Bromocriptine reduces augmented thyrotropin secretion in obese premenopausal women

An Overview on Different L-Thyroxine (l-T4) Formulations and Factors Potentially Influencing the Treatment of Congenital Hypothyroidism During the First 3 Years of Life

Congenital hypothyroidism (CH) is a relatively frequent congenital endocrine disorder, caused by defective production of thyroid hormones (THs) at birth. Because THs are essential for the development of normal neuronal networks, CH is also a common preventable cause of irreversible intellectual disability (ID) in children. Prolonged hypothyroidism, particularly during the THs-dependent processes of brain development in the first years of life, due to delays in diagnosis, inadequate timing and dosing of levothyroxine (l-thyroxine or l-T₄), the non-compliance of families, incorrect follow-up and the interference of foods, drugs and medications affecting the absorption of l-T₄, may be responsible for more severe ID. In this review we evaluate the main factors influencing levels of THs and the absorption of l-T₄ in order to provide a practical guide, based on the existing literature, to allow optimal follow-up for these patients.

Regulatory aspects of the human hypothalamus-pituitary-thyroid axis

Thyroid hormones are essential for growth, differentiation and metabolism during prenatal and postnatal life. The hypothalamus-pituitary-thyroid (HPT)-axis is optimized for these actions. Knowledge of this hormonal axis is derived from decades of experiments in animals and man, and more recently from spontaneous mutations in man and constructed mutations in mice. This review examines the HPT-axis in relation to 24 h TSH profiles in men in various physiological and pathophysiological conditions, including obesity, age, longevity, and primary as well as central hypothyroidism. Hormone rhythms can be analyzed by quantitative methods, e.g. operator-independent deconvolution, approximate entropy and fitting the 24-h component by Cosinor analysis or related procedures. These approaches have identified some of the regulatory components in (patho)physiological conditions.

Dopamine regulates renal osmoregulation during hyposaline stress via DRD1 in the spotted scat (Scatophagus argus)

Dopamine is an important regulator of renal natriuresis and is critical for the adaptation of many animals to changing environmental salinity. However, the molecular mechanisms through which dopamine promotes this adaptation remain poorly understood. We studied the effects of dopamine on renal hypo-osmoregulation in the euryhaline fish Scatophagus argus (S. argus) during abrupt transfer from seawater (SW) to freshwater (FW). Following the transfer, serum dopamine concentration was decreased, and dopamine activated expression of the dopamine receptor 1 (designated SaDRD1) in the kidney, triggering the osmoregulatory signaling cascade. SaDRD1 protein is expressed in the renal proximal tubule cells in vivo, and is localized to the cell membrane of renal primary cells in vitro. Knockdown of SaDRD1 mRNA by siRNA significantly increased Na⁺/K⁺-ATPase (NKA) activity in cultured renal primary cells in vitro, suggesting that expression of SaDRD1 may oppose the activity of NKA. We demonstrate that exogenous dopamine enhances the response of NKA to hyposaline stress after transferring primary renal cells from isosmotic medium to hypoosmotic medium. Our results indicate that dopamine regulation via SaDRD1 ignited the renal dopaminergic system to balance the osmotic pressure through inhibiting NKA activity, providing a new perspective on the hyposaline adaptation of fish.

Characterization of the Hypothalamic-Pituitary-Adrenal-Axis in Familial Longevity under Resting Conditions

Objective

The hypothalamic-pituitary-adrenal (HPA)-axis is the most important neuro-endocrine stress response system of our body which is of critical importance for survival. Disturbances in HPA-axis activity have been associated with adverse metabolic and cognitive changes. Humans enriched for longevity have less metabolic and cognitive disturbances and therefore diminished activity of the HPA axis may be a potential candidate mechanism underlying healthy familial longevity. Here, we compared 24-h plasma ACTH and serum cortisol concentration profiles and different aspects of the regulation of the HPA-axis in offspring from long-lived siblings, who are enriched for familial longevity and age-matched controls.

Design

Case-control study within the Leiden Longevity study cohort consisting of 20 middle-aged offspring of nonagenarian siblings (offspring) together with 18 partners (controls).

Methods

During 24 h, venous blood was sampled every 10 minutes for determination of circulatory ACTH and cortisol concentrations. Deconvolution analysis, cross approximate entropy analysis and ACTH-cortisol-dose response modeling were used to assess, respectively, ACTH and cortisol secretion parameters, feedforward and feedback synchrony and adrenal gland ACTH responsivity.

Results

Mean (95% Confidence Interval) basal ACTH secretion was higher in male offspring compared to male controls (645 (324-1286) ngl/L/24 h versus 240 (120-477) ng/L/24 h, P = 0.05). Other ACTH and cortisol secretion parameters did not differ between offspring and controls. In addition, no significant differences in feedforward and feedback synchrony and adrenal gland ACTH responsivity were observed between groups.

