T3 Thyrotoxicosis: Thyrotoxicosis Due to Elevated Serum Triiodothyronine Levels

Abstract

Thyrotoxicosis may be caused by elevated serum triiodothyronine (T3) in the presence of normal thyroxine (T4) concentration. Of a series of 26 selected cases, 12 have been observed in patients with toxic nodular goiter, most typically with a solitary hyperfunctioning nodule, but also with multinodular goiters. The disorder has also been observed in seven patients with classical toxic diffuse goiter (Graves' disease) including two patients with typical infiltrative ophthalmopathy. The diagnosis of T3 thyrotoxicosis should be suspected when there are clinics and laboratory evidences of hypermetabolism in the presence of normal serum T4 and the absence of thyroxine-binding globulin (TBG) abnormalities. Until T3 measurement becomes more readily available, it is advisable to employ screening with the T3 suppression test, regardless of the initial 24-hour thyroidal radioactive iodine uptake value.