A 41-year-old man had thyrotoxicosis, diffuse goiter, exophthalmos, high titer of antithyroglobulin antibodies but undetectable long-acting thyroid stimulator. Initially, he had both elevated serum thyroxine (T4) and reverse triiodothyronine (rT3) levels, but normal serum triiodothyronine (T3) and free triiodothyronine (FT3) levels and 24-hour radioactive iodine uptake. Observations prior to radioactive iodine therapy uptake. Observations prior to radioactive iodine therapy showed a persistently normal T3 level in spite of development of atrial fibrillation. Iodine excess was not present, nor were any drugs or systemic illnesses that cause preferential monodeiodination of T4 to rT3 instead of T3. The data support the concept that (1) thyroxine is not just a prohormone for triiodothyronine but is metabolically active itself; (2) Graves' disease can be a cause of T4-thyrotoxicosis. We conclude that a normal serum T3 level does not rule out thyrotoxicosis.