This assay is used for the quantitative determination of total thyroxine (T4) in human serum and plasma. T4, the primary hormone secreted by the thyroid gland, plays a vital role in regulating metabolism and supporting cellular development. It is produced in the thyroid by coupling two DIT (3,5-diiodotyrosine) molecules and is stored bound to thyroglobulin, released under the control of thyroid-stimulating hormone (TSH). Over 99% of T4 in the blood is protein-bound, and variations in binding protein levels—due to factors like pregnancy or medications—can affect total T4 measurements. Therefore, binding protein status must be considered when assessing thyroid function. T4 testing is useful for diagnosing hyperthyroidism, primary and secondary hypothyroidism, and monitoring TSH-suppression therapy.
