SIMULTANEOUS MEASUREMENT OF FREE-THYROXINE AND FREE 3,5,3'-TRIIODOTHYRONINE IN UNDILUTED SERUM BY DIRECT EQUILIBRIUM DIALYSIS RADIOIMMUNOASSAY - EVIDENCE THAT FREE TRIIODOTHYRONINE AND FREE-THYROXINE ARE NORMAL IN MANY PATIENTS WITH THE LOW TRIIODOTHYRONINE SYNDROME/

We have devised a practical, sensitive and specific method for simulta neous measurement of free thyroxine (FT4) and free triiodothyronine (F T3) in undiluted serum by direct equilibrium dialysis radioimmunoassay (RIA). Two hundred microliters serum sample was dialyzed against buff er (pH 7.4) for 20 hours at 37 degrees C and similar to 800 mu L of th e dialysate was used for measuring FT4 and FT3 simultaneously. The ass ay was set up in polystyrene tubes coated with anti-T-4 antibody and a vailable commercially for FT4 measurement (Quest-Nichols Institute, Sa n Juan Capistrano, CA). The mean +/- SE (range) FT4 concentration (ng/ dL) was 1.2 +/- 0.04 (0.7.0 to 2.30) in 54 normal subjects. It was sig nificantly increased (3.6 +/- 0.4 [1.8 to 9.6], n = 20) in hyperthyroi dism and clearly decreased (0.40 +/- 0.04 [1.10 to 0.70], n = 26] in h ypothyroidism. All nonthyroid illness (NTI) patients had normal FT4 ex cept 3, 2 of whom were on amiodarone and 1 had received heparin. Serum FT4 concentration was minimally elevated in 18 newborn cord blood ser um (1.40 +/- 0.08 [0.90 to 2.2], cf. normal p < .05). The mean serum F T3 concentration (pg/dL) was 285 +/- 10 (134 to 454) in 54 normal sera . It was clearly increased in hyperthyroidism (1033 +/- 98 [593 to 213 4], it = 20, p < .001). However, serum FT3 varied widely in hypothyroi dism (27 to 597, mean 235 +/- 24, NS) as did serum total T-3 (19 to 17 5). Interestingly, however, the mean serum FT3 concentration was norma l (273 +/- 28 [62 to 575, NS]) in 25 NTI patients. All of these patien ts had low serum total T-3 (46 +/- 5.0 [10 to 84], ng/dL; normal 84 to 160, p < 0.001), while FT3 was clearly normal in 21 of 25 patients an d low in the remaining 4 patients. Similarly, among 18 newborn cord bl ood sera serum FT3 concentration was normal in 15 and subnormal only i n the remaining 3 while all had clearly subnormal total T-3 (28 to 74 ng/dL). Conclusions: (1) A practical, sensitive, and specific assay fo r simultaneous measurement of FT4 and FT3 is described; (2) FT3 is con sistently elevated in hyperthyroidism while FT4 is elevated in most (s imilar to 85%) cases; (3) FT4 is consistently decreased in hypothyroid ism but FT3 varies widely; (4). Serum FT3 concentration is normal in s imilar to 83% of patients with the low T-3 syndrome in NTI and newborn cord blood serum. These data suggest that normal FT3 may explain clin ical euthyroidism in many patients with the low T-3 syndrome.