Description

Triiodothyronine (T3) is a thyroid hormone. It plays an important role in the body’s control of metabolism. T3 circulates in the bloodstream as an equilibrium mixture of free and serum bound hormone. Free T3 (f3) is the unbound and biologically active form, which represents only 0.2-0.4% of the total T3.
The remaining T3 is inactive and bound to serum proteins, while the distribution of T3 between these binding proteins (thyroxine binding globulin, pre-albumin, albumin) is controversially discussed.

The detection of fT3 has the advantage of being independent of changes in the concentrations and binding properties of the binding proteins. Therefore, fT3 is a useful tool in clinical routine diagnostics for the assessment of the thyroid status.

Free T3 measurements support the differential diagnosis of thyroid disorders, are needed to distinguish different forms of hyperthyroidism, and to identify patients with T3 thyrotoxico-sis.