Background:The purpose of this study was to investigate the incidence of primary hypothyroidism (HT), as well as any correlation between dosimetric parameters and thyroid dysfunctions after neck radiotherapy (RT) in head and neck cancer (HNC) patients. Materials and methods:This study retrospectively reviewed HNC patients who finished neck RT for at least 12 months and who had available back-up treatment information. Eligible patients further received a single thyroid function test (TFT). Dosimetric parameters of the thyroid glands were retrospectively evaluated in order to detect any correlation between dosevolume parameters and primary HT. Results:We reviewed 1,102 HNC patients. Accordingly, 64 patients were deemed eligible and were included in this study. The median time interval between RT completion and TFT was 21 months (interquartile range, 14-34 months), while 26 patients (40.6%) were diagnosed with HT. The thyroid volume spared from a dose of 50 Gy (VS50 Gy ) was found to be statistically significant and considered an associated factor for developing HT (p=0.047). 1 Furthermore, there was an observable trend indicating a reduction in the risk of HT when VS50 Gy was more than 5 cm 3 (p = 0.052). Conclusion:In our study, VS50 Gy was determined to be a significant predictive parameter of radiation-induced HT.