Non-thyroid Malignancies With Metastases to the Thyroid Gland: A Case Series and Review of the Literature.

Metastasis of non-thyroid malignancies (NTMs) to the thyroid gland is an uncommon phenomenon. The most frequently reported primary tumors include renal cell carcinoma (RCC), lung cancer, colorectal cancer, breast carcinoma, and melanoma. We present a case series of four patients with tumor-to-tumor metastases involving the thyroid gland. Primary tumors included colon adenocarcinoma, clear cell RCC, and malignant melanoma. Each case was evaluated for imaging findings and clinicopathologic features, treatment, and outcomes. In all cases, the metastases occurred at a subsequent time following the primary cancer diagnosis. The average time to thyroid metastasis was 41.5 months for colorectal cancer, 68 months for RCC, and 20.9 months for melanoma. Imaging characteristics such as hypoechogenicity, irregular margins, increased vascularity, and fluorodeoxyglucose avidity were present and consistent with malignant features in all cases. Subsequent pathology analysis was essential for accurate diagnosis. Treatment consisted of hemi or total thyroidectomy in all cases, followed by thyroid hormone replacement therapy in three out of the four patients. Although rare, metastases to the thyroid gland should be considered in patients with a history of malignancy who present with suspicious thyroid nodules. This case series demonstrates the diagnostic challenges and emphasizes the importance of radiologic vigilance and confirmatory histopathology in detecting and managing thyroid metastases from non-thyroid primaries.