The Thyroid Gland: Thyroid is two-lobed gland, located at the front of the larynx and trachea in the neck. Two thyroid hormones–thyroxine (T4) and trilodothyroxine (T3),increase the rate of cellular metabolism. Dietary iodine is required to make thyroxine and trilodothyroxine. Different cells within the thyroid produce a third hormone—calcitonin, which lowers blood calcium levels under certain conditions. The main effect of thyroxine is control the BMR (Base Metabolic Rate). Thyroxine (T4) contains four atoms of iodine; it is secreted in greater amounts but is less potent than triiodothyroxine (T3), which has only three atoms of iodine. Iodine is actively transported into the thyroid gland, and it may reach a concentration as much as twenty five times greater than that of the blood. The thyroid hormones bind to many different cell types, where they speed metabolism by increasing supplies of enzymes needed for cellular respiration. Under thyroid stimulation, gas exchange in the lungs is faster, the small intestine absorbs nutrients more rapidly and the fat levels in cells and in blood plasma diminish.