How they work together
- Step 1, the hypothalamus decides: it collects signals from the rest of the brain and from the blood (for example the level of a hormone or the body’s water balance) and responds by releasing a releasing hormone, such as thyrotropin-releasing hormone (TRH), corticotropin-releasing hormone (CRH) or gonadotropin-releasing hormone (GnRH). [1][2]
- Step 2, a private blood route carries the message: the releasing hormone enters a capillary bed in the infundibulum and travels down the hypophyseal portal veins to a second capillary bed in the anterior pituitary, so it arrives concentrated instead of being diluted in the whole bloodstream. [1][10]
- Step 3, the anterior pituitary answers with its own hormone: TRH makes it release TSH, CRH makes it release ACTH, GnRH makes it release LH and FSH, and growth-hormone-releasing hormone makes it release growth hormone. [1][2]
- Step 4, a gland elsewhere in the body makes the final hormone: TSH drives the thyroid to release thyroxine (T4) and T3, ACTH drives the adrenal cortex to release cortisol, and LH and FSH drive the ovaries or testes to release oestrogen, progesterone or testosterone. These three chains are the hypothalamic–pituitary–thyroid, –adrenal and –gonadal axes. [2][11][12]
- Step 5, the loop closes: the final hormone circulates back and damps down both the hypothalamus and the pituitary (negative feedback). Cortisol, for example, inhibits release of CRH and ACTH, so the level settles instead of climbing. [13][2]
- The posterior pituitary works differently: there is no releasing hormone and no portal blood. Oxytocin and vasopressin are made in hypothalamic neurons, carried down their axons through the stalk and released straight into the blood when those neurons fire. Vasopressin makes the kidneys hold on to water when the blood gets too concentrated; oxytocin drives contractions of the uterus in childbirth and the let-down of milk. [1][7][14]
- An honest limit: most hormones are not made in the brain at all. The thyroid, adrenal glands, pancreas, ovaries, testes and even fat and the gut have their own hormones, and insulin, for example, is set mainly by blood sugar acting on the pancreas, not by the pituitary. The brain is the conductor of the pituitary-driven glands, not the source of every hormone. [15][16]
Melatonin ties hormones to the sleep–wake cycle: the same suprachiasmatic nucleus in the hypothalamus that sets the body clock drives the pineal gland’s night-time melatonin. [17][8] What part of the brain controls sleep?