Hypothalamic–pituitary–adrenal axis the midbrain that mediate the general adaptation syndrome (GAS). While steroid hormones are produced mainly in vertebrates, the physiological role of the HPA axis and corticosteroids in stress response is so fundamental that analogous systems can be found in invertebrates and monocellular organisms as well. The HPA axis, HPG axis, HPT axis, and the hypothalamic–neurohypophyseal system are the four major neuroendocrine systems through which the hypothalamus and pituitary direct neuroendocrine function. The key elements of the HPA axis are: CRH and vasopressin are released from neurosecretory nerve terminals at the median eminence. CRH is transported to the anterior pituitary through
Anterior pituitary death and can lead to gigantism, a hormone disorder shown in “giants” such as André the Giant, where it occurs before the epiphyseal plates in bones close in puberty. The most common type of pituitary tumour is a prolactinoma which hypersecretes prolactin. A third type of pituitary adenoma secretes excess ACTH, which in turn, causes an excess of cortisol to be secreted and is the cause of Cushing's disease. Hypopituitarism is characterized by a decreased secretion of hormones released by the anterior pituitary. For example, hypo-secretion of GH prior to puberty can be a cause of dwarfism. In addition, secondary