Osteoporosis Causes

Written by the Healthline Editorial Team | Published on October 3, 2014
Medically Reviewed by Christine Case-Lo on October 3, 2014

What Is Osteoporosis?

Osteoporosis is the thinning of our bones as we get older. It affects one out of every two women and one out of every five men, according to the Centers for Disease Control and Prevention (CDC). Ten million Americans over the age of 50 have osteoporosis. An additional 34 million have osteopenia, which is reduced bone mass. There are a variety of risk factors that can determine your risk for the disease. Some are preventable and some are unavoidable. What causes bone thinning in the first place?

Bone Remodeling

Bone is living tissue with holes inside, kind of like a honeycomb. Bones suffering from osteoporosis have larger holes and are more fragile. Understanding thinning of the bones begins with understanding how bones are made. We repeatedly place demands on our bones. Because of these demands, bone is being remodeled constantly. Bones are like a house that the owner is never quite finished fixing.

This bone remodeling is done in two phases. First, bone is broken down by osteoclasts, a special kind of bone cell. Then, new bone is created by osteoblasts, another type of bone cell. The coordination of osteoclasts and osteoblasts is a dance that works well for most of our lives. Eventually, this coordination can break down, and the osteoclasts begin to remove more bone than the osteoblasts can create.

When we are young, we make a lot of bone. In our mid-20s we have “peaked” and reached the maximum of bone mass. After that, we start to slowly lose bone mass as more bone is dissolved than is rebuilt.

Keys to Bone Balance

Parathyroid hormone (PTH) is an important contributor to the bone remodeling process. High levels of PTH hormone can activate osteoclasts and cause too much bone breakdown. The natural release of PTH is triggered by the amount of calcium in our blood. Low calcium levels in the blood (hypocalcaemia) can result in high levels of PTH and the breakdown of bone to make sure we have enough calcium in our blood.

Calcium is needed for heart health, blood clotting, and muscle function. Our bodies will “mine” our bones for calcium if we don’t have enough in our blood. Getting enough calcium throughout your life is important to prevent bone thinning. In our teens and early adult years we are building bones. Sufficient calcium intake at that time ensures healthy bones later on. As we get older, eating enough calcium-rich foods helps reduce the amount of bone breakdown.

Vitamin D is essential for maintaining the calcium in our bones. Vitamin D helps us absorb calcium through our intestines. But many older adults do not get enough vitamin D. The National Institutes of Health (NIH) report that up to 50 percent of older adults with hip fractures have very low levels of vitamin D. Without enough vitamin D, the calcium in milk, calcium supplements, or other sources will not be properly taken up by your bloodstream. Low levels of vitamin D will also trigger a series of events that lead to activation of osteoclasts. It also leads to increased production of parathyroid hormone, which creates even more osteoclasts.

The Impact of Hormones

Older women, especially white and Asian women, are more likely to be affected by osteoporosis than men. Why is this? One reason is the impact of falling estrogen levels after menopause. Consistent estrogen is very important in maintaining the rhythm of bone remodeling. If estrogen levels decrease, this changes the levels of a lot of messaging chemicals, which help maintain a healthy balance of bone production and breakdown. The result is that osteoclasts become more active without estrogen, and more bone is broken down.

Certain medical conditions and some medications can speed up the process of osteoporosis. This is called secondary osteoporosis. It occurs most often as a result of taking glucocorticoid steroids. Steroids like cortisol and prednisone directly slow down osteoblasts and speed up osteoclasts. They make it harder for the body to absorb calcium and they also increase how much calcium we lose in urine.

Taking thyroid hormones can also increase the risk of bone thinning. Thyroid hormones speed up the bone remodeling process, and with this increase in speed, the risk of imbalance between osteoblasts and osteoclasts increases. Alcohol abuse, smoking, and having an eating disorder are also risk factors that can cause osteoporosis. These interfere with our ability to absorb needed nutrients like calcium and vitamin D.


The complex interactions between parathyroid hormone, calcium, and vitamin D control the balance of bone-making and bone-destroying cells. Maintaining levels of calcium and vitamin D are key to reducing the risk of osteoporosis. Some health conditions and medications can impact the bone remodeling process and lead to bone thinning.

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