Numerous neurological, psychological and cognitive abnormalities have been described in patients with clinically manifested hypothyroidism. Impairments in neuropsychological tests have been found in hypothyroid patients, also significant prolongation of P300 response and diffuse slowing of the background activity have been found. The main task in our study is to appraise the cognitive function in Egyptian adults with hypothyroidism. 30 hypothyroid patients were exposed to history taking, examination, laboratory investigations, MRI, EEG, P300 and neuropsychological assessment. The neuropsychological tests were impaired in hypothyroid patients and P300 latency was delayed. EEG showed slowing in the background activity in some patients concluding that there is cognitive dysfunction in hypothyroid patients.