T3 Uptake is an indirect measure of thyroid-binding globulin (TBG) saturation. It doesn't measure hormone directly. It measures how much radioactive T3 tracer a serum sample can hold, which inversely reflects TBG capacity.
T3 Uptake is primarily used to calculate the Free Thyroxine Index (T7 = Total T4 × T3 Uptake ÷ 100), a pre-Free-T4-assay method for estimating active thyroid hormone. When free T4 is directly measured, T3 Uptake adds little clinical value.
Optimal T3 Uptake: 28-38 %.
Typically re-tested every 12 weeks.