Variation of Plasma Immunoreactive Parathyroid Hormone and Calcitonin in Normal and Hyperparathyroid Man during Daylight Hours*

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Abstract
Many hormones are secreted episodically; if this were true of PTH and calcitonin (CT), it would obviously affect interpretation of clinical PTH and CT assays. To examine this issue, we carried out frequent sampling of peripheral venous blood (1-min intervals for 1.5 h, then 15-min intervals for 6.5 h) starting at 0800 h in three normal volunteers and five patients with primary hyperparathyroidism. We measured immunoreactive PTH (iPTH) using antiserum CH-12M, which has major immunochemical specificity for the intact PTH molecule and does not bind midregion or carboxyl-terminal PTH fragments. Immunoreactive CT (iCT) was measured using antiserum G-1701 (major specificity for the CT amino acid sequence region 17–21). Intraassay coefficients of variation for all assays were 12.2% or less (median, 9.2% mean, 9.4% range, 5.8–12.2%). There was remarkably little change in plasma iPTH or iCT, either minute to minute or hour to hour. In almost all studies, the variation in plasma hormone concentration was fully accounted for by assay variance (variation of intraassay reference standards was the same or greater than that of samples). If PTH and CT are secreted episodically in normal or hyperparathyroid man, the phenomenon does not produce major fluctuations of peripheral blood iPTH and iCT levels during waking hours. Our results show that the finding of normal serum iPTH in some patients with primary hyperparathyroidism cannot be accounted for by fortuitous sampling at the nadir of rhythmic PTH secretion. Multiple determinations of serum iPTH and iCT over the short term (e.g. 1 day) are unlikely to yield major improvement in diagnostic accuracy.