Central nervous system (CNS) dysfunction is an important consequence of thyroid hormone deficiency Several studies using objective behavioral measures reported that cognition is impaired in patients with hypothyroidism. The aim of the present study is to evaluate objectively the functional changes in the nervous system and possible cognitive effects of hypothyroidism by using event related potentials (ERP) in order to determine the frequencies of these changes in patients with hypothyroidism. Forty-two patients (3 males, 39 females) (mean age: 40 +/- 11) with newly diagnosed primary hypothyroidism were included in the study. In all patients the cause of hypothyroidism were chronic autoimmune thyroiditis. None of them had received thyroid hormone as a replacement therapy. In hypothyroid group both N2 and P300 latencies from FZ, CZ, PZ electrode positions were prolonged relative to controls and the differences were statistically significant; respectively (N200: P < 0.003, 0.003, 0.002--P300: P < 0.008, 0.02, 0.001) N100 and P200latencies from FZ, CZ, PZ electrode positions did not differed significantly from those of normal controls; respectively. (p < 0.140, 0.195, 0.153--p < 0.400, 0.185, 0.766) Latency and amplitudes of P300 did not correlated with FT3, FT4 or neither with TSH values in the patient group. We believe like other's that with the aid of this technique we are able to detect early effect on central information processing compatible with a reduction of processing resources and increased effort necessary for a given task in hypothyroid individuals.