A doctor uses a bone densitometer to measure the optical density of a female patient's index bone by shining light on it and measuring its transmission to diagnose osteoporosis. Osteoporosis is caused by loss of the protein matrix in bones, and is a natural part of aging. The condition is especially severe in post-menopausal women due to loss of the hormone estrogen, which helps maintain bone mass. The bones become brittle, weak and liable to fracture.