Clinical Implications of Estimation of Intact Parathyroid Hormone (PTH) Versus Total Immunoreactive PTH in Normal Subjects and Hyperparathyroid Patients*

Abstract

We describe a clinical study comparing the value of measurements of intact human PTH [hPTH(l–84)] and total PTH immunoreactivity [hPTH-(l–84) plus fragments]. A twostep immunochemical method was used to separate plasma hPTH-(l–84) from all circulating PTH fragments. The firststep involved extraction and concentration of plasma PTH using solid phase antiamino-terminal PTH antibodies. After elution, the PTH immunoextract was analyzed using a sensitive midand C-region immunoassay. Complete separation in the imraunoextraction step was proven by Sephadex G-75 gel filtration. hPTH-(l–84) values in fasting patients showed a clear distinction between those with primary hyperparathyroidism and those with nonparathyroid hypercalcemia, in contrast with small overlap in total immunoreactive PTH values. The hPTH-(l–84) values increased faster and more substantially in response to long EDTA and calcium infusion tests, compared with total PTH immunoreactivity, in normal subjects. Infusion of EDTA (10 mg/kg BW) in 5 min) elicited a readily measurable response of hPTH-(l–84) between 5 and 10 min after starting the infusion. Ingestion of 1000 mg calcium caused a decrease in hPTH-(1–84) in 1 h or less. More than 50% of patients with terminal renal failure had normal hPTH-(l–84) values despite elevated total immunoreactive PTH concentrations. We conclude that the two-step hPTH-(l–84) assay is more specific and sensitive than most regional PTH assays. Measurements of hPTH-(l-84) levels may identify disorders of parathyroid function at an early stage and provide a useful tool for the study of parathyroid physiology.