This study is designed as a prospective non-randomized longitudinal single-center cohort study. It will enroll around 120 patients undergoing total thyroidectomy with data being collected from September 2019 up to December 2019. The hypothesis is that a significant association and cut-off point in PTH levels may be established with regard to postoperativne hypocalcaemia.

Primary outcome measures are presence of hypocalcemia on the first and fifth postoperative day. Secondary outcome measures are the need for calcium supplement therapy during the first five postoperative days and amount of medication given.

Associations between variables will be assessed using Spearman's rho rank correlation coefficient, the Kruskal-Wallis test for independent samples and a logistic regression model to test statistically significant correlations between PTH and serum calcium values as a primary end point.

This study is designed as a prospective non-randomized longitudinal single-center cohort study. It will enroll around 120 patients undergoing total thyroidectomy with data being collected from September 2019 up to December 2019. The study was approved by the Hospital Board of Ethics, according to the Declaration of Helsinki Ethical Principles for Medical Research Involving Human Subjects, adopted by the 18th World Medical Assembly, Helsinki, Finland, June 1964, and as amended most recently by the 64th World Medical Assembly, Fontaleza, Brazil, October 2013. The patients will be eligible if they undergo total thyroidectomy regardless of the surgical indication, if complete serum PTH and calcium data are available through the first five postoperative days and if they sign an informed consent form. Patients with incomplete data, preoperative pathological calcium or PTH levels, or suffering from conditions affecting calcium metabolism and parathyroid function will be excluded from the study.

Demographic and clinical data including age, sex, preoperative and postoperative laboratory values (serum calcium and PTH), neck dissection procedures, and postoperative calcium supplement therapy will be noted. Primary outcome measures are presence of hypocalcemia on the first and fifth postoperative day. Secondary outcome measures are the need for calcium supplement therapy during the first five postoperative days and amount of medication given. Preoperative blood samples for serum PTH and calcium measurements will be obtained after hospital admission. Postoperative serum PTH will be sampled 1 hour after surgery and at 7 am on the first and fifth postoperative day. Serum calcium sampling will be performed daily if a patient has hypocalcemia detected on the first postoperative day. Hypocalcemia is defined as serum calcium levels < 2.00 mmol/L regardless of clinical symptoms present. Normal PTH range is defined by the Department of Laboratory Diagnostics reference range - from 1.6 to 6.9 pmol/L. The recovery of parathyroid function is defined as the return of serum PTH and serum calcium to normal values, requiring no further calcium or vitamin D supplementation. If the patient does not have laboratory or clinical signs of hypocalcemia, calcium supplement therapy will not be administered. Supplement therapy will be administered in patients with laboratory findings confirming hypocalcemia. Supplement therapy consists of either peroral elemental calcium (calcium carbonate, 1-gram unit) or calcitriol (0.5 microgram unit) or both. If postoperative calcium and PTH are normal and there are no symptoms of discomfort, the patient will be discharged on the first or second postoperative day and serum PTH and calcium sampling will be performed on an outpatient basis. If the patient did not receive treatment during hospitalization, no supplements will be prescribed after hospital discharge.

Tested variables will be noted using standard descriptors (arithmetic mean and standard deviation or median). Associations between variables will be assessed using Spearman's rho rank correlation coefficient, the Kruskal-Wallis test for independent samples and a logistic regression model to test statistically significant correlations between PTH and serum calcium values as a primary end point.

Inclusion Criteria:

• The patients were eligible if they underwent total thyroidectomy regardless of the surgical indication, if complete serum PTH and calcium data were available through the first five postoperative days and if they signed an informed consent form.

Exclusion Criteria:

• Patients with incomplete data, preoperative pathological calcium or PTH levels, or suffering from conditions affecting calcium metabolism and parathyroid function were excluded from the study.

Patients undergoing total thyroidectomy regardless of the surgical indication.