People with thyroid cancer may be at an increased risk for coronary heart disease and stroke, especially if they’ve had surgery to remove the organ, according to new findings presented at the 2017 Cancer Survivorship Symposium Advancing Care and Research in San Diego, California. Researchers say long-term hormone treatment with high doses of levothyroxine appear to be the cause, Medscape reports.

The thyroid is a small butterfly-shaped organ located at the front of the neck that creates and controls hormones that affect many functions in the body, such as metabolism, heart rate, blood pressure, temperature and weight. Thyroid cancer occurs when abnormal cells begin growing in this organ. Most thyroid cancers can be cured—through hormone therapy, radiation treatment, chemotherapy or surgery—depending on the stage and type.

For this study, researchers from Seoul National University Cancer Hospital in South Korea checked the long-term effects of the most intensive thyroid treatment, a thyroidectomy, or full removal of the thyroid gland. Scientists identified 182,419 individuals in the Korean National Health Insurance database who underwent the surgery between 2004 and 2012 and compared them with a control group of patients with normal thyroid function.

Findings showed that those with thyroid cancer registered a 15 percent increased risk for coronary heart disease and a similar 15 percent increased risk for ischemic stroke when compared with their healthy counterparts. Researchers also noted that people who received treatment with high doses of levothyroxine—a drug that replaces thyroid hormones in the body after the organ is removed—had a four- to fivefold higher risk for both cardiovascular conditions. 

The report also showed that people who took a higher dose of levothyroxine (defined as at least 170 μg each day) risked a two- to 2.5-fold higher chance of developing atrial fibrillation (an irregular heartbeat), which causes poor blood flow and can trigger cardiovascular issues later in life.

“We can conclude that screening for cancer and cancer surveillance have both benefits and harms,” said Wendy Landier, PhD, RN, an associate professor in the department of pediatrics at the University of Alabama School of Medicine and an author of the study.

Landier wrote a discussion paper about the report and said that when it comes to thyroid cancer treatment, “sometimes, less may be more.”

Click here to learn more about additional options for thyroid cancer treatment.