Abstract Background Total thyroidectomy is the treatment of choice for many thyroid diseases. The most common metabolic complications are disorders of calcium ion concentrations. In this study we aimed to determine the incidence of hypomagnesemia after total thyroidectomy and to investigate whether hypomagnesemia after thyroidectomy has any impact on post-thyroidectomy hypocalcemia. Patients and Methods This study was a prospective analytical one done between July 2019 and July 2020 including fifty patients who underwent total or completion thyroidectomy. Our patients were divided according to adjusted serum calcium levels of day one postoperative and clinical manifestations of hypocalcemia into 2 groups, group (A) for normocalcemic patients (clinical and biochemical) and group (B) for hypocalcemic patients (clinical or biochemical). And each group was subdivided into two subgroups according to serum magnesium levels (normomagnesemia and hypomagnesemia). Results These results cleared that, postoperative biochemical hypocalcemia was detected in 36% of cases and hypomagnesemia was detected in 62%, from the hypocalcemic group there were 88.8% with hypomagnesemia while in the normocalcemic group, there were 46.8% with hypomagnesemia. In the postoperative period 24% of patients had manifestations of clinical hypocalcemia; all of these patients (100%) had biochemical hypocalcemia. All of the cases (100%) with symptomatic hypocalcemia had low serum magnesium levels while as for the patients with only biochemical hypocalcemia, 66 % had hypomagnesemia. Conclusion Our results concluded that, regarding the correlation between calcium and magnesium, there was a statistically highly significant positive correlation between calcium and magnesium in the first day postoperative of thyroidectomy. Post-thyroidectomy hypomagnesemia was found to be a good predictor of postoperative biochemical hypocalcemia and is closely related to the clinical manifestations of hypocalcemia. So, correction of both hypomagnesemia and hypocalcemia is necessary for prompt resolution of symptoms of refractory hypocalcemia after total thyroidectomy.