Prolactin can cause breast tissue growth and lactation. Other substances like serotonin, gonadotropin-releasing hormone, and estrogen can also stimulate prolactin release. As a non-tropic hormone, TRH directly stimulates lactotropic cells in the anterior pituitary to produce prolactin. TRH also has a non-tropic effect on the pituitary gland through the hypothalamic-pituitary-prolactin axis. These second messengers mobilize intracellular calcium stores and activate protein kinase C, leading to downstream gene activation and transcription of TSH. PLC hydrolyzes phosphatidylinositol 4,5-P(PIP) into inositol 1,4,5-triphosphate (IP) and 1,2-diacylglycerol (DAG). Activation of Gq protein leads to the activation of phosphoinositide-specific phospholipase C (PLC). It binds to the TRH receptors on the anterior pituitary gland, causing a signal cascade mediated by a G-protein coupled receptor. TRH is a tropic hormone, meaning that it indirectly affects cells by stimulating other endocrine glands first. These cell bodies project their neurosecretory neurons down to the hypophyseal portal circulation, where TRH can concentrate before reaching the anterior pituitary. TRH is a peptide hormone created by the cell bodies in the periventricular nucleus (PVN) of the hypothalamus. TRH stimulates thyrotropin cells in the anterior pituitary to the release of thyroid-stimulating hormone (TSH).

The hypothalamus releases thyrotropin-releasing hormone (TRH) into the hypothalamic-hypophyseal portal system to the anterior pituitary gland.

Regulation of thyroid hormone starts at the hypothalamus. Iodine deficiency can cause cretinism, goiter, myxedema coma, and hypothyroidism. Decreased iodine intake can cause iodine deficiency and decreased thyroid hormone synthesis. Sources of iodine include iodized table salt, seafood, seaweed, and vegetables. Iodine is an essential trace element absorbed in the small intestine. In contrast, hyperthyroidism caused by increased thyroid gland function manifests as weight loss, heat intolerance, diarrhea, fine tremor, and muscle weakness. Hypothyroidism, caused by an underactive thyroid gland, typically manifests as bradycardia, cold intolerance, constipation, fatigue, and weight gain. Thyrotropin-releasing hormone (TRH) from the hypothalamus, thyroid-stimulating hormone (TSH) from the anterior pituitary gland, and T4 work in synchronous harmony to maintain proper feedback mechanism and homeostasis. The main hormones produced by the thyroid gland are thyroxine or tetraiodothyronine (T4) and triiodothyronine (T3). The thyroid gland, anterior pituitary gland, and hypothalamus comprise a self-regulatory circuit called the hypothalamic-pituitary-thyroid axis.

The thyroid hormone is well known for controlling metabolism, growth, and many other bodily functions.

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