Free Thyroxine (fT4) ELISA
The Calbiotech, Inc. (CBI) fT4 ELISA kit is used for the quantitative measurement of free Thyroxine (fT4) in human serum.
Over 99% of thyroxine (T4) circulates in blood is bound to carrier proteins; 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. In normal thyroid function as the concentrations of the carrier proteins alters, the total T4 level changes so that the free T4 concentration remains constant. Thus, measurements of free T4 concentrations correlate more reliably with clinical status than total T4 levels. The increase in total T4 levels associated with pregnancy, oral contraceptives and estrogen therapy result in higher total T4 levels while the free T4 concentration remains basically unchanged.
The CBI fT4 is a solid phase competitive ELISA. The samples Anti-T4 Biotin and fT4 enzyme conjugate are added to the wells coated with Streptavidin. fT4 in the patient’s serum competes with a T4 enzyme conjugate for binding sites. Unbound T4 and T4 enzyme conjugate is washed off by washing buffer. Upon the addition of the substrate, the intensity of color is inversely proportional to the concentration of fT4 in the samples. A standard curve is prepared relating color intensity to the concentration of the fT4.
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