Anti-thyroxine antibodies in autoimmune thyroiditis--effect on radioimmunoassay and binding characteristics.

Antibodies against thyroxine (T4) were detected in a patient of systemic lupus erythematosus associated with chronic thyroiditis and a patient with primary myxedema. Both patients were clincally hypothyroid with elevated serum TSH. Serum T4 values measured with solid phase radioimmunoassay were very high (over 25ug/dl), but were undetected with polyethylene glycol separation method. On the extraction of the sera with ethanol, the concordant low values of T4 were obtained with both radioimmunoassay methods and these values were compatible with clinical findings and serum TSH levels. Major portion of 125I-T4 binding of sera was found in 7S fraction on Sephadex G-200 column chromatography and T4-binding protein migrated to the gamma-globulin region on cellulose acetate electrophoresis. The 125I-T4 binding of IgGs were competitively inhibited by the addition of unlabelled T4 in both patients. In Case 1, the association constant (Ka) for binding to T4 was 6.1 ×108 l/mol and the binding capacity was 4.8ng T4/mg IgG. The anti-T4 antibody of Case 1 cross reacted with T3 and resulted in falsely high or low T3 values with radioimmunoassay. Ka and the binding capacity of case 2 were 9.2×109 l/mol and 0.11ng T4/mg IgG respectively. The clinical significance of these antibodies was discussed.