Serum concentration of 1,25-dihydroxyvitamin D in the human: diurnal variation.

The serum concentrations of many hormones exhibit regular circadian rhythms. To determine if the serum concentration of 1,25-dihydroxyvitamin D [1,25-(OH)2D], undergoes periodic changes during the day, we measured the serum concentration of 1,25-(OH)2D every 30 min for 28 h in five normal men. Serum concentrations and urinary excretion rates of calcium (Ca) and inorganic phosphate (Pi) also were measured. The results indicate that the serum concentration of 1,25-(OH)2D does not undergo any large fluctuations during the day, but, rather, is maintained within approximately 20% of its overall 24-h mean. In two of five men, part of the observed daily variation could be attributed to a low amplitude (less than or equal to 10%) circadian rhythm, with the mean acrophase occurring at 1645 h. The previously reported circadian rhythms in serum concentration and urinary excretion rates of Ca and Pi were confirmed. Although the rhythmic changes in serum 1,25-(OH)2D in two subjects were not correlated with the diurnal changes in the serum concentrations of Ca or Pi, a significant negative correlation (r = 0.61; P less than 0.001) was found between the circulating concentrations of Ca and Pi.