IgA class and subclass thyroid auto-antibodies in Graves' disease and Hashimoto's thyroiditis.

The reported prevalence of IgA class thyroid antibodies in Hashimoto's thyroiditis is variable and the IgA subclass distribution in unknown, despite recent reports of IgG subclass restriction in the thyroid auto-antibody response. Using an ELISA, IgA class antibodies were found against thyroglobulin (Tg) and microsomes (Mic) in 40-52% of patients with Graves' disease and Hashimoto's thyroiditis, and, against thyroglobulin, they were detected in the absence of IgG antibodies in 10% of the cases. Both IgA1 and IgA2 subclasses were detected in all patients with IgA class antibodies, although a significantly higher proportion of IgA2 relative to IgA1 was found in microsomal compared with thyroglobulin antibodies. In view of the high turnover rate and unique complement-fixing properties of IgA2 antibodies, this class of thyroid auto-antibody may play an important role in determining the response in thyroid auto-immunity.