We conducted a prospective study of thyroid function in 46 patients with Parkinson disease and 46 age- and sex-matched controls with other neurologic disease. There was no statistical difference in serum thyroxine (T4) and T3 resin uptake (T3U) between the two groups. Neither the duration nor the quantity of L-dopa or carbidopa/L-dopa (Sinemet) therapy influenced these assessments of thyroid function. However, 3 of 46 Parkinson patients were hypothyroid, whereas none of 46 controls was hypothyroid. There was one hyperthyroid individual in each group. Early evaluation of thyroid function in all patients with Parkinson disease is recommended because of the unexpected frequency of hypothyroidism and because hypothyroid symptoms may be masked.