triiodothyronine resin uptake test, a thyroid function test measuring how many sites on thyroxine-binding globulin 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. See also thyroxine-binding globulin.
