Normal Free Thyroxine in Critical Nonthyroidal Illnesses Measured by Ultrafiltration of Undiluted Serum and Equilibrium Dialysis

Considerable controversy exists concerning the assessment of thyroidal state in critically ill patients with decreased serum T₄ and T₃ concentrations, in part because serum free T₄ values are often low in such patients no matter what method of measurement is used. We developed an ultrafiltration method to measure free T₄ and free T₃ in undiluted serum and compared the results with those obtained using a standard equilibrium dialysis method to measure free T₄ and T₃. In 30 consecutive intensive care unit (ICU) patients, serum free T₄ values were similar to or higher than those in 12 normal subjects by both methods in most patients and were clearly distinguishable from those in hypothyroid patients. The serum total T₄ concentrations in these patients ranged from 12.9-131.3 nmol/ L (mean, 68.2; normal mean, 115.8). Free T₄ by equilibrium dialysis was highly correlated with free T₄ by ultrafiltration in the ICU group (r = 0.91; P < 0.001). Serum free T₃ levels, however, whether measured by equilibrium dialysis or ultrafiltration, were decreased in the ICU patients, confirming other reports of lowered free T₃ in critically ill clinically euthyroid patients. Our findings suggest that the use of equilibrium dialysis of undiluted serum or ultrafiltration to measure serum free T₄ concentrations will distinguish euthyroid hypothyroxinemic ICU patients from those with hypothyroidism. (J Clin Endocrinol Metab 67: 1031, 1988).