Low titer, competitive insulin autoantibodies are spontaneously produced in autoimmune diseases of the thyroid

The occurrence of multiple endocrine autoimmunity with organ-specific autoantibodies is well known. In this study we evaluated the presence of competitive insulin autoantibodies (IAA) in immune and non-immune diseases of the thyroid, utilizing a sensitive and specific radiobinding assay. We studied 37 patients with Graves' disease, 44 patients with Hashimoto's thyroiditis, 11 patients with non-immune thyroid diseases and 30 normal controls. In 5/37 (13.5%), 7/44 (15.9%) patients with Graves' and Hashimoto's diseases, respectively, but in none of those with non-immune thyroid disease or of the controls, IAA levels exceeded our upper limit of normal range (50 nunits/ml) (P < 0.01). Positive IAA levels ranged between 50 and 123 nunits/ml with fluctuation of these levels over time. Islet cell antibodies were not detected in any of the patients and the controls in the study. No association was found between propylthiouracile treatment and level of IAA. In none of 10 IAA-positive patients was the early phase insulin secretion of the intravenous glucose tolerance test below 46 mu units/ml, and in 2 subjects repeated tests after 3 years showed conserved insulin secretion. In conclusion, our findings show that 15% of patients with autoimmune thyroid diseases, produce specific IAA which do not seem to reflect aggressive beta cell destruction.