Thyroid-stimulating antibody in a patient with euthyroid Graves' disease.

We report an 11-year-old girl with euthyroid Graves' disease. She was referred to our clinic because of left exophthalmos without other symptoms suggestive of hyperthyroidism. Her serum concentration of free thyroxine (FT4) and free triiodothyronine (FT3) were normal, but thyroid-stimulating hormone (TSH) was below normal and impaired TSH response to TSH releasing hormone (TRH) was found. Although the sera were positive for anti-TSH receptor antibody (TRAb) and thyroid-stimulating antibody (TSAb), both titers were not as high as usually observed in Graves' disease. Three months later, she developed hyperthyroidism and was treated with propylthiouracil. Within 2 weeks of the initiation of therapy, all symptoms except exophthalmos disappeared, and after 2 months of treatment TRAb was negative though TSAb remained positive. TSAb is therefore a good indicator to use in the diagnosis and follow-up of euthyroid Graves' disease and should be measured in patients with exophthalmos of unknown origin, even in children.