Osteoporosis is the thinning of bone tissue and loss of bone density over time.

Osteoporosis is the most common type of bone disease. There are currently an estimated 10 million Americans suffering from osteoporosis, as well as another 18 million who have low bone mass, or osteopenia.

Osteoporosis occurs when the body fails to form enough new bone, or when too much old bone is reabsorbed by the body, or both.

Calcium and phosphate are two minerals that are essential for normal bone formation. Throughout youth, the body uses these minerals to produce bones. If calcium intake is not sufficient, or if the body does not absorb enough calcium from the diet, bone production and bone tissues may suffer.

As people age, calcium and phosphate may be reabsorbed back into the body from the bones, which makes the bone tissue weaker. Both situations can result in brittle, fragile bones that are subject to fractures, even without trauma.

Usually, the loss occurs gradually over years. Many times, a person will sustain a fracture before becoming aware that the disease is present. By the time this occurs, the disease is in its advanced stages and the damage is severe.

The leading causes are a drop in estrogen in women at the time of menopause, and a drop in testosterone in men. Women, especially those over the age of 50, get osteoporosis more often than men.

Other causes include excess corticosteroid from Cushing's syndrome, hyperthyroidism (too much thyroid hormone), hyperparathyroidism, being confined to a bed, and bone cancers.

Researchers estimate that about 20% of American women over the age of 50 have osteoporosis. In addition, another 30% of them have osteopenia, which is abnormally low bone density that may eventually deteriorate into osteoporosis, if not treated.

About half of all women over the age of 50 will suffer a fracture of the hip, wrist, or vertebra (bones of the spine).

White women, especially those with a family history of osteoporosis, have a greater-than-average risk of developing osteoporosis. Other risk factors include smoking, eating disorders, low body weight, too little calcium in the diet, heavy alcohol consumption, early menopause, absence of menstrual periods (amenorrhea), and use of certain medications, such as steroids and anticonvulsants.

There are no symptoms in the early stages of the disease.

Symptoms occurring late in the disease include:

• Fractures of the vertebrae, wrists, or hips (usually the first indication)
• Low back pain
• Neck pain
• Bone pain or tenderness
• Loss of height over time
• Stooped posture

• Bone mineral density (BMD) testing -- as performed in dual-energy x-ray absorptiometry (DEXA) -- measures the demineralization of the bones. This has become the gold standard for osteoporosis evaluation. BMD testing should be performed on all postmenopausal women with fractures, all women under 65 with an additional risk factor for osteoporosis (besides menopause), and all women 65 and over.
• A spine CT can show demineralization. Quantitative computed tomography (QCT) can evaluate bone density, but is less available and is more expensive than DEXA.
• A spine or hip x-ray may show fracture or vertebral collapse in severe cases.
• Measuring the amount of calcium in your urine can provide some evidence of increased bone turnover, but is of limited value. A number of newer tests to evaluate bone turnover are becoming available, including measurement of urinary N-telopeptide (Osteomark). In the future, these may enhance your physician's ability to diagnose early osteoporosis.