Case report and literature review of synchronous follicular and papillary thyroid carcinoma with metastasis to femur

Background Thyroid carcinoma is common, but the synchronous occurrence of bilateral tumors with discordant histology—specifically follicular thyroid carcinoma (FTC) and papillary thyroid carcinoma (PTC)—is extremely rare. It is even more uncommon for such cases to present initially with distant bone metastasis from an occult FTC, which poses a significant diagnostic challenge. Case Summary: We report a case of a 67-year-old male who presented to the orthopedic department with left hip pain. Imaging revealed bone destruction, and a biopsy confirmed metastatic thyroid carcinoma. Although the patient had no neck symptoms, a subsequent thyroid workup identified bilateral nodules. The patient underwent hip replacement followed by total thyroidectomy. Postoperative pathology confirmed a synchronous left-sided invasive FTC and a right-sided multifocal PTC. The bone metastasis was attributed to the FTC component. The patient was referred for radioactive iodine (I-131) therapy. Conclusion This case highlights that metastatic thyroid cancer should be considered in the differential diagnosis of lytic bone lesions, even in patients without palpable goiters. For rare synchronous bilateral carcinomas, the prognosis is often determined by the more aggressive subtype (FTC), necessitating a multidisciplinary approach involving surgery and systemic therapy.