Free triiodothyronine (T3) is a second- or third-level test of thyroid function. It provides further confirmation of hyperthyroidism, supplementing the tetraiodothyronine (T4), sensitive thyrotropin (sTSH), and total T3 assays. Evaluating clinically euthyroid patients who have an altered distribution of binding proteins. Monitoring thyroid hormone replacement therapy.Triiodothyronine (T3) is one of two major hormones produced by the thyroid gland, a small butterfly-shaped organ that lies flat across the windpipe at the base of the throat. The other major thyroid hormone is called thyroxine (T4) and together they help control the rate at which the body uses energy. Almost all of the T3 (and T4) found in the blood is bound to protein. The rest is free (unbound) and is the biologically active form of the hormone. Tests can measure the amount of free T3 or the total T3 (bound plus unbound) in the blood.