Free thyroxine concentrations in serum measured by equilibrium dialysis in chronic renal failure

The serum concentration of free thyroxine (FT4) is often low in patients with chronic renal failure (CRF) with low serum concentrations of triiodothyronine (T3). We evaluated the serum FT4 concentration by using both an equilibrium dialysis RIA kit (D-FT4) and a labeled-antibody kit (M-FT4) in two different groups of CRF patients, undergoing chronic hemodialysis (HD, n = 145) or not (non-HD, n = 30), and in a group of normal healthy subjects (n = 58). Thyroid peroxidase antibodies and thyroglobulin antibodies were not detected in any patient. Serum FT4 concentrations (mean +/- SD, pmol/L) by the D- and M-FT4 assays were, respectively, 21.5 +/- 4.6 and 16.6 +/- 2.0 in the healthy subjects, 17.8 +/- 4.3 and 13.9 +/- 3.6 in the non-HD patients, and 16.9 +/- 4.9 and 10.7 +/- 1.9 in the HD patients. By the D-FT4 assay, results for both CRF groups were significantly different from those for the healthy group (P <0.01), as were the results for each pair of groups by the M-FT4 assay (P <0.01). FT4 values were reported as being within the healthy reference range by D-FT4 in 73 of 113 HD subjects who had low T3 and low M-FT4 values. Serum FT4 concentrations measured by both assay kits showed a significant inverse correlation with the serum concentration of creatinine (P <0.01), but the serum concentrations of sex-hormone-binding globulin did not differ significantly among the three groups. Our results indicate that the low FT4 concentration measured by D-FT4 in patients with CRF, particularly those on HD, probably reflects the actual, mild nonthyroidal illness of renal failure.