T-3, FREE

T-3 FREE

A T3 test is performed as part of an evaluation of thyroid function. It measures the blood level of the hormone T3 (triiodothyronine), some of which is produced directly by the thyroid gland.

Most T3 in the blood, however, is produced elsewhere in the body where T4 (the major thyroid hormone produced by the thyroid gland) is chemically converted to T3. T3 helps control many body functions including growth, body temperature, and heart rate.

The FT3 (or total T3) test is usually ordered following an abnormally low TSH result and/or a clinical picture suggestive of hyperthyroidism, particularly if the free T4 test result is not elevated. Up to 10% of patients with proven hyperthyroidism (due to Graves disease or an autonomously secreting thyroid nodule) may have an elevated FT3 but normal FT4 (T3 toxicosis), particularly early in the course of disease or as an early sign of relapse after treatment. 

T3 (triiodothyronine) is one of the main hormones produced by the thyroid. Along with T4, T3 is responsible for regulating metabolism, heart rate, body temperature, mood, mental alertness and more.