta blockers, such as atenolol, and benzodiazepines have been used as a symptomatic therapy. Among the drugs with the capacity for reducing TSH secretion are TR agonists, such as triiodothyroacetic acid, D-thyroxine, triiodothyropropionic acid, and L-T3.

Risperidone-associated hyperprolactinemia

OBJECTIVE

To compare the prolactogenic effects of risperidone, clozapine, and typical antipsychotic agents in an outpatient community-based psychiatric population.

METHODS

Prolactin and thyroid-stimulating hormone (TSH) concentrations were measured in 68 outpatients with schizophrenia who were receiving antipsychotic medications and were recruited from a community mental health clinic.

RESULTS

The percentage of women with increased prolactin concentrations was significantly greater in the risperidone group (100%, 12 of 12 patients) than in the clozapine group (25%, 1 of 4) (P = 0.0071) but not in comparison with the typical antipsychotic agent group (83%, 5 of 6) (P = 0.333). The percentage of men with increased prolactin concentrations was significantly greater in the risperidone group (94%, 17 of 18) than in the clozapine group (18%, 3 of 17) (P<0.0001) and in comparison with the typical antipsychotic agent group (27%, 3 of 11) (P = 0.0003). The mean prolactin concentration (all ng/mL +/- standard deviation) was also significantly higher in patients taking risperidone (women, 125.0 +/- 56.6; men, 37.3 +/- 23.9) than clozapine (women, 22.0 +/- 25.9; men, 13.3 +/- 22.4) (female patients, P = 0.0004; male patients, P<0.0001) or typical antipsychotic agents (women, 69.0 +/- 59.8; men, 13. 3 +/- 9.1) (female patients, P = 0.036; male patients, P = 0.0003). In the risperidone group, gender affected prolactin level, with women having higher concentrations than men, but the duration of therapy did not. In this group, prolactin was inversely dependent on age. No difference was noted in TSH concentrations between medication groups.

CONCLUSION

Risperidone is a potent inducer of hyperprolactinemia in outpatients with schizophrenia in a community population. The higher and more frequently increased prolactin concentrations caused by risperidone could adversely affect patient health and compliance.

Central nervous system control of thyrotropin secretion during sleep and wakefulness

Thyrotropin (TSH) is a pulsatile secreted hormone with a pronounced circadian rhythmicity and a characteristic nightly surge based on an augmentation of pulsatile release. A number of physiological factors influence TSH secretion via an alteration in the amount of pulsatile released hormone. An increase in somatostatinergic tone during fasting appears to decrease TSH pulse amplitude and sequentially mean TSH serum levels. In contrast, blockade of dopaminergic tone by metoclopramide infusion when circulating TSH levels are low during the afternoon hours increase TSH pulse amplitude to levels comparable to the nightly TSH surge suggesting a physiological dampening of TSH pulse amplitude by dopamine during the daytime.

Effect of medroxyprogesterone acetate on thyrotropin secretion in adult and old female rats

Steroid hormones have been implicated in the modulation of TSH secretion; however, there are few and controversial data regarding the effect of progesterone (Pg) on TSH secretion. Medroxyprogesterone acetate (MPA) is a synthetic alpha-hydroxyprogesterone analog that has been extensively employed in therapeutics for its Pg-like actions, but that also has some glucocorticoid and androgen activity. Both hormones have been shown to interfere with TSH secretion. The objective of the present study was to investigate the effects of MPA or Pg administration to ovariectomized (OVX) rats on in vivo and in vitro TSH release and pituitary TSH content. The treatment of adult OVX rats with MPA (0. 25 mg/100 g body weight, sc, daily for 9 days) induced a significant (P<0.05) increase in the pituitary TSH content, which was not observed when the same treatment was used with a 10 times higher MPA dose or with Pg doses similar to those of MPA. Serum TSH was similar for all groups. MPA administered to OVX rats at the lower dose also had a stimulatory effect on the in vitro basal and TRH-induced TSH release. The in vitro basal and TRH-stimulated TSH release was not significantly affected by Pg treatment. Conversely, MPA had no effect on old OVX rats. However, in these old rats, ovariectomy alone significantly reduced (P<0.05) basal and TRH-stimulated TSH release in vitro, as well as pituitary TSH content. The results suggest that in adult, but not in old OVX rats, MPA but not Pg has a stimulatory effect on TSH stores and on the response to TRH in vitro.

Pulsatile release and circadian rhythms of thyrotropin and prolactin in children with growth hormone deficiency

