Hyperthyroxinaemia due to decreased peripheral triiodothyronine production

Two patients, a boy of 8 and a women of 60 years of age, had higher than normal levels of serum total thyroxine (T4), free T4, (FT4), FT4 index, and reverse triiodothyronine, but normal serum triiodothyronine (T3) levels. The pituitary-thyroid axis could be normally stimulated by thyrotropin-releasing hormone, suggesting euthyroidism at the pituitary level. High levels of serum T4-binding globulin decreased during T3 treatment in the boy. Studies show that in these patients a raised serum FT4 is necessary to produce in the peripheral tissues sufficient amounts of T3 for biological action. Two possible mechanisms for a basic defect underlying this newly recognised syndrome are proposed: inhibition of T4 transport into tissue cells and reduced intracellular 5'-deiodinase activity catalysing T4 to T3 conversion.