Which test provides an indirect measure of thyroxine-binding globulin (TBG)?

• A. T3 Uptake
• B. T4
• C. Free T3
• D. TSH

Answer: (A)

The test that provides an indirect measure of thyroxine-binding globulin (TBG) uses competition to gauge how many binding sites are available on TBG. In this assay, a fixed amount of labeled T3 is added and its binding to TBG depends on how much of those sites are already occupied by endogenous T4. If TBG binding sites are largely occupied by T4 (high TBG or high T4 levels), less labeled T3 can bind, so the uptake signal changes. Because the result reflects the capacity of TBG to bind thyroid hormones rather than the actual free hormone level, it serves as an indirect estimate of TBG binding sites. By contrast, measuring total T4 reveals all hormone (bound plus free) but not TBG capacity; measuring free T3 directly assesses unbound hormone, and TSH indicates pituitary signaling rather than TBG. So this uptake test is the indirect indicator of TBG.