Episodic Fluctuation in Serum Intact Parathyroid Hormone Concentration in Men

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Abstract
To evaluate the temporal features of physiological fluctuation in serum PTH concentration, we sampled peripheral blood at 4-min intervals for 24 h from five normal men (32.8 yr; range, 26-40 yr) and measured serum PTH levels using a two-site immunoreadiometric assay with the exquisite sensitivity and specificity for human PTH-(1-84) (intact PTH). The resultant 24-h time series of serum intact PTH levels were assessed by contemporary techniques in chronophysiology for rhythmic and episodic peak detection. Cosinor analysis disclosed a significant circadian rhythm in serum intact PTH concentrations in all five men, with the mean circadian amplitude and acrophase of 7.2 .+-. 4.4 ng/L and 2305 .+-. 401 h, respectively (mean .+-. SD; n = 5). No apparent fixed ultradian periodicity was found by autocorrelation and spectral analyses. Evaluation of episodic intact PTH pulsalitity by Cluster analysis revealed 23.0 .+-. 4.4 discrete PTH pulses/24 h (P < 0.01 vs. signal-free noise), which occurred at an interpulse interval of 61.6 .+-. 11.1 min. The average duration of a serum intact PTH peak was 42.8 .+-. 7.3 min, and its mean incremental amplitude was 12.6 .+-. 1.3 ng/L, which corresponded to a 31.8 .+-. 5.2% increase above the preceding nadir. Discrete PTH peaks were separated by nonpulsatile valleys which lasted for 17.9 .+-. 4.4 min. Cross-correlation between the time series of serum intact PTH and whole blood ionized calcium (Ca2+) was at its maximum (-0.5) at concurrent time points in three subjects, while significant positive correlation between serum intact PTH and simultaneous serum inorganic phosphorus concentrations was observed in four of five subjects. There was no apparent correlation between the levels of serum intact PTH and serum magnesium. Our data show that serum levels of intact PTH, the only biologically active form of PTH in the blood, is characterized by a significant circadian periodicity, spontaneous episodic pulsatility with distinct peak properties, and a significant temporal coupling with Ca2+ and inorganic phosphorus concentrations. We conclude that PTH secretion, as judged by the temporal pattern of serum intact PTH levels, is pulsatile in normal men.