region of the brain lying below the thalamus and making up the floor of the third cerebral ventricle. The hypothalamus contains a control centre for many functions of the autonomic nervous system, and it has effects on the endocrine system because of its complex interaction with the pituitary gland, which lies beneath it.

The hypothalamus and pituitary gland are connected by both nervous and chemical pathways. Nerve tracts from the hypothalamus stimulate the release of oxytocin and vasopressin from the posterior pituitary gland; these hormones cause smooth-muscle contractions in the circulatory and reproductive systems. Hormones secreted from the hypothalamus trigger the release from the anterior pituitary of growth hormone, adrenocorticotropic hormone, luteinizing hormone, and other pituitary secretions.

The hypothalamus influences caloric intake and weight regulation, establishing a stable “set point” for individual weight gain. The hypothalamus also regulates body heat in response to variations in external temperature, determines wakefulness and sleep, and regulates fluid intake and sensation of thirst.

Injuries or diseases affecting the hypothalamus may produce symptoms of pituitary dysfunction or diabetes insipidus; in the latter disorder, the absence of antidiuretic hormone (a hormone that promotes the reabsorption of water in the kidneys) produced by the hypothalamus induces the rapid loss of water from the body through frequent urination. Hypothalamic disease can also cause insomnia and fluctuations in body temperature.