The involvement of the central nervous system either in hypothyroidism or in hyperthyroidim has previously been shown on the basis of visual, auditory, somatosensory, and central motor evoked potential studies by some investigators. In to our previous study, we found that abnormal electrodermal activity in nonmedicated hyperthyroid patients was not associated with psychiatric symptoms. In this study, our purpose was to investigate whether hypothyroidism results in electrodermal abnormalities in the absence of measurable psychiatric symptoms. Electrodermal activity was recorded with a skin conductance unit connected to a personal computer. Basal levels of electrodermal activity and responsiveness to repeated acoustic stimulation were studied in 14 nonmedicated hypothyroid patients and 14 healthy controls. Psychiatric rating scores indicated that patients and healthy controls had normal levels of anxiety and depression. Hypothyroid patients had lower skin conductance levels, lower fluctation rates and prolonged onset latencies compared with controls. None of the hypothyroid patients had amplitude changes. In conclusion, hypothyroid patients may have abnormal electrodermal activity that is related to the change of hypothalamic-pituitary-thyroid axis function, without associated psychiatric symptoms.