Secondary hyperparathyroidism is an expected consequence of parathyroidectomy for primary hyperparathyroidism: A prospective study

Background. Parathyroidectomy for primary hyperparathyroidism (PHPT) can cause secondary hyperparathyroidism, with increased serum parathyroid hormone (PTH) and normal or low serum calcium concentrations. Methods. A prospective study investigated 78 consecutive patients who underwent exploration for PHPT. Serum intact PTH and total calcium concentrations were measured the evening after operation and ionized Ca⁺⁺ the following morning. These levels were reassayed 1 week later. Results. Before operation, the mean PTH level was 138 ± 15 pg/mL, total calcium concentration was 11.6 ± 0.1 mg/dL, and ionized Ca⁺⁺ concentration was 1.44 ± 0.02 mmol/L. On the night of the operation, the PTH level was 11 ± 2 pg/mL, and the total calcium concentration was 8.9 ± 0.1 mg/dL. Fifty-five patients had hypoparathyroidism, with a PTH level less than 10 pg/mL. The day after the operation, the ionized Ca⁺⁺ level was 1.14 ± 0.01 mmol/L. One week later, PTH, ionized Ca⁺⁺, and total serum calcium concentrations returned to normal levels. In 9 patients (12%), PTH levels were increased (98 ± 16 pg/mL), although ionized Ca⁺⁺ concentrations were normal (1.18 ± 0.02 mmol/L), demonstrating secondary hyperparathyroidism. Risk factors for postoperative secondary hyperparathyroidism included older age, symptomatic hyperparathyroidism, higher preoperative PTH and alkaline phosphatase levels, and lower serum phosphorous levels. In 70% of these patients, PTH levels returned to normal in 3 to 12 months. Conclusions. Secondary hyperparathyroidism occurs in 12% of patients after surgical treatment of PHPT. It is transient, possibly compensating for relative hypocalcemia.