T3 uptake (T3U) is an indirect measure of the quantity of thyroxine binding proteins (thyroid binding prealbumin, albumin, and TBG) in plasma. It is directly proportional to the degree of saturation of the binding proteins by thyroxine. T3U is decreased during pregnancy, with supra-normal doses of estrogen, early in acute hepatitis, and with genetic TBG excess. It is increased by anabolic hormones and glucocorticoids, the nephrotic syndrome, and genetic TBG deficiency. T3U is not useful alone but should be combined with T4 to calculate the FTI (also called T7).