Free triiodothyronine (FT3) is the unbound, biologically active fraction of triiodothyronine (T3) which represents 0.3% of total T3. Variations in the levels of transport proteins do not affect the level of the unbound form. FT3 helps regulate growth and development, helps control metabolism and body temperature, and, by a negative-feedback system, acts to inhibit the secretion of thyrotropin by the pituitary gland. This test is ordered primarily to help diagnose hyperthyroidism and may be ordered to help monitor the progress of a patient with a known thyroid disorder. The FT3 test is usually ordered following an abnormal TSH and T4 test. INCREASED levels are seen in primary (peripheral) and secondary (central) hyperthyroidism See Thyroxine (FT4), Free, Serum; Thyroid Stimulating Hormone (TSH), Serum; and Thyroglobulin, Serum.
Allow specimen to clot completely at room temperature. Separate serum or plasma from cells ASAP or within 2 hours of collection