Millions of people around the world take levothyroxine every day to treat an underactive thyroid, a condition known as hypothyroidism.
The thyroid is a small butterfly-shaped gland in the front of the neck that produces hormones controlling how the body uses energy.
These hormones affect many important functions, including heart rate, body temperature, digestion, and metabolism. When the thyroid does not make enough hormones, people may feel tired, gain weight, feel cold, have dry skin, or develop depression and other health problems.
Levothyroxine replaces the thyroid hormone that the body cannot make on its own. For most people, it is a safe and effective treatment that greatly improves quality of life. However, the medicine must be given in the correct amount. If the dose is too high or too low, it can upset the body’s natural hormone balance and increase the risk of health problems.
A new study from the University of Michigan suggests that getting this balance right is especially important for protecting the heart. The research was led by Dr. Maria Papaleontiou and published in JAMA Network Open. The scientists examined the medical records of more than 705,000 U.S. veterans who received thyroid hormone treatment between 2004 and 2017.
The researchers found that many patients did not have thyroid hormone levels within the recommended range. In fact, up to half of the people studied had hormone levels that were either too high or too low during treatment. These changes were linked with a higher risk of dying from heart disease.
The team focused on a blood test called thyroid-stimulating hormone, or TSH. Doctors commonly use this test to see whether thyroid hormone treatment is working properly.
When TSH is very low, it usually means there is too much thyroid hormone in the body. This often happens when a person is taking more levothyroxine than they need. Doctors call this exogenous hyperthyroidism because the extra hormone comes from medicine rather than the thyroid gland.
When TSH is very high, the opposite problem is happening. The body is not getting enough thyroid hormone, which may mean the levothyroxine dose is too low or that the medicine is not being absorbed properly. Both situations can place extra stress on the heart.
Too much thyroid hormone can make the heart beat too quickly, increase the chance of abnormal heart rhythms, and place extra strain on the cardiovascular system. Too little thyroid hormone can slow the heart, raise cholesterol levels, and increase blood pressure. Over time, these problems may increase the risk of heart disease.
The study found that people with TSH levels below 0.1 mIU/L or above 20 mIU/L had the greatest risk of dying from heart-related causes. Older adults appeared to be especially vulnerable because they are often more sensitive to changes in thyroid hormone levels.
To make sure the results were as accurate as possible, the researchers excluded patients with thyroid cancer and people taking medicines such as lithium or amiodarone, which can strongly affect thyroid function.
The findings highlight that successful thyroid treatment is not simply about taking a daily tablet. It is about making sure the dose stays at the right level over time. Doctors usually recommend regular blood tests after starting levothyroxine or changing the dose. These tests help ensure TSH stays within a healthy range and allow treatment to be adjusted if needed.
Many people taking levothyroxine feel well and may think regular check-ups are unnecessary. However, hormone levels can change without causing obvious symptoms. Regular medical appointments and blood tests help detect these changes early before they lead to serious problems.
This study also reminds us that the thyroid and the heart are closely connected. Careful monitoring, especially in older adults, may reduce the risk of heart disease while allowing patients to continue receiving the benefits of thyroid hormone replacement.
Although levothyroxine remains an important and highly effective medicine, this research shows that careful follow-up is just as important as the treatment itself.
If you care about heart health, please read studies that yogurt may help lower the death risks in heart disease.
The full study was published in JAMA Network Open.
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