Simple and sensitive method for estimation of antithyroid plasma membrane antibodies in the serum of patients with autoimmune thyroid diseases; comparison with other assays.

The ability of protein A from Staphylococcus aureus to interact with Fc fragments of IgG was used to estimate the antithyroid plasma membrane antibodies in sera of patients with Graves' disease. The results were expressed as an antithyroid plasma membrane antibodies (ATMA) index. The ATMA index estimated in 60 healthy blood donors varied from 0.57 to 1.28, with a mean value of 0.99, SD theta 0.20. The ATMA index in hyperthyroid untreated Graves' disease varied from 1.80 to 8.0, with a mean value of 4.7. Autoantibody binding to thyroid plasma membranes could be inhibited by (Fab)2 fragments obtained from the serum of patients with Graves' disease but not by (Fab)2 fragments obtained from the serum of healthy blood donors. The influence of rabbit antithyroglobulin and antimicrosomal antibodies on the ATMA index estimation has been evaluated. The ATMA index estimation was compared with the thyrotrophin binding inhibiting immunoglobulins (TBII) index and with the adenyl cyclase stimulating activity of immunoglobulins obtained from 92 hyperthyroid Graves' patients. The ATMA index was positive in 97%, the TBII index in 62% and TSI in 35% of cases. This method using protein A could also be used for estimation of ATMA in other autoimmune thyroid disorders.