We have measured mean concentrations and have appraised the pulsatile nature of thyrotropin (TSH) and prolactin (PRL) release in children with classical GH deficiency (GHD; n = 4) and neurosecretory GH dysfunction (NSD; n = 4) and have compared the results with those obtained in children with constitutional delay (control; n = 4). Blood samples were obtained at 20-min intervals for 24 h. Pulse analysis of TSH and PRL was undertaken using the Cluster pulse detection algorithm. Circadian rhythmicity of TSH and PRL was assessed using cosinor analysis. The mean 24-h concentration of GH in the control subjects was significantly higher than that obtained in the GHD and NSD groups. With regard to TSH, the mean serum concentration in the GHD and NSD group were higher than that of the control subjects. This augmentation reflects TSH pulses of large amplitude and area, and a higher interpulse valley mean rather than a difference in peak number or peak duration. No differences in mean PRL concentration or characteristics of PRL pulses were found between the control and GHD and NSD subjects. When the 24 h data sets were divided into day (0800-2000 h) and night (2000-0800 h), the mean nighttime TSH concentration was higher than the daytime concentration in the control, GHD, and NSD groups. Although there were no day versus night differences in TSH pulse frequency in either group, peak amplitude, area, and interpulse valley means were increased during the night in the control group, and peak area, duration, and amplitude mean in the NSD group. The nighttime mean PRL concentrations in the control, GHD, and NSD subjects were higher than those found during the day. This increase was accounted for by increases in PRL peak amplitude, area in the control group, and peak area, amplitude, and interpulse valley mean in the GHD and NSD groups. Cosinor analysis of the 24-h TSH and PRL data revealed clear circadian rhythmicity in all groups of subjects. These data suggest that GHD and NSD are associated with an increase in pulsatile TSH secretion due to an increase in pulse amplitude and interpulse valley mean.

Mechanisms of action of somatostatin on human TSH-secreting adenoma cells

The mechanisms of somatostatin (SRIH) action on thyroid-stimulating hormone (TSH) secretion were examined using human TSH-secreting adenoma cells. SRIH (10(-7) M) inhibited TSH secretion through a pertussis toxin-sensitive G protein. SRIH also inhibited forskolin- and 8-bromo-adenosine 3',5'-cyclic monophosphate (8-BrcAMP)-induced TSH secretion. The mechanisms of this inhibition were investigated by measuring intracellular Ca2+ concentration ([Ca2+]i) and by electrophysiological experiments. Application of 10(-7) M SRIH reduced the [Ca2+]i, whereas forskolin and 8-BrcAMP increased the [Ca2+]i. Simultaneous application of SRIH abolished the forskolin-and the 8-BrcAMP-induced [Ca2+]i increase, indicating that the SRIH-induced decrease in [Ca2+]i was independent of the reduction in intracellular cAMP. Under current clamp using the whole cell clamp, 10(-7) M SRIH hyperpolarized the membrane and arrested Ca(2+)-dependent action potentials, which accounted for the SRIH-induced decrease in [Ca2+]i. Voltage clamp experiments revealed that this membrane hyperpolarization resulted from the activation of an inward-rectifying K+ current through a pertussis toxin-sensitive G protein. Intracellular injection of cAMP (100 microM) through the patch pipette did not abolish the SRIH-induced K+ current, indicating that the activation of SRIH-induced K+ channels was independent of intracellular cAMP. From these data, we concluded that SRIH-induced membrane hyperpolarization was responsible for the [Ca2+]i decrease, which in turn inhibited TSH secretion. Application of thyrotropin-releasing hormone (TRH; 10(-7) M) caused an increase in the [Ca2+]i, composed of an initial transient increase followed by a sustained increase. SRIH inhibited the sustained increase in [Ca2+]i. SRIH also inhibited the TRH-induced decrease in the membrane conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuroendocrine regulation of thyrotropin-releasing hormone (TRH) in the tuberoinfundibular system

[...] It is now required to list each part needed for mucous excretion. They are two ducts in the brain substance, then a thin portion of membrane shaped as the infundibulum, then the gland that receives the tip of this infundibulum and the ducts that drive the mucus (pituita) from this gland to the palate and nares. [...] and I said that one (duct) [...] from the middle of the common cavity (third ventricle) descends [...] into the brain substance, and the end of this duct is [...] the sinus of the gland where the brain mucus is collected [...].

Glucocorticoid receptor colocalization with pituitary hormones in the rat pituitary gland

The presence of glucocorticoid receptor (GR) in the anterior lobe of the pituitary gland has previously been demonstrated, but the exact cell types expressing GR have not yet been characterized. In this study, we demonstrate the colocalization of GR and pituitary hormones in the rat pituitary gland by using an immunocytochemical double-labelling method. The majority of anterior lobe corticotropin-immunoreactive and growth hormone-immunoreactive cells contained GR-like immunoreactivity. Cells of the intermediate lobe showed intensive ACTH-like immunoreactivity but did not express GR. The glycoprotein hormones thyroid-stimulating hormone, follicle-stimulating hormone and luteinizing hormone were colocalized with GR to a lesser degree; approximately one-half of the cells exhibited immunoreactivity to these hormones contained GR. By contrast, only a minority of the prolactin-immunoreactive cells expressed GR. Our results suggest that glucocorticoids may differentially regulate the secretion and/or synthesis of these pituitary hormones by directly affecting the hormone-producing cells of the anterior pituitary.

Trace element deficiencies in cattle

Deficiency of cobalt, copper, iron, iodine, manganese, selenium, or zinc can cause a reduction in production. Reduced production occurs most commonly when a deficiency corresponds to the phases of growth, reproduction, or lactation. Because of environmental, nutrient, disease, genetic, and drug interactions, deficiencies of single or multiple elements can occur even when the levels recommended by the National Research Council for these nutrients are being fed. Additionally, random supplementation of trace elements above National Research Council recommendations is not justified because of the negative interaction among nutrients and potential toxicosis. Evaluation of trace element status can be difficult because many disease states will alter blood analytes used to evaluate nutrient adequacy. Proper dietary and animal evaluation, as well as response to supplementation, are necessary before diagnosing a trace element deficiency.