Clinical usage recommendations and analytic performance goals for total and free triiodothyronine measurements

The major clinical role for total triiodothyronine (TT₃) and (or) free T₃ (FT₃) is the assessment of hyperthyroidism in patients with suppressed sensitive thyrotropin (sTSH) concentrations. The assays are particularly important in hyperthyroid patients with normal free thyroxine (FT₄) concentrations to assess potential T₃ thyrotoxicosis. Other specialized uses for T₃ and FT₃ measurements are monitoring thyroid hormone replacement therapy, the evaluation of amiodarone-induced thyrotoxicosis, and predicting outcome of antithyroid drug therapy in patients with Graves hyperthyroidism. The roles of these tests in assessing heart function in cardiopulmonary bypass surgery, evaluation of patients with neuropsychiatric disorders, and monitoring of patients on anticonvulsant therapy are not well defined. These assays are not recommended for diagnosis of hypothyroidism. Analytic recommendations include CV <5.2% for T₃ and <3.8% for FT₃; <0.2% cross- reactivity with L-T₄; and < 1.0% cross-reactivity with D-T₄, D-T₃, and reverse T₃.