Thyroid is a butterfly shaped endocrine gland. This gland is of weight 20-25 grams for a man, and this gland is present in front of neck. The thyroid gland controls how quickly the body uses energy, makes proteins, and controls how sensitive the body is to other hormones. It participates in these processes by producing thyroid hormones, the principal ones being triiodothyronine (T3) and thyroxine which can sometimes be referred to as tetraiodothyronine (T4). These hormones regulate the rate of metabolism and affect the growth and rate of function of many other systems in the body. T3and T4 are synthesized from both iodine and tyrosine. The thyroid also produces Calcitonin, which plays a role in Calcium homeostasis. Hormonal output from the thyroid is regulated by Thyroid-stimulating hormone (TSH) produced by the anterior pitutary, which itself is regulated by Thyrotropin-releasing hormone (TRH) produced by the hypothalamus.