Effect of anti-thyroxin autoantibodies on radioimmunoassay of free thyroxin in serum
Konishi, J.; Iida, Y.; Kousaka, T.; Ikekubo, K.; Nakagawa, T.; Torizuka, K.
Clinical Chemistry 28(6): 1389-1391
1982
ISSN/ISBN: 0009-9147 PMID: 6896171 Document Number: 193500
Serum thyroxin was nearly or completely undetectable by radioimmunoassay in an elderly patient with Graves' disease being treated with methimazole. Abnormal binding of thyroxin to antibodies of the IgG variety was shown, the association constant of the complex being 8.0 x 10(8) L/mol and the binding capacity 6.3 nmol/g of IgG. The effect of the antibody on results of radioimmunoassay of free thyroxin was studied with three commercial kits, two of which (Clinical Assays and Damon Diagnostics) gave essentially the same values as did equilibrium dialysis. The third (Amersham International) gave falsely high results because the 125I-labeled thyroxin derivative used in the kit was bound by the autoantibody.