A bilateral headache, persisting for weeks, may be due to mild hypothyroidism which is not readily recognized by physical examination. Suggestive features of such masked hypothyroidism are asthenia, sensitivity to cold, slow resting pulse rate, and a moderate hypotension. The basal metabolic rate should be detd. in any patient complaining of persistent headaches, in whom no cause is discernible upon routine history and physical examination. This type of headache is remarkable for its favorable response to thyroid medication.