Background/aim: Thyroid cancer is the most common endocrine malignancy. Recently the incidence has been increasing faster compared to other malignancies. Different studies have shown that the incidence of breast cancer in patients followed due to thyroid cancer has increased, and vice versa. The aim of this study was to evaluate the frequency of second primary cancers in the follow-up of patients with thyroid cancer.Materials and methods: In this study, 1196 patients with thyroid cancer were evaluated in the Necmettin Erbakan University Meram Medical School?s Department of Endocrinology between 2004 and 2018. Demographic characteristics and radiological and pathological results of the patients were recorded. The presence of accompanying second malignancies in patients with thyroid cancer was investigated.Results: In our study, 985 (82.4%) women (mean age: 46.1 ± 13.3 years) and 211 (17.6%) men (mean age: 49.9 ± 14.2 years) were evaluated. The median follow-up was 63 months (2?164 months). Of the 1196 patients, 1126 (94.1%) had no additional cancer and 70 (5.9%) patients had a second malignancy. The accompanying second malignancies were breast cancer in 24 (2%) patients, skin cancer in 8 (0.7%) patients, renal cell cancer in 5 (0.4%) patients, lung cancer in 5 (0.4%) patients, colon cancer in 5 (0.4%) patients, lymphoma in 5 (0.4%) patients, endometrial cancer in 4 (0.3%) patients, and 14 cases of other rare types of cancer.Conclusion: In our study, it was found that the most common second primary malignancy in patients with thyroid cancer was breast cancer. However, other cancers (skin cancer, renal cell cancer, lymphoma, and colon, lung, or endometrial cancer) may occur in patients with thyroid cancer.