Familial non-autoimmune hyperthyroidism, a rare disorder that results from activating germline mutations in the TSH receptor gene, is inherited in an autosomal dominant fashion and has a variable age at onset. Here, we present a family, five members of which were determined to have non-autoimmune hyperthyroidism. Levels of free T3 and free T4 were high and TSH suppressed in two siblings aged 12 and 16, who were admitted due to failure to gain weight and swollen necks. Thyroid autoantibodies were negative and thyroid ultrasonography demonstrated no nodules. A similar situation was detected in their father and two other siblings, none of whom had remarkable complaints. A novel heterozygous missense mutation (c.1906T>A) in the 10th exon of the TSH receptor gene was found in the affected cases. Treatment with methimazole and propranolol was initiated in all cases. During follow-up, one case underwent total thyroidectomy, and radioactive iodine treatment was administered to another. Gain-of-function germline mutations in the TSH receptor should be considered in cases of hyperthyroidism not associated with autoimmune thyroid disease, and family screening, including asymptomatic individuals, should be done.

Keywords: <i>hipertiroidi, mutasyon, otozomal resesif, metimazol.</i>