Aim: to determine the association pre-operative vitamin D3 levels with post-operative hypocalcemia in patients undergoing total thyroidectomy. Material and methods: A Prospective study was conducted in the Department of General Surgery, Patna Medical College and Hospital, Patna, Bihar, India for 17 months. Total 80 patients who underwent total thyroidectomy for benign thyroid disorders were included for this study. Preoperatively, serum calcium and vitamin D3 levels of all patients were measured. Postoperatively, serum calcium was measured at 24 hours, 1st and 4th week. A data of patient demographics, indication for surgery, operative findings, pre- and postoperative biochemical laboratory results and clinical outcomes were collected and analysed. Postoperative calcium concentrations, total length of hospital stay and also on the use of intravenous calcium for the treatment of hypocalcemia and the need for calcium and/or calcitriol supplementation at the time of discharge. Results:80 patients were subjected to total thyroidectomy for benign disease. The mean age was 45.77 years with interquartile range of 53.68-34.89 years. Mean serum albumin and serum PTH (post-operative day 1) levels of study group were estimated as 4.31 (g/dl) and 26.39 (pg/ml) respectively. Patients were divided into two groups, one with lower preoperative vit D levels (group A, 25 hydroxy <30 ng/dl, n=20) and other group with normal preoperative vit D levels (group B, 25 hydroxy ≥30 ng/dl, n=60). The mean preoperative vitamin D concentration was 15.27±3.2 ng/ml, in group A and it was 33.65±10.88 ng/ml in group B patients .Mean preoperative serum calcium levels of the study group were estimated as 7.89 (mg/dl). Mean 24 hour post-operative calcium was 8.06 mg/dl in group A and 8.74 mg/dl in group B.During postoperative course, 24 of 80 (30%) of the study group became symptomatically hypocalceamic, which was statistically significant (p=0.034), 18 of 24 (75%) patients who developed symptomatic hypocalcemia had preoperative vitamin D3 levels between 20-30 ng/dl; another 25% (6 of 24) had preoperative Vitamin D3 levels <20 ng/dl. Post operatively, 24/80 patients developed hypocalceamic symptoms. This included 18 in group A and 6 in group B (p<0.05) which was statistically significant. The mean 24 hour postoperative calcium in group A was 8.06 mg/dl whereas 8.74 mg/dl in group B, that was significantly lower or normal in group A. No difference was found in the postoperative PTH concentrations (34±32 vs. 31±44 pg/ml, respectively, p=0.72).The total length of hospital stay was significantly longer in the group A as compared with the group B (6.7±2.5 vs 3.4±1.7 days,p=0.02). Conclusion: the preoperative serum vitamin D3 estimation seems to predict significantly postoperative occurrence of symptomatic and/or biochemical hypocalcemia following TT. Thus necessary means to supplement vitamin D, preoperatively can curb the incidence of hypocalcemia following TT, thereby reducing associated morbidity.