RESUMO
Thyroid cancer is one of the most common endocrine cancers. The most common histological subtypes are papillary and follicular variants; these are "differentiated thyroid cancers" and are associated with an excellent prognosis. The exact mechanism of thyroid cancer is not known. Several genetic alterations and environmental factors are found to be associated with this cancer. Patients with differentiated thyroid cancer are treated with postoperative radioactive iodine (RAI) therapy to ablate residual thyroid tissue and metastatic micro-foci. It is thought that after RAI, there is an increased risk of secondary malignancies such as lung, renal, and stomach cancer and lymphomas. However, the risk of secondary malignancy is not clear. They may be associated with genetic syndromes, environmental factors, and radiation exposure. The secondary malignancy may be detected incidentally during follow-up or present with signs and symptoms of that malignancy. There is no direct association between second malignancy and radiation exposure in I-131 therapies. We present a case series of five patients treated with high doses of I-131 for the remnant. The patients developed metachronous malignancies later in life.