The hypocalcemia after total thyroidectomy is one of common surgical complications. It may be asymptomatic, but patients can complain of dysfunction around the mouth or extremities, stiffness or convulsion in severe cases. The incidence of transient hypocalcemia was 6.9-46%, and permanent hypocalcemia was reported to be 0.4-33%. It has been reported that the incidence of hypocalcemia after thyroidectomy is high when the difference in blood levels of parathyroid hormone and vitamin D is large before and after surgery.

Therefore, it is a very important task to study the effect of using vitamin D3 (cholecalciferol), which has a better effect on the human body, on the prevention of hypocalcemia after total thyroidectomy with a long follow-up period after surgery. The department of surgery in Seoul National University Hospital intends to analyze the preventive effect "D-mac 30,000 IU" on postoperative hypocalcemia and safety of 'D-mac 30,000 IU" through a prospective randomized clinical trial.

The incidence of postoperative hypocalcemia of the group taking orally taking vitamin D3 (cholecalciferol) before surgery will be compared with that of the group not taking vitamin D3 before surgery. Patients who are enrolled in this RCT are allocated to the case group and the control group. Patients in the case group are taking 30,000 IU of vitamin D3 (cholecalciferol) orally, and patients in the control group are not taking any drugs.

Primary endpoint of this study is to evaluate the incidence of hypocalcemia. And secondary endpoints are to evaluate the recovery duration from postoperative hypocalcemia and the risk factors for postoperative hypocalcemia.

Inclusion Criteria:

• Patients aged 19 to 70 years old
• Patients undergoing total thyroidectomy due to thyroid disease
• Patients who consented to the study and obtained consent for the study

Exclusion Criteria:

• Patients with confirmed hypocalcemia or hypercalcemia before surgery
• Patients receiving calcium or vitamin D treatment before surgery, patients with excessive vitamin D
• Patients with a previous history of parathyroid disease or a history of cervical irradiation
• Patients with confirmed parathyroid comorbidity
• Patients with diseases or conditions that cause hypercalcemia or hypercalciuria (myeloma, bone metastases, or other malignant bone diseases)
• Patients with confirmed renal dysfunction (glomerular filtration rate <60mL/min/1.73m2) before surgery or with a history of chronic renal failure
• Renal stone disease patient, kidney stone disease patient
• Patients with bowel disease that may affect serum vitamin D levels (celiac disease, small intestine resorption disease, small bowel resection history)
• Patients taking drugs that may affect serum calcium or vitamin D levels (anticonvulsants, bisphosphonates, cisplatin, aminoglycosides, diuretics, proton pump inhibitors, glucocorticoid benzodiazepine derivatives, etc.)
• Uncontrolled hypertension, diabetes, and clotting disorders
• Cardiovascular disease (angina pectoris, heart failure, myocardial infarction, coronary artery disease, history of arrhythmia treatment, stroke, transient ischemic attack), and taking medications for arrhythmia
• Drug Abuse and Alcohol Abuse
• Patients who participated in other drug clinical trials within 30 days
• Patients with a history of allergies to drugs
• For women, pregnant and lactating patients
• Patients judged unsuitable by the person in charge of the clinical trial
• Patients with genetic problems such as galactose intolerance, lactose deficiency, or glucose-galactose malabsorption disorder