Discrepancy between uptake of serum triiodothyronine as measured by albumin and charcoal methods in nonthyroidal illnesses.

Triiodothyronine (T3) uptake as determined with albumin (AT3U) and with charcoal (CT3U) as binders were compared for patients with low-T3 syndrome. Both T3U values were identical in normal subjects and in patients with thyroidal disorders, pregnancy, low thyroxin-binding globulin (TBG), or acute hepatitis with high concentration of TBG. In the low-T3 syndrome, AT3U was significantly higher than CT3U (38.5 +/- 7.2% vs 29.4 +/- 4.9%, mean +/- SD, n = 37, p less than 0.001), and this difference increased as TBG and albumin concentrations decreased. Decreasing the concentration of TBG and albumin in serum by dilution increased the AT3U more than the CT3U value. Adding albumin to serum decreased AT3U, but not CT3U, when serum TBG and albumin concentrations were low. The free-thyroxin (T4) index as calculated by the AT3U ratio and T4 more accurately reflects the concentration of free T4 in low-T3 syndrome than does the index calculated from the CT3U ratio and T4. Thus AT3U is determined primarily by changes in TBG and albumin concentrations, so that the free-T4 index based on the AT3U ratio and T4 may be more useful than the CT3U method for evaluating free-T4 concentrations in low-T3 syndrome, which frequently has concomitant decreases in TBG and albumin concentrations.