Aging of the Endocrine System

Abstract

The coordination and communication of the living cells to support life is an extraordinary phenomenon. Understanding the underlying mechanisms is an age old fascinating quest for humankind. Some assumed that the vital spirits in the blood are responsible for regulating body functions. Scientific endeavors put forward the concept of “chemical messengers or hormones” in coordinating and communicating various systems in the human body. The hormones are released by the endocrine glands into the portal system to coordinate, communicate and affect distant organs. Discovery of the various endocrine glands, their hormonal secretions and their various specific biological actions in target organs form the concept of “endocrine system”. Interestingly, the underlying endocrine mechanism of menopause, which usually occurs from the fourth to fifth decade in females, was unfolded by the management of its symptoms with the administration of ovarin that was extracted from cow ovaries in the 19th century. The manifestation of menopausal symptoms is a consequence of endocrine dysregulation with age, thereby suggesting that aging and endocrine system are interrelated. Endocrine dysregulation may result when the aging hypothalamic-pituitary axis is blunted to inputs from the downstream endocrine effects and as well to external milieu. The diminishing response of the hypothalamus-pituitary axis results in an unregulated homeostasis in aged individuals culminating in diseases and death. The fast feedback inhibition of glucocorticoids on the hypothalamic-pituitary axis in human seems to be dampened during aging as a consequence which aged individuals poorly adapt to stress. In addition, the response of the downstream target endocrine glands to the hypothalamic-pituitary axis input seems to decrease as a person grows older. For example, the response of the thyroid gland to thyroid stimulating hormone (TSH) diminishes even in the presence of elevated TSH which often corresponds to thyroidal disorders. Different target cells respond differently to a single hormone during aging. The different regions of the adrenal gland show different responses to the peptide hormone, adrenocorticotropic hormone (ACTH) that is released from the anterior pituitary. For example, the zona reticularis, innermost layer of adrenal cortex, which produces androgens showed decreased response to ACTH whereas the zona fasciculata, middle layer, which produces cortisol shows preserved ACTH response. The endocrine axis such as hypothalamic-pituitary adrenal/thyroid/gonadal axes, enteroinsular (pancreas and gut) axis and other endocrine glands like pineal, parathyroid and others exhibit a changing pattern in the hormonal secretions associated with various physiological and pathological alterations, detrimental to aged individuals. Dietary restriction, nutraceuticals and physical exercise plausibly will have a positive impact in managing the clinical geriatric care. A general trend on the relationship of various endocrine players with aging is established although few conflicting studies on the aging of human endocrine system in terms of changes of some hormones level remains unclear. Given the current scenario of an increasing average lifespan of human beings; a bird's eye view on the elucidation of the aging endocrine system will kindle a systematic understanding on various underlying molecular mechanisms during aging. This will augment further research on the therapeutic and clinical management for healthy aging.