T3 Uptake is a thyroid function test for the diagnosis of hypothyroidism or hyperthyroidism, used with thyroxine (T4) or equivalent to provide free T4 index, (FTI). An indirect measure of binding protein, the T3 uptake shows available binding sites (ie, reflects TBG). T3 uptake is not a measurement of T3 serum. T3 Uptake should never be used alone due to the fact that its usual application is use with thyroxine (T4).