Sunitinib-Induced Thyrotoxicosis Followed by Persistent Hypothyroidism with Shrinkage of Thyroid Volume

Sunitinib, a tyrosine kinase inhibitor, has been approved for the treatment of cancers, such as advanced renal cell carcinoma (RCC). On the other hand, sunitinib treatment is known to induce thyroid dysfunction in a substantial proportion of patients treated for advanced RCC; in fact, hypothyroidism is a frequent complication. However, little is known about sunitinib-induced thyrotoxicosis and destructive thyroiditis. Here, we report a patient with RCC who developed transient overt thyrotoxicosis followed by hypothyroidism due to sunitinib treatment. A 58-year-old woman, who had been treated with chronic thyroiditis, was diagnosed as having left RCC with bone metastasis to the rib. The patient underwent resection of the left kidney and the bone metastasis lesion. However, 3 months later, bone metastasis to the rib recurred, and sunitinib treatment was started. At 6 weeks of sunitinib therapy, the patient developed transient thyrotoxicosis, followed by persistent hypothyroidism. In the thyrotoxic phase, the patient was diagnosed as having destructive thyroiditis based on an increased thyroglobulin level, a low radioactive iodine uptake, increased free thyroxine level, and suppressed thyroid-stimulating hormone level. The thyroid volume in the hypothyroid phase was 68% of that in the thyrotoxic phase. In conclusion, the present report suggests that sunitinib-induced persistent hypothyroidism may be a consequence of preceding destructive thyroiditis with transient thyrotoxicosis. The decreased volume of the thyroid during the hypothyroid phase indicates irreversible organ damage in the present patient, thereby resulting in persistent hypothyroidism. Thus, periodic surveillance of thyroid function is mandatory during sunitinib therapy.