- Last Update On : 2013-02-02
The major function of T4 is to serve as a prohormone that is converted to T3 by deiodination. The cellular action of thyroid hormone is largely mediated by the interaction of T3 with nuclear T3 receptors.
Total thyroxine (T4) is a good index of thyroid function when thyroid binding proteins are normal. Elevated T4 levels are associated with hyperthyroidism and acute thyroiditis. Decreased levels are found in hypothyroidism and chronic thyroiditis.Nonthyroid illness may cause significant confusion in the interpretation of T4.
Total T4 may be increased by the following medications; estrogen, oral contraceptives, clofibrate, fluorouracil, and exogenous thyroxine. Total T4 may be decreased by androgens, glucocorticoids, L-asparaginase, phenytoin, carbamazepine, rifampin, and dopamine.
Reference range is 4.5 - 12.4 ug/dL.
Specimen requirement is one SST tube of blood.