What causes osteoporosis?

On the basis of aetiology, osteoporosis can be classified as idiopathic or secondary.

Idiopathic osteoporosis

This is essentially osteoporosis in the absence of another condition that can predispose to osteoporosis. It is therefore mainly due to natural changes in bone density that occur over time.

The normal pattern of bone density changes over a lifetime for men and women are shown below.

Click to enlarge

In women, after the menopause there is greater rate of decline in bone density than in men for the first 5-10 years. Similarly due to women having an earlier growth phase in puberty than men, their peak bone density is less than men. These factors make women more likely to suffer osteoporosis than men, but its occurrence in men as well should be noted.

This post-menopausal change in women seems to be due to a lack of oestrogen. Low oestrogen levels have been shown to inhibit the osteoblast cells that form bone and stimulate the osteoclast cells that resorb bone (see 'What is bone remodelling?') This probably accounts for the rapid loss of bone density seen in women. Indeed, the fact it may be managed by Hormone replacement Therapy (HRT) using oestrogen supports this idea.

Other factors that predispose to idiopathic osteoporosis include:

• early menopause
• family history
• low calcium intake in the diet
• sedentary lifestyle (impact exercise such as aerobics where the bones are put under mild stress has been shown to reduce the risk of osteoporosis)
• cigarette smoking
• excessive alcohol intake
• excessive caffeine intake

Secondary osteoporosis

This is where other medical conditions or medications can cause osteoporosis. These include:

1. Endocrine disorders

• hyperparathyroidism (overactive parathyroids)
• hyperthyroidism (overactive thyroid)
• hypogonadism (underactive ovaries or testes produce low amounts of oestrogen and testosterone respectively, both of which are important for bone formation)
• Cushing's syndrome (excess blood cortisol)

2. Metabolic problems

• chronic renal failure (causes low calcium availability to the bones)
• malabsorption (where conditions such as sprue, colitis and diarrhoea result in less calcium
• being absorbed from the diet)

3. Specific conditions affecting bones

• osteomalacia (due to calcium deficiency)
• osteogenesis imperfecta (a genetic disorder disrupting bone formation)

4. Medications

• glucocorticoid therapy (e.g. hydrocortisone for Addison's)
• thyroxine (for hypothyroidism)

5. Other

• cancer (such as multiple myeloma)
• rheumatoid arthritis
• anorexia nervosa