Seventy-seven patients aged more than 60 years from the iodine deficiency area of southern Lower Saxony with hyperthyroidism, diagnosed by assessment of thyroid parameters, showed a nonspecific disease picture with surprising frequency when compared with younger patients. Predominant characteristics were weight loss (86%), general weakness (78%), tachycardia (74%), reduced appetite (66%), exertional dyspnoea (60%), apathy, lack of initiative or depression (48%), and uncharacteristic abdominal complaints (20%). A goitre was not palpable in 30 patients (39%). Laboratory diagnosis was hampered by relative or absolute TBG deficiency and conversion inhibition in the form of a low-T3 syndrome as evidence of non-thyroidal changes in generalised disorders. Neither thyroxine nor triiodothyronine were increased in ten patients (13%). Only by additional routine estimation of the thyroxine-binding index (T3 in vitro test) as indicator of free binding capacity could the diagnosis be ascertained in these cases. The poor prognosis be ascertained in these cases. The poor prognosis of hyperthyroidism in the older age group necessitates generous use of specific diagnostic laboratory investigations in cases with suspicious symptoms.