Factors Influencing the Concentration of Free and Total Thyroxine in Patients with Nonthyroidal Disease

Abstract

The free thyroxine concentration in serum can be considered to be directly proportional to the product of the dialyzable fraction (DF) as determined by equilibrium dialysis and the total concentration of thyroxine based on the serum PBI value. Previous studies from this laboratory have indicated that the DF is increased in sera of patients with a variety of nonthyroidal diseases as a result of a decrease in the concentration of thyroxine-binding prealbumin (TBPA). These observations have been extended. The relationship between TBPA and DF has been examined from both an experimental and theoretical point of view. Of 36 patients with nonthyroidal disease and an increased DF, 16 had elevated free thyroxine concentrations as well. Chromatographic studies suggested that the elevated levels of free thyroxine are not due to a systematic increase in a nonthyroxine component of the serum PBI value in such patients. Twenty patients with elevated DF values had normal concentrations of free thyroxine, presumably because of a compensatory decrease in total thyroxine concentration. Patients in this group may have serum PBI values in the hypothyroid range without clinical evidence of hypo-thyroidism. No correlation was found between free thvroxine concentration and the thyroidal clearance and 24-hr. uptake of I131. Significant increases in free thyroxine were found as the temperature of dialysis was increased from 37 to 40 C. These observations suggest that body temperature may have a direct effect on the in vivo free thyroxine concentration.