Serum Concentration of 1,25-Dihydroxyvitamin D in the Human: Diurnal Variation*

Abstract

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 (P_i) 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 (≤1O%) 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 P_i 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 P_i, a significant negative correlation (r = 0.61; P < 0.001) was found between the circulating concentrations of Ca and Pj.