Summary
Highlights
The paraventricular nucleus secretes corticotropin-releasing hormone (CRH) into the hypophyseal portal system. CRH stimulates corticotropes in the anterior pituitary to produce pro-opiomelanocortin (POMC), which breaks down into alpha MSH (melanocyte-stimulating hormone) and ACTH (adrenocorticotropic hormone).
The paraventricular nucleus also releases thyrotropin-releasing hormone (TRH), which travels through the portal system to the anterior pituitary. TRH stimulates thyrotropes to release TSH (thyroid-stimulating hormone).
The arcuate nucleus releases growth hormone-releasing hormone (GHRH). GHRH circulates to the anterior pituitary, stimulating somatotropes to produce and release growth hormone (GH).
The arcuate nucleus also releases prolactin-inhibiting hormone (PIH), commonly known as dopamine. Unlike other hormones mentioned, dopamine inhibits lactotropes in the anterior pituitary from releasing prolactin. A decrease in dopamine levels stimulates prolactin release.
The preoptic nucleus releases gonadotropin-releasing hormone (GnRH). GnRH stimulates gonadotropes in the anterior pituitary to produce FSH (follicle-stimulating hormone) and LH (luteinizing hormone). The frequency of GnRH release determines the relative amounts of FSH and LH produced; more frequent high levels of GnRH lead to LH production, while less frequent high levels lead to FSH production.
The hypothalamus also produces inhibiting hormones such as TIH (thyrotropin-inhibiting hormone), CIH (corticotropin-inhibiting hormone), GnIH (gonadotropin-inhibiting hormone), and somatostatin (growth hormone-inhibiting hormone), which oppose the actions of their releasing hormone counterparts and provide negative feedback.
Beyond normal feedback mechanisms, secondary triggers can influence hormone release. For CRH, common triggers include fevers, hypoglycemia (low blood glucose), and long-term stress, all leading to increased ACTH production.
Cold temperatures and pregnancy can act as secondary triggers for TRH release, stimulating TSH. Factors like breastfeeding, estrogen levels, birth control pills, and certain drugs can influence prolactin secretion.
Secondary triggers for GHRH, leading to growth hormone release, include hypoglycemia, high levels of amino acids, low levels of fatty acids, and exercise.