The pituitary gland is a small structure that is located just below the brain. It is attached by a stalk to the hypothalamus, the area of the brain that controls its function.

The hormones secreted by the pituitary and their functions are:

• growth hormone (GH), which stimulates growth of tissues and bone
• thyroid stimulating hormone (TSH), which stimulates the thyroid gland to secrete hormones that affect body metabolism
• adrenocorticotropic hormone (ACTH), which stimulates the adrenal gland to secrete cortisol which helps to maintain blood pressure
• prolactin, which stimulates female breast development and milk production
• luteinizing hormone (LH), which controls sexual function in males and females
• follicle stimulating hormone (FSH), which controls sexual function in males and females
• antidiuretic hormone (ADH), which controls water loss by the kidneys
• oxytocin, which stimulates contraction of the uterus during labor and milk release from the breasts.

In hypopituitarism, there is an absence of one or more pituitary hormones. Lack of the hormone leads to loss of function in the gland or organ that it controls. (For example, loss of thyroid stimulating hormone leads to loss of function in the thyroid gland.)

Hypopituitarism may be caused by tumors of the pituitary gland or hypothalamus, head trauma, brain tumor, radiation, brain surgery, stroke, or infections of the brain and the tissues that support the brain. Occasionally, hypopituitarism is due to uncommon immune system or metabolic diseases such as sarcoidosis, histiocytosis X, and hemochromatosis.

Hypopituitarism is also a rare complication following pregnancy, a condition called Sheehan's syndrome. The cause of hypopituitarism is unknown.