Free T4

T4

An endogenous prohormone released from the follicular cells of the thyroid gland.

FT(4) may be increased with radiologic contrast agents, propranolol, amiodarone, and heparin. It may be decreased with carbamazepine (Tegretol(R)). Free T(4) is a small part of total T(4). Increased free T(4) levels may occur in subjects with nonthyroid diseases. Such elevations are described as transient.[2] Low values were reported in patients with nonthyroidal illness.[3] Discrepancies in free T(4) levels between methods are recognized.

Free T(4) may be indicated when binding globulin (TBG) problems are perceived, or when conventional test results seem inconsistent with clinical observations. It is normal in subjects with high thyroxine-binding globulin hormone binding who are euthyroid (ie, free thyroxine should be normal in nonthyroidal diseases). It should be normal in familial dysalbuminemic hyperthyroxinemia