Triiodothyronine (T3) toxicity has been well documented in adults, but only 
isolated cases have been reported in children. Two girls presented with firm 
goitres and overt hyperthyroidism. In each patient, total serum thyroxine (T4) 
values by competitive protein binding were normal, however total T3 values by 
radioimmunoassay were elevated. One patient had Graves' disease, the second 
patient had Hashimoto's disease which has been only infrequently associated with 
T3 toxicity in adults. Both patients responded to therapy with propylthiouracil. 
The mechanisms by which T3 is preferentially secreted in thyrotoxic states in 
man are poorly understood, but iodine deficiency and poor iodination of 
thyroglobulin may be important factors.