Conclusions

These results suggest that familial longevity is not associated with major differences in HPA-axis activity under resting conditions, although modest, sex-specific differences may exist between groups that might be clinically relevant.

Impact of Adiposity and Fat Distribution on the Dynamics of Adrenocorticotropin and Cortisol Rhythms

Obesity impacts many hormonal systems, including pituitary hormones, as well as insulin and leptin. In this review we discuss articles which investigate the influence of obesity on the hypothalamic-pituitary-adrenal (HPA) axis. Different techniques have been used to assess the function of the HPA-axis in obesity, including measuring fasting and/or late evening levels of adrenocorticotropic hormone (ACTH) and (free) cortisol in plasma and saliva, studying feedback with dexamethasone or cortisol, and evaluating responsiveness of the system to corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) or ACTH 1-29. In addition, more elaborate studies investigated 24-h secretion patterns, analyzed with deconvolution techniques to quantitate pulsatile secretion rates of cortisol and less often ACTH. Other investigators used timed infusions of labeled cortisol for the estimation of the 24-h secretion rate, clearance rate and distribution volume. Many studies relied on the 24-h urinary excretion of free cortisol, but for quantitation of the 24-h secretion, measurement of all cortisol-derived metabolites is required. Several studies have applied modern liquid chromatography-tandem-mass spectrometry techniques to measure these metabolites. The picture emerging from all these studies is that, first, ACTH secretion is amplified, likely via enhanced forward drive; and, second, serum cortisol levels are normal or even low, associated with a normal 24-h cortisol secretion per liter distribution volume determined by deconvolution, but enhanced when based on the increased total distribution volume associated with obesity. Increased cortisol secretion was also established by isotope dilution studies and reports based on the measurement of all urinary cortisol metabolites. The responsiveness of the adrenal gland to ACTH is diminished. The studies do not address quantitative aspects of cortisol-cortisone metabolism on individual organs, including liver, central and peripheral fat, intestine, skin, and muscle.

Thyrotropin secretion in healthy subjects is robust and independent of age and gender, and only weakly dependent on body mass index

CONTEXT

Studies of the influence of sex, age, and body weight on TSH secretion are not unanimous. Most reports are based on a single TSH measurement; studies using frequent blood sampling are scarce and include a limited number of selected subjects.

OBJECTIVE

The goal was to investigate TSH dynamics in 117 healthy adults.

METHODS

TSH was measured by a sensitive immunofluorometric assay. Secretion parameters were quantified by automated deconvolution, approximate entropy [ApEn], spikiness, and diurnal properties.

RESULTS

Mean age was 43 years (range, 22-77 y). Mean body mass index (BMI) was 26.8 kg/m(2) (range, 18.3-39.4 kg/m(2)). Daily TSH secretion was 45.4 mU/L (range, 8.0-207 mU/L). There were no sex differences in secretion parameters, including pulse frequency; basal, pulsatile, and total secretion; pulse mode; half life; pulse regularity; ApEn; spikiness; and nycthemeral properties. BMI was positively related to basal secretion. Total secretion correlated negatively with free T₄ (R = 0.225; P = .018). The onset of the nocturnal surge was delayed by increasing BMI and advanced by increasing age. ApEn and spikiness correlated positively with age, especially in men. The 9 am sample correlated strongly with the total 24-hour secretion, explaining two-thirds of the variability.

CONCLUSION

This study shows that the 24-hour TSH secretion in healthy volunteers is stable and robust and not influenced by sex, BMI, and age. ApEn in the elderly, especially men, is increased, pointing to a less tight feedback control. Furthermore, aging is associated with advance shifting of the TSH rhythm, which is a phenomenon also observed in other biological rhythms.

Thyrotropin secretion patterns in health and disease

Thyroid hormones are extremely important for metabolism, development, and growth during the lifetime. The hypothalamo-pituitary-thyroid axis is precisely regulated for these purposes. Much of our knowledge of this hormonal axis is derived from experiments in animals and mutations in man. This review examines the hypothalamo-pituitary-thyroid axis particularly in relation to the regulated 24-hour serum TSH concentration profiles in physiological and pathophysiological conditions, including obesity, primary hypothyroidism, pituitary diseases, psychiatric disorders, and selected neurological diseases. Diurnal TSH rhythms can be analyzed with novel and precise techniques, eg, operator-independent deconvolution and approximate entropy. These approaches provide indirect insight in the regulatory components in pathophysiological conditions.

