hypothalamus

The portion of the diencephalon lying beneath the thalamus at the base of the cerebrum, and forming the floor and part of the lateral wall of the third ventricle. It includes the optic chiasm, mammillary bodies, tuber cinereum, infundibulum and hypophysis (pituitary gland), but for physiological purposes, the hypophysis is considered a distinct structure.
The hypothalamic nuclei activate, control and integrate many of the involuntary functions necessary for living. The various hypothalamic centers influence peripheral autonomic mechanisms, endocrine activity and many somatic functions, e.g. a general regulation of water balance, body temperature, sleep, thirst and hunger, and the development of secondary sex characteristics.
Because of its influence on the release and inhibition of pituitary hormones, the hypothalamus indirectly plays an important role in the regulation of protein, fat and carbohydrate metabolism, body fluid volume and electrolyte content, and internal secretion of endocrine hormones. The hormones synthesized and secreted by the special neurons of the hypothalamus are called hypothalamic releasing and inhibiting hormones or factors. They act directly on the tissues of the pituitary gland. Some of the major hypothalamic factors are: thyroid-stimulating hormone releasing hormone (TRH), which activates the release of the thyroid-stimulating hormone (TSH) from the anterior lobe of the pituitary gland; corticotropin-releasing factor (CRF); growth hormone releasing factor (GHRF); gonadotropin releasing hormone (GnRH); and prolactin-inhibiting factor (PIF). In addition, there are other stimulating and inhibiting factors that influence the release and retention of other anterior pituitary hormones.
The hypothalamic hormones or factors are secreted directly into the veins in the lower part of the hypothalamus and are transported directly to the tissues of the pituitary gland. This transportation network is called the hypothalamic–hypophyseal portal system. The secretion of the hypothalamic hormones is a part of a regulatory negative feedback system that continuously operates to maintain homeostasis.

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