During the first incubation triiodothyronine of the patient samples and calibrators bind to the monoclonal anti-triiodothyronine antibodies immobilized on the solid phase of the microtiter plate. Following an incubation period of 45 min, unbound components are separated from the solid-phase immune complexes by a wash step. Sample Types: serum. Range: 1 - 63 pmol/L. Sensitivity: 0.5 pmol/L. Time: 1.5 hours ? Circulating thyroid hormones (thyroxine, T4 and triiodothyronine, T3) are distributed into two compartments; a major protein bound one, and a minor free fraction (0.03 % of T4 and 0.3 % of T3). Variations in total thyroid hormone in blood may result from either changes of binding protein concentrations, or thyroid hormone synthesis. Thyroid disorders exist only if a net change of free unbound fractions occurs persistently. Therefore the true determination of thyroid status will be the measurement of the free hormones.