Dopamine and G protein-coupled receptor kinase 4 in the kidney: role in blood pressure regulation

Complex interactions between genes and environment result in a sodium-induced elevation in blood pressure (salt sensitivity) and/or hypertension that lead to significant morbidity and mortality affecting up to 25% of the middle-aged adult population worldwide. Determining the etiology of genetic and/or environmentally-induced high blood pressure has been difficult because of the many interacting systems involved. Two main pathways have been implicated as principal determinants of blood pressure since they are located in the kidney (the key organ responsible for blood pressure regulation), and have profound effects on sodium balance: the dopaminergic and renin-angiotensin systems. These systems counteract or modulate each other, in concert with a host of intracellular second messenger pathways to regulate sodium and water balance. In particular, the G protein-coupled receptor kinase type 4 (GRK4) appears to play a key role in regulating dopaminergic-mediated natriuresis. Constitutively activated GRK4 gene variants (R65L, A142V, and A486V), by themselves or by their interaction with other genes involved in blood pressure regulation, are associated with essential hypertension and/or salt-sensitive hypertension in several ethnic groups. GRK4γ 142Vtransgenic mice are hypertensive on normal salt intake while GRK4γ 486V transgenic mice develop hypertension only with an increase in salt intake. GRK4 gene variants have been shown to hyperphosphorylate, desensitize, and internalize two members of the dopamine receptor family, the D(1) (D(1)R) and D(3) (D(3)R) dopamine receptors, but also increase the expression of a key receptor of the renin-angiotensin system, the angiotensin type 1 receptor (AT(1)R). Knowledge of the numerous blood pressure regulatory pathways involving angiotensin and dopamine may provide new therapeutic approaches to the pharmacological regulation of sodium excretion and ultimately blood pressure control.

Thyrotropin secretion in mild and severe primary hypothyroidism is distinguished by amplified burst mass and Basal secretion with increased spikiness and approximate entropy

CONTEXT

Twenty-four-hour TSH secretion profiles in primary hypothyroidism have been analyzed with methods no longer in use. The insights afforded by earlier methods are limited.

OBJECTIVE

We studied TSH secretion in patients with primary hypothyroidism (eight patients with severe and eight patients with mild hypothyroidism) with up-to-date analytical tools and compared the results with outcomes in 38 healthy controls.

DESIGN AND METHODS

Patients and controls underwent a 24-h study with 10-min blood sampling. TSH data were analyzed with a newly developed automated deconvolution program, approximate entropy, spikiness assessment, and cosinor regression.

RESULTS

Both basal and pulsatile TSH secretion rates were increased in hypothyroid patients, the latter by increased burst mass with unchanged frequency. Secretory regularity (approximate entropy) was diminished, and spikiness was increased only in patients with severe hypothyroidism. A diurnal TSH rhythm was present in all but two patients, although with an earlier acrophase in severe hypothyroidism. The estimated slow component of the TSH half-life was shortened in all patients.

CONCLUSION

Increased TSH concentrations in hypothyroidism are mediated by amplification of basal secretion and burst size. Secretory abnormalities quantitated by approximate entropy and spikiness were only present in patients with severe disease and thus are possibly related to the increased thyrotrope cell mass.

Drugs that suppress TSH or cause central hypothyroidism

Many different drugs affect thyroid function. Most of these drugs act at the level of the thyroid in patients with normal thyroid function, or at the level of thyroid hormone absorption or metabolism in patients requiring exogenous levothyroxine. A small subset of medications including glucocorticoids, dopamine agonists, somatostatin analogues and rexinoids affect thyroid function through suppression of TSH in the thyrotrope or hypothalamus. Fortunately, most of these medications do not cause clinically evident central hypothyroidism. A newer class of nuclear hormone receptors agonists, called rexinoids, cause clinically significant central hypothyroidism in most patients and dopamine agonists may exacerbate 'hypothyroidism' in patients with non-thyroidal illness. In this review, we explore mechanisms governing TSH suppression of these drugs and the clinical relevance of these effects.

Thyrotropin secretion profiles are not different in men and women

CONTEXT

The hypothalamo-pituitary-thyroid axis in women may differ from that in men. Previous reports have shown an almost 2-fold increased response to TRH in females compared with males.

OBJECTIVE

We analyzed TSH profiles in healthy men and women to delineate differences in the hypothalamo-pituitary-thyroid system.

SUBJECTS AND INTERVENTION

The subjects, 24 men (mean age 44 +/- 3 yr) and 22 women (mean age 42 +/- 3 yr) underwent a 24-h study with blood sampling intervals of 10 min. Premenopausal women were investigated in the early follicular phase of the cycle.

METHODS

Serum TSH concentration profiles were analyzed with a newly developed automated deconvolution program, approximate entropy, and cosinor regression.

RESULTS

Basal and pulsatile TSH secretion, and also pulse frequency, hormone half-lives, and secretory mode were indistinguishable in the two genders. There were no differences in diurnal variation, and the times of maximal secretion coincided. Approximate entropy, reflecting secretory regularity, was not different between men and women. In women but not men, TSH secretion was dependent linearly on age.

CONCLUSIONS

TSH secretion is gender invariant and depends on age in women only.