To explore why sera of some patients with nonthyroidal illnesses (NTI) show a very high dialyzable fraction (DF) of thyroid hormones, aliquots of sera of five normal subjects and seven severely ill NTI patients with high DF of T4 (DFT4) were added to a pooled normal serum and DFT4 and DF of T3 (DFT3) were determined by equilibrium dialysis after adjustment of the final volume of the test specimens to 5 ml with 0.15 M phosphate buffer (pH 7.4). The control DFT4 of 1.2% with 100 pi pooled normal serum decreased to values of 0.48–0.57% with the addition of 200 /il and to 0.31–0.36% with the addition of 400 nl normal sera to the pooled normal serum. There was a much smaller decrease or an actual increase in DFT4 when NTI sera were added to the pooled normal serum. DFT4 ranged between 0.71–4.8% with the addition of 200 µ1 and between 0.56–4.4% with the addition of 400 /xl NTI sera to the pooled normal serum. Similar differences between sera of patients and normal subjects were observed in studies of DFT3. Thes...