Purpose:Despite routine supplementation of calcium and vitamin D pre- and post-thyroidectomy, there is a high percentage of persistently hypocalcaemic patients. There is also a trend of low magnesium levels post thyroidectomy, the magnitude of is greater, in patients developing hypocalcaemia suggesting a strong relationship between magnesium levels and hypocalcaemia. Methods:Eligible patients (n =78) undergoing total thyroidectomy for thyroid cancer were randomly assigned to receive oral supplements of 1,500 mg/day elemental calcium, 1 mcg/day alfacalcidol with or without 340 mg elemental magnesium for 2 weeks.. Serum ionized calcium levels and symptoms of hypocalcaemia were followed for 28 days as well as serum magnesium levels. Results:The percentage of symptomatic hypocalcaemia was higher in the control group without statistical significance (p value=0.525). The Mg level was significantly different between groups at days 7 and 14 postoperative only but there was no significant difference in the ionized calcium levels at days 1, 2, 3, 7, 14 and 28 between both groups. The mean ionized Ca2+ of the study group at day 28 post -operative was statistically significantly lower than at day 14 post-operative. Conclusion:Magnesium replacement therapy resulted in a non-significant reduction in the incidence and duration of symptomatic hypocalcaemia post total thyroidectomy..