Thyroxine is a hormone synthesized and secreted by thyroid follicular cells, released into the blood circulation in free form, and most of it quickly binds to plasma proteins. The bound part of triiodothyronine (T3) and tetraiodothyronine (T4) has no biological activity, its high and low can not reflect the functional state of the thyroid gland, only a small part does not bind to proteins, in a free state, called serum free triiodothyronine (FT3), serum free thyroxine (FT4). Both are the most direct indicator of thyroid function status and are not affected by changes in blood thyroxine-binding globulin concentration and binding capacity.