Background The purpose of this study was to evaluate the clinical usefulness of routine oral calcium and vitamin D supplements in the prevention of hypocalcemia after total thyroidectomy. Methods Ninety patients who underwent total thyroidectomy were randomly assigned to routinely receive or not receive a supplement containing oral calcium (3 g/d) and vitamin D (1 g/d) for 2 weeks. Hypocalcemic signs and symptoms, serum calcium, and parathyroid hormone (PTH) levels were monitored and compared between the 2 groups. Results The incidences of symptomatic and laboratory hypocalcemia were significantly lower in the oral calcium/vitamin D group than in the group not receiving the supplement: 3 of 45 patients (7%) versus 11 of 45 (24%) and 6 of 45 (13%) versus 16 of 45 (36%), respectively ( P ≤ .02). The hypocalcemic symptoms were minimal in the supplement group but more severe in the group not receiving the supplement. Serum calcium levels decreased in both groups after surgery but recovered earlier in the supplement group. No hypercalcemia or PTH inhibition developed in the supplement group. Conclusion Routine administration of a supplement containing oral calcium and vitamin D is effective in reducing the incidence and severity of hypocalcemia after total thyroidectomy.