The pituitary gland

The pituitary sits in a bony cavity, the sella turcica, and is attached to the hypothalamus by a stalk. Anatomically it has two parts: the adenohypophysis and the neurohypophysis.

The adenohypophysis is made of the pars distalis and the pars intermedia. The pars distalis, usually called the anterior pituitary, makes six hormones: growth hormone (GH), prolactin (PRL), thyroid stimulating hormone (TSH), adrenocorticotrophic hormone (ACTH), follicle stimulating hormone (FSH) and luteinizing hormone (LH).

The pars intermedia secretes only one hormone, melanocyte stimulating hormone (MSH). In humans it is almost merged with the pars distalis.

The neurohypophysis (pars nervosa), or posterior pituitary, stores and releases oxytocin and vasopressin. Both are actually made in the hypothalamus and carried down axons to the neurohypophysis.

Too much GH causes abnormal growth of the body, gigantism; too little causes stunted growth, pituitary dwarfism. Excess GH in adults, especially in middle age, causes acromegaly: severe disfigurement, mostly of the face, which can lead to serious complications and early death if unchecked. It is hard to detect early and often goes unnoticed for years until outward features change.

PRL controls the growth of the mammary glands and milk formation in them. TSH stimulates the thyroid to make and secrete thyroid hormones. ACTH stimulates the adrenal cortex to make and secrete steroid hormones called glucocorticoids.

LH and FSH stimulate the gonads and are called gonadotrophins. In males, LH stimulates the testis to make and secrete androgens, and FSH together with androgens regulates spermatogenesis.

In females, LH induces ovulation of fully mature (Graafian) follicles and maintains the corpus luteum, which forms from what is left of the Graafian follicle after ovulation. FSH stimulates the growth and development of the ovarian follicles.

MSH acts on melanocytes, the melanin-containing cells, and regulates skin pigmentation. Oxytocin makes smooth muscles contract; in females it drives the vigorous contraction of the uterus at childbirth and milk ejection from the mammary gland.

Vasopressin acts mainly on the kidney: it stimulates the distal tubules to resorb water and electrolytes, reducing water lost in urine (diuresis), so it is also called anti-diuretic hormone (ADH). If ADH synthesis or release is impaired, the kidney cannot conserve water well, causing water loss and dehydration: diabetes insipidus.