THE EFFECTS OF MENOPAUSE AND AGE ON CALCITROPIC HORMONES - A CROSS-SECTIONAL STUDY OF 655 HEALTHY WOMEN AGED 35 TO 90

Although women lose 30% of their skeletal mass after the menopause, th e mechanism of this loss is uncertain, Clearly estrogen deficiency is important but whether this works only through direct effects on the sk eleton is uncertain, To examine these mechanisms further we have evalu ated calcium-related metabolic factors in 655 healthy women, Fasting b lood samples were collected from all subjects who were up to 35 years past the menopause, and fasting urine and 24-h urine samples were coll ected in 365 women who were up to 25 years past the menopause, In the first 15 years postmenopause, there was a rise in total plasma calcium due to a rise in albumin, Bone resorption (hydroxyproline creatinine ratio), bone formation (alkaline phosphatase), and the urine calcium c reatinine ratio all rose at menopause and remained elevated for the ne xt 25 years, There was a transient further rise in bone resorption for the 10 years following menopause, Neither PTH nor the free calcitriol index changed for the first 10 years following menopause, Ten years p ast the menopause, although total calcitriol rose, the free calcitriol index fell due to a rise in vitamin D binding protein, PTH began to r ise at 15 years past menopause, GFR fell gradually over the 25 years f ollowing menopause, Thus following menopause there is an increase in b one turnover and increased urine calcium loss independent of any effec t of PTH or calcitriol, suggesting a direct effect of estrogen deficie ncy on bone and kidney, At 10 years postmenopause, the free calcitriol index fell and PTH began to rise, these changes being associated with persistent elevation of bone resorption perhaps due to increased PTH action on the bone, The cause of the fall in the free calcitriol ratio may be due to progressive renal deterioration and phosphate retention , The cause of the rise in PTH may be due to a negative external calci um balance due to a reduction in gut calcium absorption, a consequence of relative deficiency of 25 hydroxyvitamin D and the fall in calcitr iol together with persistent renal calcium loss.