hypothalamic-pituitary axis

The hypothalamus produces hypophysiotropic hormones—CRH, TRH, GnRH, GHRH, dopamine (PIH), and somatostatin—that travel via the hypothalamo-hypophyseal portal system to the anterior pituitary. These stimulate or inhibit release of anterior pituitary hormones: ACTH, TSH, LH/FSH, GH, and prolactin, respectively. Each pituitary hormone then acts on a peripheral target gland (adrenal cortex, thyroid, gonads) to produce end-hormones (cortisol, thyroid hormone, sex steroids) that exert negative feedback on both the hypothalamus and pituitary, maintaining hormonal homeostasis. The posterior pituitary, in contrast, stores and releases ADH and oxytocin, which are synthesized in the hypothalamus (supraoptic and paraventricular nuclei) and transported via axons—this is a neurohypophyseal, not portal, system. Clinically, understanding this axis is crucial for diagnosing endocrine disorders: primary vs secondary vs tertiary dysfunction is distinguished by measuring both the target hormone and the trophic hormone (e.g., low cortisol with high ACTH suggests primary adrenal insufficiency, while low cortisol with low/inappropriately normal ACTH suggests secondary/tertiary causes). Feedback loops can be tested pharmacologically, e.g., dexamethasone suppression test for Cushing syndrome, or CRH stimulation test. Disorders like Cushing disease (pituitary ACTH-secreting adenoma), Sheehan syndrome (postpartum pituitary infarction), and hypothalamic amenorrhea illustrate axis dysfunction at different levels. Chronic stress and glucocorticoid use suppress the HPA axis, causing adrenal atrophy and risk of adrenal crisis upon abrupt steroid withdrawal.