Melatonin and the Thyroid Gland

In this review, data from reports on relationships, observed between melatonin – the main pineal hormone - and the thyroid gland, are briefly summarized. The prevailing part of the survey is devoted to melatonin influence on thyroid growth processes and function. Much evidence has been accumulated, suggesting an inhibitory action of melatonin on thyroid growth and secretion; this effect has been revealed by using different experimental models: short-term and/or chronic melatonin administration in vivo to various animal species, pinealectomized animals, light restriction, which is known to increase the activity of the pineal gland, etc., as well as by employing in vitro conditions. Oxidative stress plays a crucial role in physiological and pathological processes in the thyroid gland. Accordingly, it has been documented that oxidative stress accompanies certain thyroid disturbances or diseases. Up-to-date literature, although not abundant, indicates that melatonin can protect against oxidative damage in the thyroid and in other organs. Furthermore, much data has been gathered, indicating – in experimental conditions – a mutual relationship between the pineal gland and the thyroid. The confirmation of these relations in clinical studies in humans meets numerous difficulties, resulting – among others – from the fact that - nowadays - human beings, as well as animal species, used in experimental studies, have been living far away from their natural and original habitats. It makes almost impossible to compare the results of studies on the pineal-thyroid interrelationship, obtained in particular experiments performed in different species.