BACKGROUND: The thyroid gland, a vital endocrine organ, the thyroid gland synthesizes and secretes hormones such as triiodothyronine, tetraiodothyronine, and calcitonin. The pineal gland produces and releases melatonin, which is stimulated in the dark and suppressed in the presence of light. Aim: This study aims to assess how melatonin influences adult male rats' thyroid performance. METHODS: Eighteen adults male Wistar albino rats (Rattus norvegicus) (approximately 12-14 weeks old and weighing 220±10 g) were randomly divided into three groups, each containing six rats, and were treated for 30 days as follows: The control group (A) got distilled water (D.W.). A1 group: received orally 10mg/kg body weight (B.W.) of melatonin daily, A2 group: received orally 20 mg/kg B.W. of melatonin daily for 30 days. At the end of the study, blood was drawn by cardiac puncture, and serum was collected to assess thyroid hormone levels: triiodothyronine (T3), thyroxine (T4), thyroid- stimulating hormone (TSH), and total antioxidant capacity (TAC). RESULTS: The results showed T3 levels increased dramatically in the A2 group, whereas T4 and TSH levels remained constant. On the other hand, there is a significant increase in total antioxidant capacity in the A1 groups as compared with the control and A2-treated groups. The histological section of the thyroid glands revealed the normal histological structure of the thyroid in the A1 group as compared to the A2 group. Besides, the figures of the thyroid gland A2 group showed severe vascular degeneration and necrosis of thyroid follicles. CONCLUSIONS: Treatment of melatonin at higher doses may influence thyroid hormone regulation, highlighting its potential physiological effects, while the melatonin at a dose of 10 mg/kg plays a key role in regulating thyroid functions and enhancing antioxidant status