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 supplementatio n are believed to affect the incidence of the diagnosis of hyperthyroidism. Seasonal variation of the incidence of hyperthyroidism was investigated be fore 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 acc ording to etiology (Graves' disease, autonomous nodular goiter) and grade o f hyperthyroidism (preclinical or clinical form). The so formed groups were divided according to the month of diagnosis into further 12 subgroups. Sig nificant differences in month to month variation were found in the patients suffering from Graves' disease and autonomous nodular goiter with preclini cal hyperthyroidism after increased iodine supplementation. The peak incide nce of the diagnosis occurred in June, July, and August. Our results indica te that improved iodine supplementation may be the main cause of seasonal v ariations in the incidence of thyrotoxicosis.