A Comparison of Methods for Assessing Thyroid Function in Nonthyroidal Illness *

Abstract

We studied various tests of thyroid function in sick patients with nonthyroidal illness (NTI) in order to determine the utility of each test for differentiating these patients from a group with hypothyroidism. We evaluated each test in 22 healthy volunteers who served as controls, 20 patients with hypothyroidism, 14 patients admitted to medical intensive care unit whose serum T₄ was less than 5 μg/dl, 13 patients with chronic liver disease, 32 patients on chronic hemodialysis for renal failure, 13 ambulatory oncology patients receiving chemotherapy, 16 pregnant women, 7 women on estrogens, and 20 hyperthyroid patients. On all samples, we measured serum T₄, the free T₄ index by several methods, free T₄ by equilibrium dialysis, free T₄ calculated from thyronine-binding globulin (TBG) RIA, free T₄ by three commercial kits (Gammacoat, Immophase, and Liquisol), T₄, rT₄, and TSH (by 3 different RIAs). Although all of the methods used for measuring free T₄ (including free T₄ index, free T₄ by dialysis, free T₄ assessed by TBG, and free T₄ assessed by the 3 commercial kits) were excellent for the diagnosis of hypothyroidism, hyperthyroidism, and euthyroidism in the presence of high TBG, none of these methods showed that free T₄ was consistently normal in patients with NTI; with each method, a number of NTI patients had subnormal values. In the NTI groups, free T₄ measured by dialysis and the free T₄ index generally correlated significantly with the commercial free T₄ methods. Serum rT₄ was elevated or normal in NTI patients and low in hypothyroid subjects. Serum TSH provided the most reliable differentiation between patients with primary hypothyroidism and those with NTI and low serum T₄ levels.