Major postoperative adverse events in thyroid or parathyroid surgery are uncommon. Various authors have developed risk indices to identify physical deficits in patients undergoing elective surgery. The literature is less clear about patients undergoing thyroid or parathyroid surgery. In this retrospective cohort study, the authors have reported frailty-related factors associated with short-term postoperative adverse events in patients undergoing thyroid or parathyroid surgery. Major postoperative adverse events included deep neck space infection, pulmonary embolism, acute kidney injury, cerebrovascular accident, coma, myocardial infarction, cardiac arrest, sepsis, septic shock, failure to wean off ventilator, reintubation, blood transfusion, return to the operating room or death within 30 days of surgery. Of 154,895 patients analysed in this study, 2.1% had major postoperative adverse events within 30 days of surgery; older age, male sex, current smoking, dyspnoea, anaemia, leukocytosis, hypoalbuminaemia, functional dependence, recent weight loss, obesity, anticoagulation use, inpatient status and length of surgery were independently associated with major adverse events or death. The authors developed the Cervical Endocrine Surgery Risk Index (CESRI) which outperformed other models to determine major adverse events. It can provide an acceptable measure of postoperative risk that surgeons may use to counsel patients awaiting thyroid or parathyroid surgery.