Familial Nonautoimmune Hyperthyroidism Due to a TSH Receptor Gene Pathogenic Variant.

Familial nonautoimmune hyperthyroidism (NAH) is a rare type of autosomal dominant hyperthyroidism caused by constitutively active pathogenic variants of the thyrotropin receptor () gene. Although affected family members present with varied levels of hyperthyroid features, even when the same pathogenic variant is present, total thyroidectomy followed by radioiodine therapy is recommended for long-term management. Herein, we present the case of an 18-year-old proband and her family members with NAH (TSHR-I640V), who presented with diverse thyroid dysfunctions: fluctuations between euthyroid and subclinical hyperthyroidism, mild hyperthyroidism, and overt hyperthyroidism. Almost all affected adult family members, except the proband, showed no progression of hyperthyroidism or thyroid enlargement. A family history of thyrotropin receptor antibodies (TRAb)-negative hyperthyroidism is important for the identification of NAH in adults before genetic testing can be performed. Ablative therapy is not necessary when familial NAH presents with late-onset mild hyperthyroidism without coexisting diseases.