Hypothalamic–pituitary–thyroid (hPt) axis

Thyroid hormones, among other functions, control basal metabolic rate by increasing metabolic heat generation. Thyroid hormone secretion is controlled by the hypothalamus and pituitary, and is influenced by numerous factors, like ambient temperature.

Thyrotrophin discharging hormone (TRH) is a tripeptide synthesized in PVN neurons. TRH secreted by axon terminals of such cells in the median prominence is carried by the hypothalamic–pituitary portal system to the anterior pituitary, stimulating thyrotrophs to secrete thyroid stimulating hormone (TSH). The TSH is a glycoprotein consisting of two chains, α &β. It is liberated into the systemic circulation and stimulates growth and division of cells in the thyroid gland, and the synthesis & secretion of the thyroid hormones. There are two thyroid hormones that is thyroxine (T4) & triidothyronine (T3), name for the number of iodine atoms they have.

Thyroid hormone receptors (TRs) are associates of the steroid receptor superfamily. They form heterodimers with retinoid X receptors. They vary from glucocorticoid receptors in that the heterodimer joins to hormone-responsive elements in the DNA in the lack of ligand. These receptors have a higher affinity for T3 as compare to T4. The T4, that forms the bulk of the secreted hormones, is a prohormone that is converted to T3 by the neuronal cytosolic enzyme, 5’-deiodinase II. On binding of the T3, the thyroid hormone receptor activates gene transcription.

Thyroid hormone outcome is controlled by negative feedback acting at numerous levels of the HPT axis. A drop in thyroid hormone deliberation causes the raised secretion of TSH by thyrotrophs of the anterior pituitary. The TRH secretion from the hypothalamus is also subject to feedback reservation by both T4 &T3.

Pulses of TRH secretion drive pulsatile TSH output. The amplitude and frequency of the pulses is entrained into a circadian rhythm by the suprachiasmatic nucleus, rising throughout the night, falling during the morning, and enduring low throughout the afternoon. This circadian rhythm is sensitive to dark and light, but not affected by sleep patterns.

Thyroid hormone secretion is raised in cold revelation. Temperature-sensitive neurons in the preoptic hypothalamus that get input from skin thermoreceptors plan to brainstem noradrenergic neurons. These, in return, synapse with the TRH-secreting cells of the PVN. The cold exposure activates the noradrenergic neurons, annoying a rise in TRH secretion in a few minutes. The resultant increase in the concentrations of thyroid hormones improves metabolic rate, helping to maintain the core temperature.