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Circadian Variation in Ionized Calcium and Intact Parathyroid Hormone: Evidence for Sex Differences in Calcium Homeostasis^*

MONA S. CALVO, RICHARD EASTELL, KENNETH P. OFFORD, ERIK J. BERGSTRALH and MARY F. BURRITT

Division of Endocrinology, Metabolism, and Internal Medicine (M.S.C., R.E.), Sections of Biostatistics (K.P.O., E.J.B.) and Clinical Chemistry (M.F.B.), Mayo Clinic, Mayo Foundation and Mayo Medical School Rochester, Minnesota 55905

Address requests for reprints to: Mary F. Burritt, Ph.D., Mayo Clinic, 200 First Street SW, Rochester, Minnesota 55905.

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 0.01), and the patterns differed between sexes, notably during early morning, when serum iCa levels were lowerin 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 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 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.

^* This work was supported by a grant from the Department of Laboratory Medicine and Pathology Research Committee and Research Grants AG-04875 and RR-585 from the NIH. These data were presented in part at the First Joint Meeting of the International Conference on Calcium Regulating Hormones and the American Society for Bone and Mineral Research, September 1989, Montreal, Quebec, Canada.

Present address: Food and Drug Administration, Division of Nutrition, Clinical Nutrition Branch, HFF-265,200 C Street, Washington, D.C.20204.

Present address: Department of Human Metabolism and Clinical Biochemistry, Medical School, Beech Hill Road, Sheffield, England.

Received March 19, 1990.

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