A 58-year-old man presented with hyperthyroidism. By palpation, the left lobe of the thyroid was diffusely enlarged, and the right lobe was normal. Radioactive iodide scintiscan demonstrated homogeneous uptake that was localized to the enlarged left lobe, with near total suppression of uptake on the right. TSH stimulation led to clear visualization of a normal appearing right lobe, findings most consistent with an autonomously functioning solitary nodule on the left. Left hemithyroidectomy cured the hyperthyroidism. However, no single dominant nodule was found in the left lobe. Rather, there was diffuse thyroidal hyperplasia of the micronodular variety, consistent with multinodular toxic goiter. Thus, in this patient with a usually diffuse form of thyroid disease, the autonomously functioning hyperactive follicles were localized predominantly to one thyroid lobe. This variant expands the clinical spectrum of multinodular toxic goiter, and further emphasizes the extent of asymmetry in the distribution of hyperactive follicles than can occur in this disorder.