Thyroid gland is largest gland in endocrine system. Thyroid gland situated in neck. Main function of thyroid gland is producing hormones – T3[tri iodothyronin] and T4[tetraiodothyronine] which controls body metabolism, thyroid gland also produce calcitonin which plays a role in calcium homeostasis. The hormonal output from thyroid is regulated by thyroid stimulating hormone [TSH ] produced by anterior pituitary, which itself regulated by thyrotropin releasing hormone [TRH] produced by hypothalamus.