The influence of fatty acids on serum thyroxine determination by competitive protein-binding radioassay.

Thyroxine concentration, determined by competitive protein-binding radioassay (T4(D)), increased in serum stored at 20 degrees C and 37 degrees C. The increase correlated well with the increase in nonesterified fatty acids (NEFA). In vitro addition of NEFA to serum and in vivo increase in NEFA induced by post-prandial heparin injection resulted in a proportional increase in T4(D) values. Protein-bound iodine and thyroxine concentration, as determined by radioimmunoassay, were not influenced by storage and changes in NEFA concentration. NEFA interfere with T4(D), leading to elevated results.