Triiodothyronine (T3) and thyroxine (T4) are the two active thyroid hormones found in the blood stream. About 80% of T3 is produced by the deiodination of T4 in the peripheral tissue and the other 20% is produced directly from the thyroid gland. T3 is transported through the peripheral blood stream bound to serum proteins, namely thyroxine binding globulin, thyroid binding prealbumin and albumin. About 0.3% of the total T3 is unbound and is therefore considered the free fraction. T3 has an influence on oxygen consumption and heat production in virtually all tissues. The hormone also plays a critical role in growth, development and sexual maturation of growing organisms. T3 is one parameter used in the clinical diagnosis and differentiation of thyroid disease, particularly hyperthyroidism. In most hyperthyroid patients, both serum T3 and serum T4 levels are elevated. Serum T3 levels are a sensitive indicator of the impending hyperthyroid state often preceeding elevated T4 and free thyroxine index values. Serum T3 levels are clinically significant in both the diagnosis of thyroid disease and in the detection of T3-thyrotoxicosis. However, it has been demonstrated that T3 levels may be affected by a number of medications, acute and chronic stress, and a variety of acute and chronic non-thyroidal illnesses. It is therefore necessary to differentiate those results that are due to thyroid dysfunction from those related to non-thyroidal diseases.
- References to Triiodothyronine (T3)