Intellectual impairment after hyperthyroidism.

Electroencephalography (EEG) and neuropsychological tests empirically shown to be sensitive to diffuse cerebral damage were performed in 26 patients 10 years after successful treatment of hyperthyroidism and in a control group with non-toxic goitre. In the hyperthyroid state 81% had abnormal EEG before treatment, and 10 years after treatment 68% still had abnormal EEG compared with 41% in the control group (P less than 0.05). In 7 out of 11 neuropsychological tests the previously hyperthyroid patients showed significant impairment compared with the control group. Twenty-three per cent of the patients displayed marked to severe intellectual impairment, 31% moderate and 41% slight or no impairment compared with 0%, 31% and 69%, respectively, in the control group (P less than 0.05). Four patients had been granted disability pension on the basis of the intellectual dysfunction. Signs of intellectual impairment indicating irreversible brain dysfunction after thyrotoxicosis thus seem to be a frequent, although hitherto not generally recognized, finding.