Hyperthyroidism Due to Functioning Metastatic Bone Lesions of Follicular Thyroid Carcinoma Treated With Lenvatinib.

A 71-year-old woman was diagnosed with unresectable metastatic follicular thyroid carcinoma (FTC) and thyrotoxicosis. She was negative for the presence of thyroxine receptor antibody and thyroid-stimulating antibody. Whole-body scintigraphy revealed increased ^99mTc-pertechnetate uptake in metastatic bone lesions but not in the thyroid nodule. Since radioactive iodine therapy was not applicable because the canalis vertebralis had been invaded, treatment with lenvatinib was initiated, along with methimazole and potassium iodide. The serum level of thyroid hormone decreased. The patient developed hypothyroidism, which continued after the methimazole was stopped, suggesting that lenvatinib suppressed the hyperthyroidism. To our best knowledge, this is the first report of a patient with functioning bone lesions of metastatic FTC in whom hyperthyroidism was controlled by lenvatinib without radioactive iodine therapy.