A high prevalence of thyroid hypofunction has been found in bipolar patients. However, the samples used in previous studies included a high percentage of patients in treatment with lithium and carbamazepine. Since the use of these drugs may explain the high prevalence of thyroid disturbances found in bipolar patients, we designed the present study to assess thyroid function in a sample of bipolar patients who had not been treated previously with lithium or carbamazepine. Patients included in the sample met Research Diagnostic Criteria for bipolar affective disorder. Assessment included determination of serum levels for total tyroxine (T4), total triiodothyronine (T3), and thyrotropin both basally and in response to infusion of 500 mg of Protilerin. The rate of thyroid hypofunction in the total sample (9.2%) was considerably lower than that reported in other studies with bipolar patients undergoing lithium therapy. Five patients (9.2%) showed some thyroid hyperfuncion parameter. Our results do not show significant differences in thyroid function indices between long-term and short-term duration of illness, between outpatients and inpatients, between high and low number of episodes, and between rapid- and non-rapid-cycling cases. Comparison between bipolar I and bipolar II patients shows a statistically significant difference in the values of TSH levels, with the bipolar II group having a higher mean value. Our data suggest that thyroid dysfunction is not related to gender, duration of illness, number of episodes, or rapid-cycling course of illness. The higher TRH-stimulated TSH levels in the bipolar II group could be considered a differential biological feature.