Serum levels of parathyroid hormone (PTH) and calcium were studied in 132 patients, divided into three groups: unilateral thyroid-lobectomy, (sub)total thyroidectomy for non-toxic goitre and (sub)total thyroidectomy for hyperthyroidism. It was found that all types of thyroidectomy, even unilateral lobectomy, had a temporary influence on the PTH and calcium metabolism in the early postoperative period, albeit to different degrees. The effect was most pronounced after surgery for hyperthyroidism, which carries the greatest risk for postoperative hypocalcemia. The pathogenesis of transient postoperative hypocalcemia as well as the implications for the surgical management are discussed.