Silent thyroiditis with thyroid-stimulation-blocking antibodies (TSBAb).

A 24-year-old man showed thyrotoxic symptoms with hypokalemic periodic paralysis. Serum thyroid hormone levels were high and thyrotropin (TSH) was undetectable. 123I-thyroidal uptake was suppressed. TSH-binding inhibitor immunoglobulin (TBII) was positive. After a month without any treatment, he became hypothyroid. Thyroid hormone level was decreased and TSH was increased to above the normal range. 123I-thyroidal uptake was increased. TBII activity was still positive. From the clinical findings, a diagnosis of silent thyroiditis was made. Sera obtained in the hypothyroid state revealed the presence of thyroid-stimulation-blocking antibodies (TSBAb), but there were no thyroid-stimulating antibodies (TSAb). These results suggest that the hypothyroidism in this patient was due to the presence of TSBAb with TBII activity.