The Thyroid Gland and Oxidative Stress; the Role of Melatonin

Reactive oxygen species (ROS) and free radicals participate in physiological and pathological processes in the thyroid gland. For example, the role of hydrogen peroxide (H₂O₂) is crucial for thyroid hormone biosynthesis. Also other free radicals or reactive species, formed from iodine or tyrosine residues, are produced during thyroid hormone synthesis in physiological conditions. In turn, much evidence has been accumulated, showing that thyroid diseases, e.g., Graves' disease, non-toxic goitre formation or thyroid cancer, are accompanied by enhanced oxidative stress. The presence of some antioxidants has been found in the thyroid. Melatonin (N-acetyl-5-methoxytryptamine) -the main secretory product of the pineal gland is a well-known antioxidant and free radical scavenger, widely distributed in the organism. Mutual relationships between the pineal gland and the thyroid have-for a long time-been a subject of intensive research. The abundant to-date's evidence relates mostly to the inhibitory action of melatonin on the thyroid growth and function. Recently, experimental models have been developed, showing - among others-protective effects of melatonin against oxidative damage to lipids in the thyroid gland. Thus, free radicals and antioxidants, melatonin included, may participate in both physiological and pathological processes in the thyroid, what has already been partially documented.