Free thyroxine estimates in nonthyroidal illness: comparison of eight methods.

Eight methods for estimating free serum T4 were compared for 26 patients with nonthyroidal illness (NTI) and 16 hypothyroid patients with comparable total T4 (TT4) concentrations. Free T4 values were determined by equilibrium dialysis, enzyme immunoassay (Abbott), antibody-coated tube (Clinical Assays), antibody-coated microfine silica (Corning Immunophase), microencapsulated antibody (Damon), and free T4 index using the T3 uptake ratio or thyroxine-binding globulin method. Equilibrium dialysis, Clinical Assays and Abbott methods usually provided free T4 estimates in the normal range in NTI patients with low TT4 values and differentiated them from hypothyroid patients with comparable TT4 levels. In contrast, the other methods gave decreased free T4 estimates in the low TT4-NTI groups and often did not distinguish them from hypothyroid patients. The normal free T4 estimates by equilibrium dialysis, Clinical Assays, and Abbott methods in the low TT4-NTI patients are consistent with the previous findings of normal T4 disposal rates in these patients. These three methods may assist the clinician in differentiating the low T4 state of NTI from overt thyroxine deficiency of hypothyroidism.