Hypothalamus-Pituitary-Thyroid Axis

The hypothalamus-pituitary-thyroid (HPT) axis determines the set point of thyroid hormone (TH) production. Hypothalamic thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of pituitary thyrotropin (thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis. Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas the opposite occurs when circulating THs are in excess. Other neural, humoral, and local factors modulate the HPT axis and, in specific situations, determine alterations in the physiological function of the axis. The roles of THs are vital to nervous system development, linear growth, energetic metabolism, and thermogenesis. THs also regulate the hepatic metabolism of nutrients, fluid balance and the cardiovascular system. In cells, TH actions are mediated mainly by nuclear TH receptors (210), which modify gene expression. T3 is the preferred ligand of THR, whereas T4, the serum concentration of which is 100-fold higher than that of T3, undergoes extra-thyroidal conversion to T3. This conversion is catalyzed by 5'-deiodinases (D1 and D2), which are TH-activating enzymes. T4 can also be inactivated by conversion to reverse T3, which has very low affinity for THR, by 5-deiodinase (D3). The regulation of deiodinases, particularly D2, and TH transporters at the cell membrane control T3 availability, which is fundamental for TH action.

© 2016 American Physiological Society. 

Ortiga-Carvalho TM, Chiamolera MI, Pazos-Moura CC, Wondisford FE.


Photo: https://cdn1.medicalnewstoday.com

Similar articles:

The HPA Axis Hypothalamus-Pituitary-Adrenal

Life is a continuum of constant change. The body, the ...

The process of aging changes the interplay of the immune, endocrine and nervous systems

The immune, endocrine and nervous systems interact with each other ...

How adrenal axis works

The adrenal axis (hypothalamic-pituitary-adrenal axis) refers to a complex set ...

The endocrine system and ageing

Complex changes occur within the endocrine system of ageing individuals ...

The Hypothalamic-Pituitary-Gonadal Axis

Regulation of the reproductive axis begins at the level of ...


Our site collects information using cookies to be more convenient and customized to your needs interests. The purposes of the use of cookies are defined in Policy the processing of personal data .If you agree to continue to receive cookies, please click the "Accept" button. If you don't agree or want to resolve this issue later, please change your browser cookie settings.