thyrotoxicosis is confirmed by high serum total or free triiodothyronine (t3/ft3) and thyroxine (t4/ft4) and low serum thyrotropin (tsh) concentrations. in endemic areas, a predominant increase of t3 with normal or low t4 and ft4 can be found, referred to as "t3-thyrotoxicosis". one variant, known as "free t3-toxicosis" has recently been described presenting subnormal serum tsh, normal ft4 and total t3 concentrations, but high serum ft3. thyrotoxicosis increases bone turnover, especially resorption, with bone loss and increases fracture rates; involves the cardiovascular system both in systemic hemodynamics and t3-mediated effects on cardiac myocite-specific gene expression. detecting t3- thyrotoxicosis is important because of the high morbidity, increased risk of osteoporosis and arrythmias. thus, the early diagnosis and treatment of the hyperthyroidism can prevent complications. we studied three patients with criteria for t3-toxicosis - one of them with ft3 toxicosis. they live in areas with adequate iodine intake and all three present with diffuse goiter. because the screening for thyroid diseases only measures ft4 and tsh, we suggest that patients with subnormal tsh levels and normal ft4 should be worked up with measurement of total t3. if normal, a ft3 can be obtained to identify ft3 toxicosis. the study of such cases must continue to provide valuable insights into the thyrotoxicosis.