Thyroid hormone medicine is taken by millions of people, but getting the dose right may be more important for the heart than many people realize.
A large study found that people whose thyroid levels became too high or too low during treatment had a greater risk of dying from heart-related causes.
The findings point to the importance of regular blood tests and careful dose changes.
The thyroid is a small gland in the neck that makes hormones that help control how the body uses energy.
These hormones affect many organs, including the heart. When the thyroid does not make enough hormone, a condition called hypothyroidism, people may feel tired, gain weight, feel cold, or experience other health problems.
Doctors often treat hypothyroidism with a synthetic thyroid hormone called levothyroxine. It replaces hormone that the body is not making in sufficient amounts and is one of the most commonly prescribed medicines in the United States. The correct dose, however, can vary from person to person and may change over time.
Researchers wanted to know whether the intensity of thyroid hormone treatment was linked to the risk of dying from cardiovascular disease. The team studied health information from 705,307 adults in the U.S. Veterans Health Administration who received thyroid hormone treatment between 2004 and 2017. Patients were followed for a median of four years.
The researchers paid particular attention to thyrotropin, commonly called TSH, which is measured with a blood test and helps doctors judge whether thyroid hormone levels are balanced. A TSH level that becomes too low during treatment can be a sign that a person is receiving too much thyroid hormone. A very high TSH level can suggest that the person is not receiving enough.
During the study, 75,963 patients, or 10.8% of the group, died from cardiovascular causes. People with thyroid levels outside the normal range had a higher risk of cardiovascular death than people whose levels remained within the normal range. The researchers took other major heart risks, including smoking, high blood pressure, diabetes, and previous heart disease, into account.
The strongest links appeared at the extremes. Patients with TSH levels below 0.1 mIU/L had a 39% higher risk of cardiovascular death compared with patients in the normal range. Those with TSH levels above 20 mIU/L had an even greater increase in risk.
Older adults appeared to be especially vulnerable. This matters because people may need different thyroid hormone doses as they age, and older adults are already more likely to have heart and blood vessel problems. Regular testing may therefore be particularly important in this group.
The researchers excluded people with thyroid cancer because doctors sometimes intentionally keep their TSH levels low to reduce the chance of the cancer returning. They also excluded people taking lithium or amiodarone because these medicines can affect thyroid test results. These steps helped the researchers focus more clearly on thyroid hormone treatment itself.
The study does not prove that an incorrect thyroid hormone dose directly causes cardiovascular death. It was an observational study, and most of the patients were male veterans, so the findings may not apply equally to every population. Still, the results suggest that avoiding both over-treatment and under-treatment could be an important part of protecting heart health.
The research was conducted by investigators including Dr. Maria Papaleontiou at the University of Michigan and was published in JAMA Network Open on May 12, 2022. The study suggests that thyroid hormone treatment should not simply be started and forgotten. Regular monitoring and dose adjustments may help patients keep their thyroid levels in a healthy range while reducing avoidable risks to the heart.
If you care about heart health, please read studies about the best time to take vitamins to prevent heart disease, and calcium supplements could harm your heart health.
For more health information, please see recent studies that blackcurrants can reduce blood sugar after meal and results showing how drinking milk affects risks of heart disease and cancer.
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