The dilemma of the normal baseline parathyroid hormone level using the intraoperative PTH assay.

OBJECTIVE: To analyze patients with "normal" baseline quick intraoperative parathyroid hormone (QPTH) levels during parathyroidectomy and to determine the prevalence of this finding, the usefulness of the assay in this situation, and to explain the possible causes for this phenomenon. STUDY DESIGN AND SETTING: Patients who underwent parathyroidectomy using QPTH in a tertiary hospital. METHODS: Retrospective analysis of 39 patients treated surgically for primary hyperparathyroidism using QPTH. RESULTS: Of the patients, 14 (36%) had normal baseline QPTH. 8 patients with localizing sestamibi scans had a single adenoma, and excision resulted in a mean decrease of 85.4% in QPTH. Six patients had nonlocalizing sestamibi scans, 1 patient had an 84% drop in QPTH level after removal of a single adenoma, and 5 patients had hyperplasia requiring > or =3 glands excision. At 11.36 months' mean follow-up, 13 patients (93%) were normocalcemic. CONCLUSIONS: A "normal" baseline QPTH level was found in 36% of patients. A 50% decrease in QPTH remains predictive of biochemical cures in patients with localizing sestamibi scans. The likely explanation for this variability in "normal" levels between different assays is the variability in detection of the 7-84 PTH fragment, which results in an overestimation of the PTH level. Assays such as the QPTH, which are more sensitive for the biologically active PTH molecule [(1-84) PTH] than other laboratory PTH assays will tend to have lower PTH levels that can be within the normal range. EBM RATING: B-3.