The Influence of Fatty Acids on Serum Thyroxine Determination by Competitive Protein-Binding Radioassay

Abstract

Thyroxine concentration, determined by competitive protein-binding radio-assay (T₄(D)), increased in serum stored at ²20°C and 37°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 postprandial 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.

Key Words:

• Fatty acids
• nonesterified
• lipolytic activity
• serum
• radioligand assay
• storage
• serum
• thyroxine