Interference of fatty acids in the competitive protein-binding assay for serum thyroxine

An increase in apparent thyroxine values obtained by competitive protein-binding assay on storage of sera is well documented. We find that the major source of this positive bias is probably the unsaturated nonesterified fatty acids. Nonesterified fatty acids cause a positive basis in the competitive protein-binding assay for serum thyroxine because they inhibit the binding of radioactive thyroxine by the serum-binding reagent (probably thyroxine-binding globulin). This inhibition by fatty acids may be due to the formation of a fatty acid/thyroxine complex. The degree of inhibition caused by the fatty acids depends on the length of the carbon chain of the fatty acid and the degree of saturation. Short-chain fatty acids are more potent inhibitors of thyroxine binding than those with a longer chain, and unsaturated fatty acids are more potent inhibitors of thyroxine binding than are saturated fatty acids. The polyunsaturated fatty acid, arachidonic acid, was the most potent inhibitor of all the fatty acids tested. Triglycerides (triacylglycerols) insignificantly inhibit thyroxine binding in this assay.