Can Levothyroxine Cause Osteoporosis?

I have Hashimoto’s hypothyroidism and have been on levothyroxine since I was in my 20s. I was upset to read that people who take levothyroxine are more likely to have osteoporosis. What am I supposed to do? I need levothyroxine to feel good and function well.

—Anonymous

I saw these reports in the news like you did and immediately went looking for the source data. I have thousands of patients with hypothyroidism, and I myself take levothyroxine — a synthetic thyroid hormone — for Hashimoto’s hypothyroidism. I was eager to better understand this new data. I was frustrated to find that the source of these stories was a press release from the Radiological Society of North America and not an actual scholarly publication.

The press release highlights a retrospective study using data from the Baltimore Longitudinal Study of Aging, which enrolls patients over age 65. The authors identified 81 patients who were taking levothyroxine for unknown reasons and 364 patients who were not taking levothyroxine and compared their bone density. They found that the bone density of the patients taking levothyroxine was on average lower than those of the patients not taking levothyroxine.

This study demonstrates an association, not a causal relationship. Because the patients weren’t randomized to take levothyroxine or not take levothyroxine, we cannot know if the levothyroxine is causing low bone density. Presumably all the patients taking levothyroxine have hypothyroidism and those not taking levothyroxine do not. It may be that hypothyroidism itself contributes to lower bone density.

Additionally, the difference in bone density may be statistically significant, but the authors don’t report whether this translates into a clinically meaningful difference. Put another way, does this decrease in bone density lead to more fractures? We don’t know. We also don’t know if these results apply to younger individuals — all the patients included in the study were over age 65.

Finally, as with any medication, we have to consider the benefits and the risks of taking levothyroxine. Thyroid hormone is critical to our body’s normal functioning. Not having enough thyroid hormone can lead to weight gain, constipation, fatigue, dry skin, hair loss, and even heart failure. Replacing thyroid hormone with levothyroxine is the only option for treating hypothyroidism. If you have hypothyroidism, the benefits of taking levothyroxine will very likely outweigh the possibility of losing a small amount of bone.