The purpose of our study is to verify if PTH assay on the first postoperative day is a reliable early predictor of the onset of hypocalcemia.Between October 1999 and May 2000, a prospective trial involved 162 patients who underwent total or near total thyroidectomy at our institute. On the basis of PTH assay on first day we divided the patients in three groups: group A 28 patients with PTH 16 pg/ml.In group A: 22 of 28 patients (78.5%) developed postoperative hypocalcemia and 20 (71.4%) needed replacement therapy; in group B: 14 of 34 (41.1%) had postoperative hypocalcemia and 10 (29.4%) received treatment; in group C: 23 of 100 (23%) became hypocalcemic after surgery but only 5 (5%) require calcium-vitamin therapy. A statistically significant correlation (p = 0.0017) was identified between post-operative PTH levels and lowest blood calcium values detected after surgery. The correlation between the drop in blood calcium levels after surgery and postoperative PTH (delta Ca) was statistically even more significant (p = 0.0002); the lower the postoperative PTH, the higher the absolute value of the delta Ca.The authors suggest a clinical approach and pharmacological treatment protocol based on the outcome of PTH assay on the first post-operative day; a solution that is only apparently more costly because it in fact aims to ensure a more timely recourse to blood calcium monitoring or replacement therapy and also an earlier discharge of the patient.