Seasonal variations in the diagnosis of hyperthyroidism

Seasonal variations in the frequency of thyrotoxicosis are known for a long time. Both, variations in the temperature and in the iodine supplementation are believed to affect the incidence of the diagnosis of hyperthyroidism. Seasonal variation of the incidence of hyperthyroidism was investigated before and after increase of salt iodination in 1990/1991. In the period from January 1987 to December 1995 hyperthyroidism was diagnosed for the first time on 4711 patients in our department. This population was subdivided according to etiology (Graves' disease, autonomous nodular goiter) and grade of hyperthyroidism (preclinical or clinical form). The so formed groups were divided according to the month of diagnosis into further 12 subgroups. Significant differences in month to month variation were found in the patients suffering from Graves' disease and autonomous nodular goiter with preclinical hyperthyroidism after increased iodine supplementation. The peak incidence of the diagnosis occurred in June, July, and August. Our results indicate that improved iodine supplementation may be the main cause of seasonal variations in the incidence of thyrotoxicosis.