Free T3 (Triiodothyronine) test is ordered to assess thyroid function. This hormone helps the body to regulate energy and has also a role in controlling weight, body temperature, muscle strength, and nervous system.

The T3, Free (FT3) test measures serum triiodothyronine (T3) not bound to thyroid hormone-binding proteins. It is used, primarily in concert with measurement of thyroid-stimulating hormone (TSH) and free T4 (FT4), in the diagnosis and management of hyperthyroidism. It also helps to clarify thyroid hormone status in the presence of a possible thyroid hormone-binding protein abnormality.