- Last Update On : 2013-02-02
T3 is the most potent of the thyroid hormones. Approximately 80% of circulating T3 is derived from the peripheral conversion of T4 and 20% from direct thyroid secretion. Most patients with hyperthyroidism have increased levels of both T3 and T4. Occasionally, however, T3 is the only hormone increased in hyperthyroidism. This condition is called T3 toxicosis.The major clinical need for T3 measurement is to identify patients with T3 thyrotoxicosis. T3 measurements are not needed for the diagnosis of hypothyroidism.
T3 levels are decreased in patients with severe systemic illnesses. Therefore, a patient with hyperthyroidism and other major diseases may have minimally increased, normal or decreased T3 concentration. Approximately 3% of patients receiving the antiarrhythmic drug, amiodarone, become thyrotoxic. Most of these patients have elevated T3 levels.
Reference range is 60 - 230 ng/dL.
Specimen requirement is one SST tube of blood.