T4 Free (FT4)

The T4 Free (FT4) test measures serum thyroxine (T4) that is not bound to thyroid hormone-binding proteins (thyroid hormone-binding globulin [TBG], transthyretin, albumin) and is thus readily available to target cells. This test is used with thyroid-stimulating hormone measurement (TSH, test code 899) in the diagnosis of hyperthyroidism (usually displaying elevated FT4) and hypothyroidism (low FT4), including congenital and acquired hypothyroidism.

The FT4 test is used to assess thyroid function, monitor thyroid conditions, and assess treatment effectiveness. It has largely replaced the total T4 test (test code 867), which measures both bound and free T4 because the FT4 test is less affected by levels of thyroid hormone-binding proteins.

This test uses a T4 analog, rather than equilibrium dialysis, to determine the amount of free T4. It differs from the T4 Free, Direct Dialysis test (test code 35167). The analog FT4 test is most useful in the general population as a follow-up test for abnormal TSH results. It may also be used instead of TSH testing to assess thyroid function in patients with (1) known pituitary abnormalities; (2) chronically suppressed TSH, as often seen in patients for months after successful treatment for Graves disease [1]; or (3) a need to assess thyroid hormone dose changes sooner than the 6-8 weeks required for TSH to reach a new stable level. When the levels of thyroid hormone-binding proteins are very abnormal (e.g., congenital TBG deficiency or excess; preterm infants [2]) or the affinity of binding proteins is altered [3] (e.g., familial dysalbuminemic hyperthyroxinemia), the T4 Free, Direct Dialysis test will provide more accurate results.