The hypothalamus controls and coordinates physiological events to maintain a stable physiological state despite living in a changing environment. Large or rapid shifts in function are the responsibility of the hypothalamus, while smaller adjustments are performed by the autonomic nervous system.

The hypothalamus forms from the diencephalon early during development.

The hypothalamus is located in the ventral side of the brain, on both sides of midline.

The hypothalamus is important for general behaviors relating to emotion and reproduction, as well as basic regulatory functions such as heart rate, blood pressure, food and water intake, and body temperature.

Many regions of the brain supply the hypothalamus with information, but only the olfactory system, and a small part of the visual system, send sensory information directly to the hypothalamus.
  T h e h ypothalamus has many cellular subdivisions, called nuclei. The hypothalamus maintains homeostasis by monitoring information about environmental changes and increasing or decreasing activity in different body systems in response to these changes.

When you are studying late at night and become chilled, your hypothalmus may take steps to maintain your normal body temperature:

• The hypothalamus may signal the autonomic nervous system to generate body heat through muscular activity (you shiver).
• The hypothalamus may use the autonomic nervous system to 'suggest' that external means of increasing heat be used (turn up the thermostat, or make a cup of hot chocolate)

Different regions of the hypothalamus control different functions, usually in pairs - one nucleus may cause an increase in activity, while a second nucleus may cause a decrease in the same activity. By balancing the output of these two nuclei, the hypothalamus determines the level of function.

Although we seldom are aware of the activity of the hypothalamus, even limited damage to this region causes profound changes - and often death. Pathologic changes in the hypothalamus may lead to many different symptoms, including diabetes insipidus (excessive urination & thirst due to lesions of the supraoptic nuclei), disturbances in body weight regulation (lesions of the ventromedial nucleus = morbid obesity & hyperphagia), extremes in body size (due to pituitary over or under activity), and disturbed temperature regulation (lesion of the anterior or posterior hypothalamic nuclei).

The hypothalamus is unique in that its control over physiological functions occurs through neural connections in the nervous system as well as through the secretion of chemicals into the cerebrospinal fluid and circulatory systems.

Most regions of the hypothalamus contain specialized cells, called tanacytes, which protect the hypothalamus from direct contact with the cerebrospinal fluid or the blood.

Other regions, such as the pituitary, are designed to allow hypothalamic neurons to release substances directly into the bloodstream using the portal blood vessels. These substances can then travel to parts of the body far from the brain. This means the hypothalamus acts as a gland, and is part of the neuroendocrine system ('neuro' = brain; 'endocrine' = gland).