According to the free radical theory, aging is caused by the damaging effects of oxygen radicals. These agents are produced continuously and they become toxic when their intra-cellular levels become elevated. Although well-known for the damage they cause, recent studies indicate that free radicals may also have a beneficial side regulating some of the processes within a cell. With aging, however, it seems that the ability to control endogenous concentrations declines, and rising levels become progressively more harmful. In addition to the primary changes, the effects of free radical damage should also be examined at the secondary level. Because of its importance in regulating homeostatic mechanisms, it is possible that in this regard the hypothalamus may play a crucial role in the aging process. Progressive loss of function in this centre may lead to systemic changes which cause widespread disruption throughout the organism.

Apart from well known areas of overlap between endocrinology and psychiatry (e.g. studies, in psychiatric disorders, of neurohormones and of the response to manipulations of hypothalamic-pituitary-target gland axis, and analysis of behavioural and psychological disturbances in endocrinological disorders) there is a more intimate intrinsic relationship between the brain and the endocrine system which is less well known or studied. Many of the extracranial endocrine glands have autonomic innervation. Like the pituitary gland which is under direct neural (as well as humoral) diencephalic control, the extracranial endocrine glands are under direct neural control, integrated by the hypothalamus and “head ganglion of the autonomic nervous system”. Yet it is only in the case of the pancreatic islets that this integration has been clearly defined. It is postulated that by this innervation the somatic endocrine glands can respond to homeostatic needs with a rapid initial secretion before the more sustained outpouring of humoral agents typically regulated by blood-borne constituents including pituitary hormones. This is a vast area awaiting further investigation.