triiodothyronine resin uptake test

triiodothyronine resin uptake test,

a thyroid function test measuring how many sites on thyroxine-binding globulin (TBG) are occupied by endogenous triiodothyronine (T₃) and how many sites remain available. An excess of radioactive exogenous T₃ is added to the sample, followed by the addition of a resin that also binds T₃. A portion of the radioactive T₃ binds to sites on TBG not already occupied by endogenous thyroid hormones, and the remainder binds to the resin. The amount of labeled hormones bound to the resin (the T₃ resin uptake) can be subtracted from the total that was added, and the remainder is the amount that was bound to the unoccupied binding sites on the TBG.