Background: Second malignancies, particularly solid tumors, are an important cause of mortality and serious morbidity among cancer survivors who were successfully cured of their first malignancy. It is suggested that there is an association between a first neoplasm and thyroid cancer, and part of it is attributed to exposure to radiotherapy in the treatment of the first neoplasm. Objective: To determine the relationship of radiotherapy as a risk factor in the appearance of thyroid cancer in patients previously treated for the first neoplasm. Methodology: A retrospective study of cases and controls was carried out; of patients diagnosed with a first malignancy from 2007 to 2017. From this group of patients, those who presented thyroid cancer as the second primary malignancy were defined as cases and those who did not present as controls. The risk factor to evaluate was the exposure or not to radiotherapy, in addition to other variables such as the radiotherapy site and the first neoplasm. Results: An association was found between radiotherapy and thyroid cancer OR 6,580 [95% CI 1,219-4,764], female sex OR 6,188 [95% CI 1,232-7,907], sinus as first neoplasm OR 6,188 [95% CI 1,194- 4,941] and as independent factors radiotherapy at the thoracic level OR 6,298 [95% CI 12,581-15,370] and the first soft tissue neoplasm OR 6,271 [95% CI 1,549-25.37]. The median appearance of thyroid cancer as the second primary was 63.80 months and without significant differences between the time between exposure or not of radiotherapy p = 0.899. Conclusion: There is a possible association between thyroid cancer as a second malignancy and having received radiation therapy to the chest