The causes of transient hypocalcemia after thyroid surgery are not fully understood. In 95 consecutive patients undergoing total thyroidectomy (n = 30), subtotal thyroidectomy (n = 14), or hemithyroidectomy (n = 51), we serially measured total calcium, parathyroid hormone (PTH), and proteins before surgery and 6, 24, 48, 72, and 96 hours after surgery, and we calculated the corresponding ionized calcium levels. In the whole population, there was a statistically significant decrease of PTH, total calcium, and proteins at nearly every time of blood withdrawal, when compared with the preoperative levels. The PTH decreased earlier and total calcium levels were significantly lower after total thyroidectomy than after hemithyroidectomy (at 48, 72, and 96 hours). Ten patients had on 2 occasions serum calcium levels below or equal to 2 mmol/L and were defined as having severe hypocalcemia. Severe hypocalcemia was found in 8 patients after total thyroidectomy, compared with 2 after hemithyroidectomy (p < .05), and was present in 3 of the 5 patients with thyroid carcinoma, compared with 7 of the 90 patients with nonmalignant thyroid diseases (p < .01). Despite careful preservation of the parathyroid glands and their blood supply, thyroidectomy was often followed by transient hypocalcemia, the determinants of which are hypoparathyroidism and hemodilution. No patients had persistent symptoms of hypocalcemia from 2 to 3 months after surgery.