Comparison of methods for measuring free thyroxin in nonthyroidal illness

Patients with severe nonthyroidal illness (NTI) often have decreased serum thyroxin (T4) concentrations and sometimes have decreased free T4 (FT4) and increased tumor necrosis factor-alpha (TNF-alpha) concentrations. We evaluated four commercial methods for measuring FT4 [Abbott IMx, Amersham Amerlex MAB, Nichols, and Diagnostic Products Corporation (DPC) RIA] and one method for TNF-alpha in 41 NTI patients, 24 euthyroid control subjects, and 10 hypothyroid patients. Free T4 index (FTI) was also measured by the Abbott IMx method. Euthyroid subjects results were in the stated normal ranges. NTI FT4 was subnormal in 2.4%, 61%, and 29% by the Nichols, DPC, and Amerlex methods, respectively. The DPC, Amersham Amerlex, and Abbott IMx methods gave significantly lower FT4 values for the NTI group than for the controls. There were good correlations between the various FT4 methods in the NTI group. FTI concentrations correlated well with FT4 for the euthyroid and hypothyroid groups but poorly for the NTI group. The Abbott IMx and Nichols RIA methods yield values in the hypothyroid range for only a small proportion of NTI patients.