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

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Abstract
We have devised a practical, sensitive and specific method for simultaneous measurement of free thyroxine (FT4) and free triiodothyronine (FT3) in undiluted serum by direct equilibrium dialysis radioimmunoassay (RIA). Two hundred microliters serum sample was dialyzed against buffer (pH 7.4) for 20 hours at 37°C and ∼800 μL of the dialysate was used for measuring FT4 and FT3 simultaneously. The assay was set up in polystyrene tubes coated with anti-T4 antibody and available commercially for FT4 measurement (Quest-Nichols Institute, San 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 significantly increased (3.6 ± 0.4 [1.8 to 9.6], n = 20) in hyperthyroidism and clearly decreased (0.40 ± 0.04 [1.10 to 0.70], n = 26] in hypothyroidism. All nonthyroid illness (NTI) patients had normal FT4 except 3, 2 of whom were on amiodarone and 1 had received heparin. Serum FT4 concentration was minimally elevated in 18 newborn cord blood serum (1.40 ± 0.08 [0.90 to 2.2], cf. normal p <.05). The mean serum FT3 concentration (pg/dL) was 285 ± 10 (134 to 454) in 54 normal sera. It was clearly increased in hyperthyroidism (1033 ± 98 [593 to 2134], n = 20, p <.001). However, serum FT3 varied widely in hypothyroidism (27 to 597, mean 235 ± 24, NS) as did serum total T3 (19 to 175). Interestingly, however, the mean serum FT3 concentration was normal (273 ± 28 [62 to 575, NS]) in 25 NTI patients. All of these patients had low serum total T3 (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 and low in the remaining 4 patients. Similarly, among 18 newborn cord blood sera serum FT3 concentration was normal in 15 and subnormal only in the remaining 3 while all had clearly subnormal total T3 (28 to 74 ng/dL). Conclusions: (1) A practical, sensitive, and specific assay for simultaneous measurement of FT4 and FT3 is described; (2) FT3 is consistently elevated in hyperthyroidism while FT4 is elevated in most (∼85%) cases; (3) FT4 is consistently decreased in hypothyroidism but FT3 varies widely; (4). Serum FT3 concentration is normal in ∼83% of patients with the low T3 syndrome in NTI and newborn cord blood serum. These data suggest that normal FT3 may explain clinical euthyroidism in many patients with the low T3 syndrome.