Temporal Interrelationships Between the Circadian Rhythms of Serum Parathyroid Hormone and Calcium Concentrations

The temporal relationships between the circadian rhythms of serum PTH, total calcium (Ca_t), and phosphate (Pi) and plasma ionized calcium (Ca_i concentrations were determined in 9 normal men. Blood samples were collected every half hour for 24 h. Serum PTH was measured by an RIA specific for the midregion of the molecule. The mean circadian pattern for each variable was derived by calculating the average value across all men at concurrent time points. After the data were smoothed by the method of running means, the correlations between PTH and mineral values from concurrent time points were calculated, as were cross-correlations to 12 lag periods (6 h). Spectral and cross-spectral analyses were performed on the same data set. Both statistical methods yielded consistent results: 1) at concurrent time points (0 lag), high correlations were found between serum PTH and Ca_t (r = -0.74), PTH and Pi (r = 0.79), and PTH and Ca_i (r = -0.53); and 2) when the PTH series was lagged by 2 h, the PTH/Ca_i correlation improved to -0.70. A direct PTH/Ca_i correlation of 0.50 was found when the Ca_i series was lagged about 4.5 h. No improvement in the correlations were found when the other series were lagged. Spectral analyses also detected significant interrelations between PTH and Ca_i at 2 and 3.5 h. These data describe the timing of the bidirectional interaction between serum PTH and plasma Ca_i under steady state conditions in normal adult men; changes in Ca_i concentrations precede inverse changes in PTH levels by 2 h, whereas changes in PTH precede similar directional alterations in Ca_i by about 4 h (J Clin Endocrinol Metab 67: 1068, 1988).