Received January 31, 2020
Accepted August 01, 2020

Our aims were to evaluate critically the evidence in research of the effects of melatonin on hypothyroidism and gonadal development. According to the World Health Organization (WHO), thyroid disorders due to iodine deficiency affect about 740 million people worldwide. Hypothyroidism is a thyroid dysfunction characterized by hypometabolism of the gland and can be identified when serum levels of T3 and T4 are reduced or physiologically normal, and the TSH level is elevated. This disorder occurs mainly in adult women and in the reproductive phase, with prevalence in 2% of the world's female population with profound repercussions on gestation and fetal formation. During the gestational period, the thyroid is initially stimulated by high concentrations of chorionic gonadotrophin (hCG), thus maintaining maternal euthyroidism during pregnancy and lactation is fundamental for fetal growth and development. Besides, the hormones produced by this gland are involved in the formation of various organs, such as the skin, brain and gonads. It is reported that hypothyroidism is associated with several menstrual abnormalities, anovulation and hyperprolactinemia, resulting in a high rate of abortions, premature births, placental rupture, and weight-related neonatal deficits. In addition, previous studies have shown that hypothyroidism can affect both ovarian morphology (number of ovarian follicles) and testicular morphology (changes in the testicular-lumen epithelium). Melatonin is an hormone known to modulate the estrous cycle and pregnancy and studies show that the exogenous application of melatonin increased T4 levels in female rats and controlled the decrease in serum levels of T3, reverting the sigs of hypothyroidism.