Clinical Significance

The hormone thyroxine (T4) is the main product secreted by the thyroid gland and is an integral component of the hypothalamus‑anterior pituitary‑thyroid regulating system. It increases the basal metabolic rate and plays an essential role in the development of all human body cells. Thyroxine is formed in a coupling reaction from two DIT molecules (3,5‑diiodotyrosine) in the thyroid gland. It is stored bound to thyroglobulin in the lumina of the thyroid follicles and is secreted as required under the influence of thyroid‑stimulating hormone (TSH). 

The major part (> 99 %) of total thyroxine in serum is present in protein-bound form. As the concentrations of the transport proteins in serum are subject to exogenous and endogenous effects, the status of the binding proteins must also be considered in the assessment of the thyroid hormone concentration in serum. If this is ignored, changes in the binding proteins (e.g. due to estrogen-containing preparations, during pregnancy or in the presence of a nephrotic syndrome etc.) can lead to erroneous assessments of the thyroid metabolic state. 

The determination of T4 can be utilized for the following indications: the detection of hyperthyroidism, the detection of primary and secondary hypothyroidism, and the monitoring of TSH-suppression therapy. 

_{Source: Roche cobas Elecsys T4 method sheet, V 3.0 2023-11}