Parathyroid hormone: what are we measuring and does it matter?

Immunometric assays claiming to determine intact parathyroid hormone (PTH) generally cross-react with N-truncated forms such as PTH(7-84). Laboratories need to examine the relevance of new assays with probable PTH(1-84) specificity. It is logical that assays should measure what they state they do. However, it seems unlikely that use of older 'intact' PTH assays will affect the clinical interpretation of results in primary hyperparathyroidism or vitamin D deficiency. It is plausible that appropriate application of new PTH assays could improve outcome in chronic renal failure. However, it has never been suggested that straightforward replacement of existing assays with new PTH(1-84) assays will lead to this improved outcome. A better understanding of PTH fragments and their interaction with PTH receptors may shed light on the relevance of different PTH assays. In the meantime, older technologies will continue to work well for the vast majority of patients.