Why your dose of thyroid medication should be checked every year

Key tips

  • Hyperthyroidism – an overactive thyroid – is associated with a slightly increased risk of dementia in older adults.
  • New research suggests that overmedication with levothyroxine, a medication for a belowactive thyroid, can also be linked to dementia.
  • Endocrinologists are calling for more research on when and how often levothyroxine is prescribed in older adults.

A recent study found that patients who take too much of a particular thyroid medication are at greater risk of dementia. Researchers said cognitive decline was previously known to occur in people whose bodies made too much thyroid hormone, but the new study links the risk of dementia to levothyroxinea medication that supplements thyroid hormone in people with levels that are too low.

The results are particularly important because millions of people take additional thyroid hormone medication, study author Jennifer Mammen, MD, PhD, an assistant professor of medicine at Johns Hopkins Medicine, told Verywell.

Here’s what experts want you to know about the link between thyroid medications and dementia and when you should talk to your doctor about your dosage if you’re taking levothyroxine.

The importance of thyroid hormones

The thyroid gland is in the front of the neck and makes thyroid hormone, which affects almost every organ in your body. The hormone helps control weight, body temperature, muscle strength, and even mood, David Cooper, MD, head of the thyroid clinic at Johns Hopkins Medicine in Baltimore, told Verywell. Cooper was not involved with the new study.

“Without enough thyroid hormone in the blood, many body functions slow down,” Cooper said.

The pituitary gland in the brain produces thyroid stimulating hormone (TSH). If the levels of thyroid hormone in the blood are too low, the pituitary makes more TSH to tell the thyroid to work harder. If thyroid hormone levels are too high, the pituitary gland makes little or no TSH.

Your doctor can do a blood test to indirectly check your thyroid hormone levels through your TSH levels. If your TSH level is high, you may need to take additional thyroid hormone. If your TSH is low, you may have too much thyroid hormone. These blood tests can also help your doctor know if your thyroid medication needs to be adjusted or stopped.

TSH levels and the brain

In the new study, researchers looked at the records of about 65,000 patients in the Johns Hopkins Community Physicians Network between 2014 and 2022 who were 65 and older. None of the patients had a diagnosis of cognitive decline or a low TSH level within six months of their first doctor’s visit.

Since previous research had shown that people whose bodies make too much thyroid hormone have a higher risk for dementia, the researchers wanted to know if the risk was also higher in people who were taking too much thyroid medication. .

During the study, some of the patients taking additional thyroid hormone medication had a low TSH level. In the follow-up period, 7.2% (4,779) of patients who took an excessively high dose of levothyroxine had a new diagnosis of cognitive disorder.

The study had limitations—for example, the population was primarily white and female. But the researchers think the findings add to a growing body of evidence that there could be cognitive risks for older adults who take thyroid medication — and who don’t even need to take it.

Who is at risk?

Based on the results, Mammen recommends that TSH levels be checked every year, especially in elderly patients, to see if their dose of thyroid medication should be changed or stopped.

“People are often under a certain dose of thyroid hormone when they are in their 40s and 50s, or even younger, but the dose can often be lowered as people get older,” said Mammen.

The study also showed that patients who were women were more likely to have a low TSH level and be overtreated.

In addition to cognitive decline, too much thyroid hormone is linked to other health risks, including atrial fibrillation (AFib) — an irregular heartbeat that can lead to blood clots and stroke — and a risk of fractures (in especially in older women), according to Cooper.

A commentary by endocrinologists from the University of Michigan and the Mayo Clinic published with the study explained that levothyroxine is one of the top three most prescribed medications in the United States, with 24 million people receiving a prescription for the drug in 2020. The authors emphasized that with so many people taking the medication, the results of the new study that uncovers the risks are important.

The comment also noted that levothyroxine tends to be prescribed more often to older patients and that several studies have suggested that the medication is sometimes prescribed without medical evidence that it is necessary.

In response to the research, the authors of the comments wrote that “increasing awareness of the risks associated with the use of levothyroxine should promote more cautious prescribing practices,” such as not giving levothyroxine to older adults that thyroid levels are just a little low.

What this means for you

Thyroid hormone is one of the most commonly prescribed medications, but taking too much comes with health risks and may even be linked to an increased risk of dementia. If you take thyroid medication, talk to your doctor at least once a year about your dosage. You should have your TSH levels checked to see if your dose needs to be adjusted.

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts in our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable and trustworthy.

  1. Adams RJ, Oh ES, Sevil Yaşar, Lyketsos CG, Mammen JS. Endogenous and exogenous thyrotoxicosis and risk of incident cognitive disorders in older adults. JAMA Intern Med. Published online October 23, 2023. doi:10.1001/jamainternmed.2023.5619

  2. Wieland DR, Wieland JR, Wang H, et al. Thyroid disorders and dementia risk. Neurology. 2022;99(7):e679-e687. doi:10.1212/wnl.0000000000200740

  3. MedlinePlus. Levothyroxine.

  4. American Thyroid Association. TSH reference ranges should be used to safely guide thyroid hormone treatment in hypothyroid patients.

  5. Papaleontiou M, Brito JP. Disentangling the association between excess thyroid hormone and cognition in older adults. JAMA Intern Med. Published online October 23, 2023. doi:10.1001/jamainternmed.2023.5618/

by Fran Kritz

Fran Kritz is a freelance health reporter with a focus on consumer health and health policy. He is a former staff writer for Forbes Magazine and US News and World Report.

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