T4 (Thyroxine) ELISA Assay kit is a one-step enzyme immunoassay based on competition between horseradish peroxidase-labeled thyroxin and free thyroxin of serum sample for monoclonal anti-thyroxin antibodies immobilized on the inner surface of microwells. Sample Types: serum. Range: 0.1 - 100 pmol/l. Sensitivity: 1.0 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 are existing only if a net change of free unbound fractions occurs persistently. Therefore the clinical utility of total T4 and T3 is dependent on the knowledge of functional levels of binding proteins, which can only be measured at length.