The pituitary is a brain structure that functions in close association with the hypothalamus to regulate other parts of the endocrine system. It is made up of the anterior and posterior pituitary. Also called the hypophysis, it is a small, pea-sized gland that sits just below the brain. It is attached to the hypothalamus, the brain structure that connects the endocrine and nervous systems. These structures are attached via the pituitary stalk, called the infundibulum. The hypothalamus plays a major role in maintaining homeostasis, which is a state of metabolic equilibrium, or balance, to maintain a consistent internal environment in the body. It sends both hormonal and neural signals to the pituitary. In turn, the pituitary acts as a "master gland" that sends hormonal signals via the bloodstream to regulate several other parts of the endocrine system. In humans, the pituitary gland contains two distinct lobes: the anterior pituitary and the posterior pituitary. The anterior pituitary produces hormones, but the posterior pituitary itself is not glandular. It secretes what is produced by the hypothalamus.
The anterior pituitary is connected to the hypothalamus via the hypophyseal portal system, a system of blood vessels that connects the two structures to facilitate hormonal signaling. The anterior pituitary (also called the adenohypophysis), comprises 80 percent of the gland and contains five different cell types (thyrotropes, corticotropes, gonadotropes, somatotropes, and lactotropes), each of which produces and secretes hormones that act on different targets:
- Thyrotropes are stimulated by hypothalamic thyrotropin-releasing hormone (TRH) and are inhibited by hypothalamic somatostatin. They produce and secrete thyroid-stimulating hormone (TSH), which stimulates the thyroid gland, which in turn regulates metabolism throughout the body. Increased activity of the thyroid gland and subsequent rising levels of thyroid hormones in the blood cause a down-regulation, or decrease, in the activity of the thyrotropic cells of the pituitary. This causes them to stop producing and secreting TSH. This type of response is called negative feedback and functions to regulate normal hormone levels.
- Corticotropes are stimulated by corticotropin-releasing hormone (CRH) from the hypothalamus. They release adrenocorticotropin (ACTH), a pituitary hormone that stimulates the adrenal glands to produce cortisol in response to stress and low blood sugar.
- Gonadotropes are stimulated by hypothalamic gonadotropin-releasing hormone (GnRH). They produce and secrete both luteinizing hormone and follicle-stimulating hormone, which both play major roles in the regulation of reproductive functions.
- Somatotropes are stimulated by growth hormone–releasing hormone (GHRH) from the hypothalamus. They are inhibited by hypothalamic somatostatin. They produce and secrete human growth hormone (HGH), a pituitary hormone that stimulates the growth of muscles and bones in children and appropriate body composition in adults.
- Lactotropes respond to several hypothalamic hormones to stimulate the release of prolactin (PRL), which stimulates breast milk production.