Normal PTH levels in primary hyperparathyroidism: still the same disease?

Previous studies have suggested that primary hyperparathyroidism (pHPT) with only normal parathyroid hormone (PTH) levels is a milder, less symptomatic form of pHPT. This study investigates symptoms, laboratory values, imaging, and outcomes of sporadic pHPT patients with normal PTH values.

We reviewed our prospectively collected database of 861 patients with sporadic pHPT who underwent parathyroidectomy between December 1999 and June 2010. Patients with only normal PTH values for 6 months before surgery were compared to a randomized control group of sporadic pHPT patients with elevated PTH, matched 1:2 for age and gender.

Fifty-eight (7%) patients had only normal PTH values within 6 months of surgery. The mean PTH was 55.1 pg/ml in the normal PTH group and 151.3 pg/ml in the control group (n=116). There was no difference in preoperative calcium values, subjective symptoms, bone health, or the frequency of single-gland disease (SGD; 88% vs. 91%) between the two groups, but the normal PTH group had higher preoperative vitamin D values (30.8 vs. 21.4 ng/ml; P<0.001), smaller adenomas (405 vs. 978 mg, P<0.001), and more frequently underwent bilateral neck exploration (57% vs. 49%). There was a trend toward lower sensitivity of preoperative imaging in the normal PTH group.

Patients with pHPT and either elevated or normal PTH levels present with similar symptoms and calcium levels. The majority of patients with normal PTH have SGD, although adenomas are smaller. This may explain why patients with normal PTH values have less sensitive imaging and more frequently require four-gland exploration.