Isolated Triiodothyronine (T3) hyperthyroidism in childhood is rate. The incidence among hyperthyroid children is about 10%. The authors present a case history of a 12 2/12 year old girl with T3-thyrotoxicosis. Cause, diagnosis and therapy are thoroughly discussed. The importance to estimating serum T3 concentration for establishing the diagnosis of T3 hyperthyroidism is emphasized.