Radiotherapy is one of the treatments for nasopharyngeal cancer (NPC). However, this treatment might produce an unfavorable effect on the thyroid gland, which eventually results in less production of thyroid hormone. This is condition is known as hypothyroidism. The development of hypothyroidism in each patient with post-radiative NPC differs according to several factors. This study aims to analyze the rate of development of hypothyroidism in post-radiated NPC patients. This aim is achieved by identifying subgroups of patients with different hazard rates of developing hypothyroidism, and further identify factors explaining hypothyroidism in each subgroup. Data on ninety-seven NPC post-radiation patients taken from one of the hospitals in Jakarta were analyzed. Survival tree with the relative risk tree algorithm was proposed to analyze the data. We identified three subgroups of patients with relatively slow, medium, and fast developing of hypothyroidism. For the slow subgroup, 26% of the patients developed hypothyroidism at 150+ weeks post-radiation, while it only took less than 30 weeks for those in fast-growing subgroup; and 70 until 130 weeks for the medium subgroup. We also found that sweat production and Zulewski's total score were the important factors in explaining the development rate of hypothyroidism.