Thyroxine, or T4, also called tetraiodothyronine, contains four atoms of iodine. Triiodothyronine, or T3, contains only three atoms of iodine. The more common hormone is T4, which accounts for nearly 90 percent of the secretions from the thyroid. The amount of T3 in the body is concentrated and very effective. Both hormones have similar functions. Enzymes in the liver can convert T4 to T3.
The thyroid and parathyroid glands are located in the neck: 1 = Superior parathyroids; 2 = Inferior parathyroids; 3 = Trachea; 4 = Isthmus; 5 = Thyroid gland; and 6 = Thyroid cartilage. Anatomical Chart Co.