Endocrine secretions under abnormal light-dark cycles and in the blind

Both endogenous and exogenous factors are involved in regulation of endocrine secretions. Among the exogenous ones, light plays an important role both in animals and in humans. Pineal gland mediates light action on the endocrine system, by means of variations of melatonin (MT) secretion. Here we discuss about the influence of abnormal light-dark cycles and in particular of blindness on pineal and pituitary secretions and on those of correlated glands. MT secretion is usually inhibited by light: thus it reaches the highest levels at night. Exposure to short or long photoperiod causes variations in circadian or infradian MT rhythmicity. Blind patients can show higher daytime levels with a phase-advanced or phase-delayed circadian rhythm. Lack of light stimulus affects cortisol rhythm shifting the zenith of secretion and inducing a free-running rhythm. Blindness can abolish nocturnal growth hormone (GH) peak and impair the GH response to some stimuli; moreover it impairs the growth of affected patients. Light stimulus influences favorably gonadal function both in animals and in man. In animals, sexual activity and gonadal function decline during the seasons with reduced luminosity. A similar finding has been described in women living in a region with a strong seasonal contrast in luminosity. Blindness can impair luteinizing hormone, follicle-stimulating hormone, prolactin and testosterone secretion in prepubertal boys causing pubertal delay or more severe hypogonadism; it can affect pubertal development and fertility in women. Light can influence thyroid function in animals. Lack of light stimulus in blind man seems to cause different effects on thyroid function before and after puberty. Increase of free thyroid hormone levels has been found in prepubertal but not in adult blind patients, probably due to a resetting of the threshold for thyrotropin feedback suppression after puberty in these patients.