Well-differentiated thyroid cancers (WDTC) are relatively a common type of thyroid cancer. They are slow growing and usually have a good prognosis with timely and the right treatment. There are three types of WDTC: papillary thyroid carcinoma, follicular thyroid carcinoma and Hürthle cell carcinoma. The lymph nodes are the most common sites for metastasis followed by the brain and bones. A systematic review was conducted to examine the uncommon sites of metastasis of well-differentiated thyroid cancers. MEDLINE, CINAHL, Cochrane Library, PubMed, Web of Science, Scopus, NCBI/PUBMED, Google Scholar were searched for articles reporting unusual sites of metastasis (<2% frequency) of WDTC. Most of these metastases are diagnosed late resulting in poor prognosis. The studies that reported common metastatic sites were not included in the review. A total of 654 articles were searched and after screening, 105 articles were included. The following unusual sites of metastasis were found: choroid plexus, retropharyngeal node metastases, ovaries, kidneys, breast, pancreas, choroid and orbit, pituitary, cardiac and the testis. Metastasis to retropharyngeal lymph nodes is rare, even though lymph nodes are a common metstatic sites. Spread to pancreas is often asymptomatic or may manifest as abdominal pain. 

Careful clinical diagnosis aided by advanced imaging techniques and immune-histopathological assays, can help to diagnose the type of tumor cells in host organs, which is essential to plan treatment strategies. Knowledge of the unusual sites of metastasis will help to recognise early clinical signs for early intervention. 

Alqahtani A. Int J Surg Res Pract 2021;8:122.