Fluoride therapy in osteoporosis: acute effects on parathyroid and mineral homoeostasis

1. Acute metabolic effects of sodium fluoride therapy were analysed among 41 osteoporotic patients already receiving large calcium supplements, 33 of whom underwent simultaneous metabolic balance studies. 2. Mean serum calcium fell transiently within 24-48 h by 0.03 +/- 0.07 (SD) mmol/l (P less than 0.01) and phosphorus by 0.06 +/- 0.08 (SD) mmol/l (P less than 0.001). In a subgroup, ionized calcium fell and biologically active parathyroid hormone (bio-PTH) rose more than fivefold (P less than 0.01). Urine calcium rose after an insignificant fall. 3. Pretreatment calcium and phosphorus balances were significantly positive and did not change overall during the first 8 days of treatment. However, on analysing balances in two groups relative to serum changes, in patients whose serum levels changed least sodium fluoride increased faecal calcium (P less than 0.025) and phosphorus (P less than 0.01) and reduced calcium balance (P less than 0.01), giving a mean balance difference between the two groups of 2.1 mmol daily (P less than 0.001). 4. Very small changes in serum levels therefore indicate well-marked metabolic responses: sodium fluoride acutely stimulates bio-PTH activity and must also enhance mineral uptake from circulation into tissue(s). By separate and opposing action(s) it inhibits intestinal calcium and phosphorus absorption, predominantly in those whose serum levels remain stable. All these effects may be relevant to long-term therapeutic results.