Subacute Thyroiditis with Thyroid-Stimulation Blocking Antibodies: A Case Report

「Abstract」. A 43-year-old female showing signs and symptoms of subacute thyroiditis was referred to our hospital for further evaluation of her thyroid function because TSH-binding inhibitor immunoglobulins (TBII) were detected in her serum. A diagnosis of subacute thyroiditis was made based on high levels of thyroid hormones with suppressed TSH level, suppressed radioactive iodine uptake (RAIU), and positive inflammatory findings and ultrasonogram results. She was at first thyrotoxic, then transiently hypothyroid, and finally became euthyroid. The assay results for TBII and thyroid-stimulation blocking antibodies (TSBAb) were positive in the thyrotoxic phase, and the TBII and TSBAb activities were decreased and finally became undetectable during the course of her illness. In contrast, the assay results for thyroid-stimulating antibodies (TSAb) were negative throughout the investigation period. The presence of TSBAb may have induced the development of hypothyroidism because she became euthyroid after the disappearance of TSBAb from the serum, but the duration of the hypothyroid phase was the same as that in typical cases of subacute thyroiditis. Inflammatory or destructive changes in the thyroid gland are thought to be involved in the production of the TSH-receptor antibodies in the patient.