Organizations osteoporosis

TABLE 53-3. World Health Organization Definition of Osteoporosis... 

The diagnosis of osteoporosis based on a low-trauma fracture or central hip and/or spine DXA using World Health Organization T-score thresholds. Normal bone mass is a T-score greater than -1, osteopenia is a T-score of-1 to -2.4, and osteoporosis is a T-score at or below -2.5. 

WHO (1994) Assessment of fracture risk and its application to screening for postmenopausal osteoporosis. Report of a WHO Study Group. World Health Organ Tech Rep Ser 843 1-129... 

The bisphosphonates are synthetic organic compounds that are incorporated directly into the hydroxyapatite of bone and then inhibit osteoclastic bone resorption. This antiresorptive action makes them useful in the pharmacological treatment of hypercalcemia, osteoporosis, and Paget s disease. 

The active forms of the D vitamins are la,25-dihydroxy-vitamin Dj and 25-hydroxy-vitamin Dj. They are formed by enzymatic hydroxylation in the liver microsomes and then in the kidney mitochondria by a ferredoxin flavoprotein and cytochrome P-450. The 1,25-dihydroxy vitamin is then transported to the bone, intestine, and other target organs (kidneys, parathyroid gland). Consequently, it can be considered a hormone since it is produced in one organ but used elsewhere. It mobilizes calcium and phosphate and also influences the absorption of these ions in the intestine, thus promoting bone mineralization. The hormone is also active in relieving hypoparathyroidism and postmenopausal osteoporosis, which, for example, results in the brittle bones of elderly women. 

Other target organs for the action of 1,25-dihydroxyvitamin D include the kidneys, bone, muscle,vwand skin. The hormone promotes reabsorption of both Ca2+ and inorganic phosphate by kidney tubules. In bone it binds to a specific receptor where it promotes the mobilization of calcium ions. This effect may result in part from stimulation of calcium-activated ATPase of the outer membrane of bone cells. Dissolution of bone also requires the presence of parathyroid hormone (PTH), the 83-residue hormone secreted by the parathyroid gland. In women past the age of menopause and in elderly men the production of 1,25-dihydroxyvitamin D decreases.w This may be a cause of the serious bone loss (osteoporosis) frequently observed. Treatment with 1,25-dihydroxyvitamin D3 or a synthetic analog seems to be helpful to such individuals. /Xy See also Chapter 30, Section A,5. 

---