Nasopharyngeal cancer is an abnormal cell growth that develops around the nasopharynx. Treatment of nasopharyngeal cancer patients includes chemotherapy or radiotherapy. Both treatments have side effects in patients. In this study, we will focus on hypothyroidism as a side effect of radiotherapy in the treatment of patients with nasopharyngeal cancer. Hypothyroidism is a condition when the thyroid gland is unable to produce enough thyroid hormone. The main goal of this study is to identify the factors associated with hypothyroidism. To achieve this goal, classification tree and logistic regression methods will be used. Classification tree is used to obtain important variables in the classification of subject classes. Then, logistic regression is used to quantify the risk of variables that appear in the classification tree, hypothyroidism risk factors, and hypothyroidism marker factors. Based on the analysis, it was found that the factors associated in this study were variable symptom, physical sign, smoking habits, gender, age, BMI (Body Mass Index), TSH (Thyroid Stimulating Hormone) and fT4 (free thyroxine) hormones, and also all items on Zulewski score, except items delayed ankle reflex and slow movements. These factors associated tended to increase the risk of hypothyroidism, except for the fT4 hormone and BMI. © 2021 Journal of Physics: Conference Series.