Subacute thyroiditis is an inflammatory disease of the thyroid gland that presents with transient thyrotoxicosis and usually resolves spontaneously. Lymphadenopathy and splenomegaly may be seen in thyrotoxicosis. Our patient reported severe pain on palpation of the thyroid gland and was diagnosed as having subacute thyroiditis. He had a high sedimentation rate and reduced thyroid uptake of 131I. The spleen extended 8 cm below the costal margin. Abdominal Doppler ultrasound examination was unremarkable except for the splenomegaly. Subacute thyroiditis should be kept in mind as a cause of massive splenomegaly.