Measurement of biologically active intact PTH is essential for accurate clinical assessment. Today most laboratories use commercially available U.S. Food and Drug Administration (FDA)–approved two sites or sandwich immunometric assays (IMAs) on different automated immunoassay platforms. These assays are designed to capture the intact molecule by using well-characterized N-terminal and C-terminal specific monoclonal or polyclonal antibodies, one used as a capture antibody bound to solid support and the other tagged with nonisotopic ligand and used as a signal antibody. These assays improved detection of intact PTH by decreasing the detection of PTH fragments and are called intact PTH assays (I-PTH). Measurement of I-PTH provided better clinical correlation than older N-terminal or C-terminal radioimmunoassays. Most I-PTH IMAs have low detection limit and high reproducibility, facilitating the diagnosis of hypoparathyroidism and differentiating primary hyperparathyroidism from hypercalcemia of malignancy.

If primary hyperparathyroidism is the cause of hypercalcemia, the serum PTH level is usually increased; in general, the elevation is proportional to the degree of hypercalcemia. Primary hyperparathyroidism is unlikely to be the cause if the serum calcium level is substantially increased and the PTH level is in the low to normal range. The PTH assay can also determine whether hypocalcemia is due to hypoparathyroidism or due to a nonparathyroid mechanism such as vitamin D deficiency. In patients with hypoparathyroidism, the serum PTH level is inappropriately low or even normal, despite the presence of hypocalcemia. In hypocalcemia due to nonparathyroid mechanisms, PTH secretion is stimulated (secondary hyperparathyroidism) and thus serum levels are high. In pseudohypoparathyroidism, PTH levels are high but hypocalcemia develops because of resistance to the effects of PTH (see Plate 3-12).

Although these I-PTH assays provide accurate measurement of PTH secretion and show excellent diagnostic sensitivity for primary hyperparathyroidism and of hypercalcemia of malignancy, their performance for management of secondary hyperparathyroidism in patients with renal insufficiency has been questioned owing to the presence of higher levels of a non-[1-84]PTH molecular form. This molecular form of PTH was later identified as an N-terminally truncated segment, the [7-84]PTH peptide. This molecular form accumulates in renal failure and is detected by I-PTH assays (see Plate 3-11). This accounts for the larger portion of I-PTH in patients with renal failure than in normal subjects and contributes to the major proportion of nonsuppressible fraction of I-PTH. This [7-84]PTH fragment is capable of binding to PTH receptor but has no biologic activity. Therefore it competes with I-PTH for receptor binding and serves as a PTH antagonist. In renal failure, a twofold increase in I-PTH is accompanied with about a sevenfold increase in [7-84]PTH fragment, leading to overestimation of I-PTH levels and of PTH-associated osseous abnormalities in uremia. After realizing this shortcoming of the I-PTH IMA assays, the efforts were made to develop the next generation of assays that employ the detection antibody that has specificity for the first four amino acids in the PTH molecule (see Plate 3-11). These assays are called “whole/total PTH assay” as well as “bioactive PTH assays.” The specificity of these assays was confirmed by their inability to detect synthetic PTH fragments lacking one or more N-terminal amino acids. Although there is excellent correlation between these two assays in normal individuals and in patients with primary hyperparathyroidism, PTH concentrations are 40% to 50% lower in patients with end-stage renal disease by “bioactive/whole PTH” assays than those obtained using the I-PTH IMA. In patients with end-stage renal disease, bioactive/whole-PTH may provide more accurate assessment of need for vitamin D and/or calcium treatment.