Levothyroxine is one of the most commonly prescribed medicines in the United States.
It is a laboratory-made version of a hormone normally produced by the thyroid, a small gland in the neck that helps control metabolism, energy use and many other body functions.
Doctors often prescribe levothyroxine to people with hypothyroidism, a condition in which the thyroid does not make enough hormone. Treatment replaces the missing hormone and can relieve problems such as tiredness, feeling cold, weight gain and difficulty concentrating.
Getting the dose right is important because both too much and too little thyroid hormone can affect the body. Research from the University of Michigan suggests that abnormal thyroid levels during treatment may also be linked to a greater risk of dying from heart disease.
Doctors commonly use a blood test for thyroid-stimulating hormone, or TSH, to help determine whether thyroid treatment is working properly. TSH is produced by the pituitary gland in the brain and sends signals telling the thyroid how much hormone to make.
When thyroid hormone levels are too high, TSH usually falls. When thyroid hormone levels are too low, TSH usually rises, so unusually high or low TSH readings can indicate that treatment may need to be adjusted.
The study was led by Dr. Maria Papaleontiou and was published in JAMA Network Open. Researchers examined information from more than 705,000 U.S. veterans who received thyroid hormone treatment between 2004 and 2017.
The researchers found that abnormal thyroid levels were common during treatment. At different points, many patients had TSH readings that were either too high or too low instead of remaining within the normal range.
Both situations were associated with greater cardiovascular death risk. The strongest links appeared among people with very low TSH levels below 0.1 mIU/L and those with very high levels above 20 mIU/L.
Having very low TSH can be a sign that a person is receiving more thyroid hormone than the body needs. Too much thyroid hormone can make the heart beat faster and may contribute to abnormal heart rhythms and other cardiovascular problems.
Very high TSH, meanwhile, can suggest that the body is not receiving enough thyroid hormone. An underactive thyroid can affect cholesterol, blood vessels and other factors connected with cardiovascular health.
Older adults appeared particularly vulnerable to the effects of abnormal thyroid levels. This is important because older people are more likely to have existing heart conditions and may be more sensitive to changes in thyroid hormone levels.
To make the comparison more reliable, the researchers excluded some people whose thyroid levels could be affected for special reasons. For example, patients with thyroid cancer were excluded because doctors sometimes intentionally keep their TSH low as part of cancer treatment.
People taking certain medicines that can interfere with thyroid function, including lithium and amiodarone, were also excluded. These steps helped the researchers focus more closely on the relationship between thyroid hormone treatment and cardiovascular death.
The findings do not mean that levothyroxine itself is unsafe or that patients should stop taking it. For people whose thyroid cannot produce enough hormone, replacement treatment can be essential for maintaining normal body function and preventing complications.
Instead, the research highlights the importance of finding an appropriate dose and checking thyroid levels regularly. Because a person’s thyroid hormone needs can change with age, health conditions, weight and other medicines, a dose that once worked well may eventually need adjustment.
Patients taking levothyroxine should continue the medicine as prescribed and discuss unusual test results or symptoms with their healthcare provider. Keeping thyroid hormone levels in a healthy range may help patients receive the benefits of treatment while reducing avoidable risks to the heart.
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 about how drinking milk affects risks of heart disease and cancer, and results showing strawberries could help prevent Alzheimer’s disease.
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