Development of Chronic Thyroiditis During Cyclosporin A Treatment.

Cyclosporin A (CsA) is a calcineurin inhibitor used as an immunosuppressant. Although CsA effectively suppresses T cells, excessive suppression of regulatory T cells may exacerbate autoimmune diseases. Here, we report a case of chronic thyroiditis developing during CsA treatment. A 64-year-old woman, on CsA for 2 years for aplastic anemia, presented with a nodule in the right thyroid lobe, raising concern for malignant lymphoma. Right hemithyroidectomy confirmed mucosa-associated lymphoid tissue lymphoma without chronic thyroiditis in the adjacent normal tissue. Owing to the localized lesion, the patient was monitored with a reduced dose of CsA. Initial thyroid ultrasonography showed a normal left lobe; however, hypoechoic areas appeared 1-year postsurgery, followed by diffuse thyroid enlargement and further expansion of these hypoechoic areas. Postoperative fluorodeoxyglucose positron emission tomography showed progressive uptake in the left lobe, and thyroid autoantibodies, initially negative, became positive. Five years later, suspected lymphoma recurrence prompted a residual thyroidectomy, which confirmed mucosa-associated lymphoid tissue lymphoma with chronic thyroiditis. This case suggests that excessive suppression of regulatory T cells by CsA may induce chronic thyroiditis. Further studies on chronic thyroiditis in patients treated with CsA may enhance our understanding of its pathogenesis.