Estimation of the maximal T4-binding capacity of TBG using the Triosorb test in serum treated with dextran-coated charcoal.

A new method for the estimation of the maximal T4-binding capacity of TBG (max. TBG) in serum (dextran-coated charcoal-triosorb method) was devised. Thyroxine (T4) concentration in test sera were determined by the Tetrasorb method and then T3 resin sponge uptake (T3 RSU) was determined after extraction of 70% of the endogenous T4 with dextran-coated charcoal. The binding capacity of unsaturated TBG in treated sera was estimated from the reciprocal of T3 RSU. Max. TBG was calculated from the arithmetic sum of the unsaturated binding capacity of TBG and the serum T4 concentration multiplied by 0.3. The normal range for max. TBG was 20.1 +/- 2.6 (mean +/- SD) mug T4/100 ml (14 approximately 24) and it was 19.2 +/- 3.5 mug T4/100 ml in hyperthyroidism which was lower compared with hypothyroidism (21.1 +/- 2.6 mug T4/100 ml). Max. TBG in 8 hyperthyroid patients did not change significantly after treatment in half of the case, although in the other half it was slightly increased. Increased values of max. TBG were observed in pregnancy (32.9 +/- 4.4) and in a hyperthyroid state associated with pregnancy (31.4 +/- 4.7). Decreases were observed in TBG deficiency (less than 5.6 mug T4/100 ml). These procedures can be applied for the determination of both the unsaturated binding capacity of TBG and that of max. TBG.