Hyperthyroidism and hypothyroidism in monozygotic twins: detection of stimulating and blocking THS receptor antibodies using the FRTL5-cell line.

We present a rare history of monozygotic twin girls who at the age of 10 yr presented with hyper- and hypothyroidism, respectively. Both girls had antibodies against thyrotropin receptors as measured by a radioreceptor assay (230 U/l and 1400 U/l; reference range less than 7 U/l). The sera were further examined in a bioassay, based on a rat thyroid cell line (FRTL5), in which release of cAMP into medium was measured. The TSH receptor antibodies of the hyperthyroid twin displayed a ratio of radioreceptor to biological activity typical of Graves' disease. In contrast, the antibodies of the hypothyroid twin acted as pure antagonists at the TSH receptor level. The occurrence of TSH receptor antibodies in these monozygotic twins suggests a similar pathogenesis of Graves' disease and that of hypothyroidism due to thyroid blocking antibodies.