Familial thyroxine-binding globulin deficiency associated with hyperthyroidism

A 24-year-old woman with familial thyroxine-binding globulin (TBG) deficiency associated with hyperthyroidism is reported. Thyroid-stimulating hormone (TSH)-binding inhibitor immunoglobulin (TBII) was negative, whereas thyroid-stimulating antibody (TSAb) was positive. Serum thyroxine-binding globulin (TBG) levels were extremely low, and remained low even after the normalization of thyroid function with methimazole (MMI) treatment. The serum TBG level of the mother of patient was also below the normal lower limit. Genetic analysis revealed single nucleotide deletion, common among Japanese with complete TBG deficiency (TBG-CD), from the allele-specific amplification of the TBG genes of the patient.