In a large cohort study, 6% of patients with primary hyperparathyroidism had PTH levels 50 ng/L or lower.

When hypercalcaemia is attributed to primary hyperparathyroidism, the serum parathyroid hormone (PTH) level is occasionally within the normal range. In these cases, the PTH level is assumed to be ‘inappropriately normal,’ because hypercalcaemia from other causes should suppress PTH to subnormal levels.

In this single-centre study, researchers used a database of 2300 patients who underwent surgery for primary hyperparathyroidism and compared those with preoperative PTH levels of 50 ng/L or lower with those with preoperative PTH levels above 50 ng/L. (Note: Normal PTH range is 12 to 88 ng/L at this institution, but the authors chose a cut-off of 50 for this study.) The following findings were noted:
• 6% of patients had preoperative PTH levels of 50 ng/L or lower
• median PTH levels were 40 ng/L and 107 ng/L in the normal- and elevated-PTH groups, respectively; however, mean preoperative serum calcium was similar in the two groups (about 2.73 mmol/L)
• demographic and clinical characteristics (including prevalence of kidney stones and bone pain) were similar in the two groups
• the two groups had similar cure rates with surgery (97%) and similar mean postoperative serum calcium (2.35 mmol/L)
• compared with patients with elevated-PTH levels, those with normal PTH levels were significantly more likely to have hyperplasia or two adenomas and less likely to have a single adenoma.

Comment: Primary hyperparathyroidism with normal PTH levels is not rare; on average, the clinical presentation is similar to that of patients with high PTH levels. For primary care physicians, the take-home point is that a ‘normal’ PTH level does not exclude primary hyperparathyroidism as the cause of hypercalcaemia. For parathyroid surgeons, the higher prevalence of multiglandular disease in the normal-PTH level group is of obvious relevance.

ALLAN S. BRETT, MD
Clinical Professor of Medicine, University of Colorado School of Medicine, Aurora, USA.

Wang R, et al. Hypercalcemia with a parathyroid hormone level of ≤50 pg/mL: is this primary hyperparathyroidism? Surgery 2023; 173: 154-159.

This summary is taken from the following Journal Watch title: General Medicine