Evaluation of a standardized short-time calcium suppression test in healthy subjects: interest for the diagnosis of primary hyperparathyroidism.

Objective: The diagnosis of primary hyperparathyroidism (PHP) can be difficult in patients with normal plasma calcium or parathyroid hormone (PTH) levels. We perfected a standardized short-time i.v. calcium loading test in healthy controls (HC) and compared the results with those of patients with PHP.

Methods: Sixteen HC received 0.33 mmol/kg calcium gluconate intravenously for 3 h. Plasma calcium and serum PTH levels (assayed with immunoluminescent sandwich methods) were measured before, at the end of the infusion and 3 h later. Results were compared with those of 16 PHP patients.

Results: In HC, basal total plasma calcium (mean +/- s.e.m.) was 2.33 +/- 0.02 mmol/l. At the end of calcium loading, calcemia reached 3.21 +/- 0.05 mmol/l and decreased to 2.94 +/- 0.08 mmol/l 3 h later. In PHP patients, basal plasma calcium was 2.54 +/- 0.03 mmol/l and reached similar values as in HC during the testing. Basal serum PTH levels were 32.5 +/- 3.3 ng/l in HC and 86.9 +/- 6.3 ng/l in PHP. At the end of calcium loading, they dropped to 8.8 +/- 0.6 ng/l (HC) and to 31.4 +/- 4.2 ng/l (PHP). Three hours later, they were 11.6 +/- 0.8 and 39.8 +/- 4.0 ng/l respectively. There was a cut-off in serum PTH values between the two groups at the end of calcium loading and 3 h later.

Conclusion: The standardized short-time PTH suppression test appears reliable to differentiate healthy subjects from PHP whose serum PTH levels remain >14 and >23 ng/ml respectively at the end of loading and 3 h later. This well-tolerated and easily performed test could be used for the diagnosis of PHP in patients suspected for the disease despite the normality of some basal biological markers.