We report two cases of euthyroid Graves' disease in women who had opht
halmopathy without previous history of hyperthyroidism. Enlargement of
extraocular muscles was observed by magnetic resonance imaging (MRI).
The patients had no thyroid enlargement and their serum concentration
s of free T-4, free T-3, and TSH were normal. The sera were negative f
or antithyroid microsomal and thyrogobulin antibodies, and anti-TSH re
ceptor antibodies measured by radioreceptor assay. T-3 suppression tes
t results were normal. Only thyroid-stimulating antibody (TSAb) measur
ed by sensitive bioassay was positive. These findings indicate that se
nsitive TSAb is the most useful laboratory test in the diagnosis of eu
thyroid Graves' disease.