Millions of people around the world take levothyroxine every day to treat an underactive thyroid. In the United States, it has remained one of the three most commonly prescribed medicines for more than 10 years.
The drug replaces thyroid hormone when the thyroid gland cannot produce enough on its own. For most people, it is safe and very effective. However, new research suggests that getting the dose exactly right is more important than many people realize.
The new study was published in JAMA Network Open and was led by Dr. Maria Papaleontiou from the University of Michigan.
The researchers found that people whose thyroid hormone treatment left their hormone levels too high or too low had a greater risk of dying from heart disease. The findings highlight the need for regular blood tests and careful adjustment of thyroid medicine, especially in older adults.
The thyroid is a small butterfly-shaped gland located in the front of the neck. Although it is small, it controls many important jobs in the body. Thyroid hormones help regulate heart rate, body temperature, energy use, and metabolism.
When the thyroid does not produce enough hormones, a condition called hypothyroidism develops. People may feel tired, gain weight, feel cold more easily, have dry skin, or experience slow thinking and depression.
Levothyroxine replaces the missing hormone and helps bring the body’s hormone levels back to normal. Doctors usually monitor treatment by measuring thyroid-stimulating hormone, also called TSH, through a simple blood test. TSH is made by the brain and tells the thyroid how much hormone to produce. When treatment is correct, TSH stays within a healthy range.
Problems can happen when treatment is not balanced. If the medicine dose is too high, TSH becomes very low. This means the body is receiving too much thyroid hormone.
The heart may beat faster, work harder, and develop abnormal heart rhythms over time. If the dose is too low, TSH becomes too high because the body is still not getting enough thyroid hormone. Untreated hypothyroidism can also increase the risk of heart disease by affecting cholesterol levels, blood vessels, and heart function.
To better understand these risks, the researchers examined medical records from more than 705,000 U.S. veterans who received thyroid hormone treatment between 2004 and 2017. This makes it one of the largest studies ever carried out on thyroid hormone therapy and heart health.
The researchers found that as many as half of the patients had TSH levels outside the recommended range during treatment. Some had levels that were too low, while others had levels that were too high.
Compared with people whose TSH stayed within the normal range, these patients were more likely to die from heart-related causes. The highest risks were seen in people whose TSH dropped below 0.1 mIU/L or rose above 20 mIU/L. Older adults appeared to be especially sensitive to these changes.
The research team carefully designed the study to avoid confusing results. They excluded people with thyroid cancer because doctors often intentionally keep TSH very low to reduce the chance of the cancer returning. They also excluded patients taking lithium or amiodarone, two medicines that can strongly affect thyroid function.
The results show that treatment intensity is something doctors and patients can change. Unlike many heart disease risk factors, thyroid hormone dosage can be adjusted after regular blood testing. Keeping TSH within the recommended range may help lower the risk of serious heart problems and heart-related death.
The study also reminds people that thyroid treatment should never be adjusted without medical advice. Missing doses, taking extra tablets, or changing the timing of medication can affect hormone levels. Regular follow-up appointments allow doctors to make small dose changes that keep treatment safe and effective.
Heart disease remains the leading cause of death in many countries, and scientists continue to search for ways to reduce this burden. This new research suggests that careful thyroid management should be part of that effort.
While levothyroxine remains an essential medicine for millions of people, the greatest benefits come when treatment is carefully monitored and tailored to each individual. By keeping thyroid hormone levels stable, doctors may be able to improve both thyroid health and long-term heart health.
If you care about heart health, please read studies about how eating eggs can help reduce heart disease risk, and herbal supplements could harm your heart rhythm.
For more health information, please see recent studies that olive oil may help you live longer, and vitamin D could help lower the risk of autoimmune diseases.
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