The thyroid gland makes thyroid hormone 1, 2, 3 and 4 (T1, T2, T3, T4); so designated by the number of iodine molecules present. The predominant hormone, however, is T4 which must be converted to T3 to be biologically active. In fact, it is the FREE (not bound to a protein) portion of T4 (FT4) that is converted to the FREE portion of T3 (FT3.) Less than 1% of circulating thyroid hormone is Free. Measuring the FT4 and FT3, at specific times of day, not only suggest whether optimal amounts are being MADE or TAKEN, but also help in establishing if the conversion rate is optimal.