In humans and other mammals, thyroid hormone regulates bioenergetics; helps maintain normal blood pressure, heart rate, and muscle tone; and regulates digestive and reproduc- tive functions. If the level of thyroid hormone in the blood drops, the hypothalamus responds by initiating a hormone cascade pathway . The hypothalamus secretes thyrotropin-releasing hormone (TRH), causing the anterior pituitary to secrete a tropic hormone known as either thyroid-stimulating hormone (TSH) or thyrotropin. TSH stimulates release of thyroid hormone by the thyroid gland, an organ in the neck consisting of two lobes on the ventral surface of the trachea. As thyroid hormone accumulates, it increases metabolic rate, while also initiating negative feed- back that prevents its overproduction.