The Autobio f-T4 assay is designed for the quantitative determination of free thyroxine (f-T4) concentration in human serum.
Thyroxine, the principal thyroid hormone, circulates in blood almost completely bound to carrier proteins. The main carrier is thyroxine-binding globulin (TBG). However, only the free (unbound) portion of thyroxine is responsible for the biological action. Further, the concentrations of the carrier proteins are altered in many clinical conditions, such as pregnancy. Under normal thyroid condition, as the concentrations of the carrier proteins alter, the total thyroxine level changes so that the free thyroxine concentration remains constant. Thus, measurement of free thyroxine concentrations correlate better with clinical status than total thyroxine levels.
For example, the increase in total thyroxine associated with pregnancy, oral contraceptives and estrogen therapy occasionally result in total T4 levels over the limits of normal while the free thyroxine concentation remains within the normal reference range. Masking of abnormal thyroid function can also occur in both hyper and hypothyroid conditions by alterations in the TBG concentration. The total T4 can be elevated or lowered by TBG changes such that the normal reference levels result is observed. Again, the free thyroxine concentration typically uncovers the patient’s actual clinical status.
