Abstract
BACKGROUND Most thyroid nodules are fortunately benign; however, up to 15% of thyroid nodules turn out to be malignant. Follicular thyroid cancer is the second most common type of thyroid malignancy, originating from the follicular cells lined by cuboidal epithelium, with a peak incidence between the ages of 40 and 60 years. The feature that differentiates follicular carcinoma from adenoma is the presence of capsular invasion. While distant metastases occur in 10% to 15% of patients with follicular thyroid cancer, only 2% to 13% of patients with thyroid cancer develop bone metastases. Metastasis to the knee in thyroid cancer is extremely rare. This report describes a rare case of limping and knee pain as presenting symptoms of metastasis of follicular thyroid cancer to the knee joint observed during clinical practice and addresses its implications. CASE REPORT A 60-year-old woman presented with right knee swelling, disabling pain, and difficulty with walking in the last 3 months. Magnetic resonance imaging showed a large mass, and a computed tomography scan-guided biopsy confirmed it to be a distant metastasis of follicular thyroid cancer. The patient underwent total knee replacement and thyroidectomy and was postoperatively treated with radioactive iodine therapy. She was in good condition at her 1-year follow-up, with no recurring pathology. CONCLUSIONS This case reports on the metastasis of follicular thyroid cancer to the knee, which is exceedingly rare. However, it should be considered in the differential diagnoses of lytic bone lesions, as early diagnosis and management yield a more favorable prognosis for patients.