Free Thyroxine Estimates in Nonthyroidal Illness: Comparison of Eight Methods *

Abstract

Eight methods for estimating free serum T₄ were compared for 26 patients with nonthyroidal illnesses (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 TT_4– 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.