To the Editor.
—In the September 1984 Archives, Cobler et al1 hypothesized that hyperthyroidism would be an infrequent cause of atrial fibrillation in older persons because atherosclerotic, hypertensive, and degenerative heart disease are more frequent with aging.They reported a prevalence rate of atrial fibrillation in 316 patients of 7.9%. Their reported prevalence rate of hyperthyroidism in the presence of atrial fibrillation in women was 31% and 11% in men. Only in women was this prevalence significantly higher compared with their patients who had atrial fibrillation and normal levels of thyroxine measured by radioimmunoassay (T4RIA) or no history of hyperthyroidism.Their reported high prevalence rate of hyperthyroidism and atrial fibrillation in women led us to do the very same study. Our chronic care institution has 512 residents with an average age of 85 years. Since May 1981, all new admissions had T4 (RIA) measurements. From May