Circadian variation in ionized calcium and intact parathyroid hormone: evidence for sex differences in calcium homeostasis.

The rate of bone loss with age and the incidence of osteoporosis are greater in women than men, which led us to question whether subtle sex differences may occur in the circadian variation of serum ionized calcium (iCa) and PTH. We measured iCa hourly and intact PTH every 2 h for 26 h in 25 women (21-69 yr) and 24 men (20-67 yr) consuming self-selected diets. Urine was collected at 0800-1600, 1600-2400, and 2400-0800 h. Serum iCa levels followed a circadian rhythm in both sexes (P less than or equal to 0.01), and the patterns differed between sexes, notably during early morning, when serum iCa levels were lower in women (P = 0.02). Urinary calcium excretion and fractional excretion of calcium declined in both sexes at night (2400-0800 h), but the decline in men was significantly greater (P = 0.02). Similarly, the percent reduction in urinary calcium excretion at night was greater in men than in women (34% vs. 17%; P less than or equal to 0.05). In women, 26-h mean serum iCa values correlated significantly with total daily calcium intake (r = 0.44; P = 0.03). Serum intact PTH levels showed a significant circadian pattern in both sexes (P less than or equal to 0.001). The patterns of serum intact PTH differed between the sexes (P = 0.05), with an earlier and greater increase at night in men. This blunted nocturnal rise in PTH in women may explain the poor nocturnal adaptation to fasting found in women who, despite lower calcium intake, did not reduce urinary calcium loss at night as effectively as men.