Also known as: Free T3, T3, Free, T3, Free, Serum, Triiodothyronine, Free, Unbound T3.

Tri-iodothyronine (T3) normally represents only approximately 5% of the thyroid hormone and like thyroxine is almost entirely bound to the carrier proteins, with only 0.25% of the total being in the free state. Measurement of free T-3 is of value in confirming the diagnosis of hyperthyroidism, when an elevated free or total thyroxine level is found. Abnormal total and free tri-iodothyronine concentrations can appear in T3 toxicosis, in the presence of normal thyroxine levels. Free T3 levels are not affected by carrier protein variation.