T3 Uptake provides what kind of relationship to thyroxine-binding globulin (TBG)?

• A. Inverse relationship with TBG
• B. Direct relationship with TBG
• C. No relationship to TBG
• D. Proportional to TBG levels and T4

Answer: (A)

T3 uptake measures the availability of binding sites on thyroxine-binding globulin (TBG) for thyroid hormones. It uses a fixed amount of labeled T3 to compete for those sites. If TBG is high (or if endogenous T4 is high and occupying many sites), fewer unoccupied sites remain, so less labeled T3 can bind and the uptake value drops. If TBG is low (or endogenous T4 is low), more sites are unoccupied and more labeled T3 binds, producing a higher uptake. So the relationship is inverse: as TBG binding sites become more occupied, T3 uptake decreases.

This is why the option describing an inverse relationship with TBG best fits. It’s not a direct or no relationship, and it isn’t proportional to both TBG levels and T4.