Thyroxine (T4), the principal thyroid hormone, circulates in blood almost completely bound to carrier proteins. However, only the free (unbound) fraction of thyroxine is considered to be biologically active. The main carriers of thyroxine are thyroxine-binding globulin (TBG), pre-albumin and albumin. The measurement of free thyroxine (fT4) levels correlate better with the clinical status than total thyroxine levels. The free T4 assay is a one-step competitive ELISA system that is rapid and easy to perform compared to equilibrium dialysis and ultrafiltration methods, which are cumbersome and time-consuming. This system employs a highly specific monoclonal antibody and a non-analog tracer that was proved experimentally to have no significant binding to TBG and albumin. In the euthyroid, normal population the free T4 concentration is 7 – 22 pg/ml. The level of free T4 is decreased in hypothyroidism while in thyrotoxic patients the level of free T4 is increased. This assay is used at times with other thyroid tests for in vitro diagnostic purposes and for assessing patients who are receiving thyroid treatments (follow-up).
- References to Thyroxine