T3 Resin Uptake
The principle of the T3 uptake test is illustrated in the figure to the right. The test is conducted by adding a known, excess amount of 125I labeled T3 to a measured volume of serum. The added radiolabeled T3 binds to and saturates the unoccupied TBG sites and does not displace the more tightly bound T4. The excess, unbound 125I T3 is then separated from the serum bound 125I T3 by adding an adsorbent, such as ion exchange resin beads and centrifuging. The serum is decanted, the beads are washed and then counted in a scintillation counter.
Results were interpreted in the past as illustrated in the figure to the right, assuming normal TBG concentrations. Hyperthyroid cases exhibit a decreased concentration of unoccupied TBG sites and an increased T3 resin uptake. Hypothyroid patients have increased unoccupied TBG sites and decreased T3 resin uptake.
The simpler T3 uptake test was as reliable as the total T4 determination by radioimmunoassay, but is obviously also influenced by variations in TBG concentrations. A euthyroid individual with an elevated TBG concentration would have a low T3 resin uptake and, thus, appear hypothyroid. A euthyroid individual with a low TBG concentration would have a high T3 resin uptake and, thus, appear hyperthyroid, etc. When TBG is abnormal, TT4 and T3RU results indicate opposite conditions. Contrary indications about thyroid status from the two tests, thus, suggest that TBG concentration is abnormal and that each individual result is unreliable.