Objectives.To study the frequency of islet cell (ICA) and glutamic acid decarboxylase (GAD‐Ab) antibodies in patients with hyperthyroidism of different types at diagnosis before treatment and in the euthyroid state following treatment.Setting.Department of Endocrinology, Malmö University Hospital, Malmö, Sweden.Subjects and design.Blood samples were collected at diagnosis from 129 hyperthyroid patients, and about 6 months later, from 78 of the patients (euthyroid state). Ninety‐two patients had Graves' disease (69 females and 23 males, median age 49 years, range 17–85 years), and 37 patients had toxic nodular goitre/solitary toxic adenoma (34 females and three males, median age 69 years, range 24–86 years).Interventions.Most patients were treated by radioactive iodine following the first blood sample.Main outcome measures.ICA and GAD‐Ab in serum.Results.At diagnosis of Graves' disease, ICA were detected in two out of 92 (2.2%) patients, two out of 85 (2.4%) without diabetes mellitus and in the euthyroid state in one patient. None of the patients with toxic nodular goitre/solitary toxic adenoma had detectable ICA. At diagnosis of Graves' disease, GAD65‐Ab as well as GAD67‐Ab were detected in 11 out of 85 (13%) patients without diabetes. As many as six out of 11 GAD67‐Ab‐positive patients were GAD65‐Ab negative. In the euthyroid state, GAD65‐Ab were found in six out of 51 (12%) and GAD67‐Ab in eight out of 51 (16%) of the non‐diabetic Graves' disease patients. The frequencies of GAD65‐Ab and GAD67‐Ab in toxic nodular goitre/solitary toxic adenoma, diabetes excluded, were 3 and 0%, respectively, in the hyperthyroid state.Conclusion. The frequency of ICA in patients with hyperthyroidism is not increased as compared to the background population. GAD‐Ab seems to be associated with Graves' disease and not with hyperthyroidism. The presence of GAD67‐Ab in GAD65‐Ab negative sera from patients with Graves' disease indicates autoreacti