Use

The T3, Free (FT3) test measures serum triiodothyronine (T3) not bound to thyroid hormone‑binding proteins (thyroid hormone‑binding globulin, transthyretin, albumin). It is used, primarily in concert with measurement of thyroid‑stimulating hormone (TSH) and free T4, in the diagnosis and management of hyperthyroidism and to clarify thyroid hormone status in the presence of a possible thyroid hormone‑binding protein abnormality. Up to 10% of patients with hyperthyroidism may have elevated FT3 but normal FT4 (“T3 toxicosis”).