Levothyroxine is one of the most commonly prescribed medications in the United States.
It’s used to treat thyroid problems, especially hypothyroidism, which happens when the thyroid gland doesn’t make enough hormones. The medicine works by replacing the missing thyroid hormone in the body.
But a large new study from the University of Michigan warns that this treatment must be carefully managed. If it’s not, it could increase the risk of death from heart problems. Since heart disease is the number one cause of death in the U.S., this is an important concern.
The study, led by Dr. Maria Papaleontiou and published in JAMA Network Open, followed over 705,000 U.S. veterans who were treated with thyroid hormone from 2004 to 2017.
What the researchers found was surprising. Nearly half of the patients had thyroid hormone levels that were either too high or too low—and both situations can be dangerous for the heart.
The researchers focused on TSH (thyrotropin), a hormone that controls how the thyroid works. If TSH levels are too low, it means the body has too much thyroid hormone, often due to taking too much levothyroxine.
This condition is called exogenous hyperthyroidism. If TSH levels are too high, it means the thyroid hormone level is too low, usually from not taking enough medicine.
Both problems can harm the heart. The heartbeat may become too fast or too slow, and blood pressure may be harder to manage. People with TSH levels below 0.1 or above 20 (measured in mIU/L) had the highest risk of dying from heart-related causes. The risk was even greater in older adults, who are more sensitive to changes in hormone levels.
The study excluded patients who had other health issues that could affect thyroid function, such as thyroid cancer or medications like lithium or amiodarone. This helped the researchers focus on the effects of thyroid hormone treatment alone.
These findings show that treating thyroid disease isn’t just about replacing missing hormones. It’s about getting the balance just right. If treatment is too strong or too weak, it can cause new problems—especially for the heart.
The good news is that this risk can be reduced. By checking TSH levels regularly and adjusting the medication when needed, doctors can help keep the heart safe. This is especially important for older adults, who may need lower doses.
This research supports what doctors already know: the thyroid and the heart are closely connected. It also shows that one-size-fits-all treatment doesn’t work for everyone. Personalized care, regular blood tests, and open communication with your doctor are key.
If you or someone you know is taking levothyroxine or another thyroid medicine, don’t skip your check-ups. Even if you feel fine, your hormone levels might not be. Keeping your TSH within a healthy range can protect your thyroid—and your heart.
Other studies also show that diet and supplements matter. For example, eating eggs may lower the risk of heart disease, while some herbal supplements could be harmful to your heart rhythm. Vitamin C may help with heart failure, while vitamin E might not. Even espresso coffee may affect cholesterol levels.
This new study is a strong reminder that small changes in treatment can have a big impact. With careful monitoring, people with thyroid problems can stay healthier and avoid serious heart issues.
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