Suppression Linked to Heart Risks After Thyroid Cancer

heart disease, thyroid cancer, tsh suppression, levothyroxine
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One of the routine practices in treating patients with thyroid cancer is—after surgical removal of the thyroid, and in some cases radioactive iodine (RAI)—to prescribe a level of thyroid hormone replacement drugs that suppresses the thyroid stimulating hormone (TSH) level. Essentially, patients are given higher dosages of medication (known as supra-physiologic doses) in order to produce low, nearly undetectable TSH levels.

Keeping patients at levels considered to be in the hyperthyroid range can prevent recurrence of the thyroid cancer.

A research study that was presented at the 2017 Cancer Survivorship Symposium is challenging this standard practice by confirming evidence that there are long-term heart risks associated with this practice.

Hyperthyroidism that is not due to medication is already a known risk factor for atrial fibrillation and  ischemic stroke. It is also linked to angina pain and heart attacks in patients who already have underlying heart conditions.

In this study, the researchers measured the cardiac impact of hyperthyroidism due to suppressive doses of levothyroxine, a synthetic form of the thyroxine (T4) hormone, in more than 182,000 patients with thyroid cancer. They have found that suppressive doses of levothyroxine increase the risk of heart disease and ischemic stroke in thyroid cancer survivors.

Specifically, they found:

  • After thyroid cancer, there was an increased rate of heart disease and stroke
  • The risk was higher in patients who had received a total thyroidectomy
  • The risk was higher in those taking a higher dosage of levothyroxine (and who therefore had a lower TSH level)
  • There was a higher risk of atrial fibrillation, which was associated with higher doses of levothyroxine

    According to the researchers, only a small part of the stroke risk was due to increased rates of atrial fibrillation. Instead, hyperthyroidism’s ability to also cause high blood pressure and changes in heart function—among other factors—were to blame.

    Another 2013 study looked at the heart disease risk in patients who had differentiated thyroid cancer. The study found that 19 percent of the patients died as a result of heart issues—including heart attack, stroke, abdominal aortic aneurysms, and pulmonary embolisms—compared to 7.4 percent who died from the thyroid cancer itself. The risk of death from heart disease was correlated with TSH levels: The lower the TSH level, the higher the risk.

    Implications for Thyroid Cancer Patients

    These findings are important for thyroid cancer patients, particularly when you are facing a total thyroidectomy, or are on—or your physician is recommending—suppressive doses of levothyroxine after surgery.

    First, since the research showed greater risk associated with total thyroidectomy, you should discuss whether a total thyroidectomy is necessary as part of your thyroid cancer treatment. Recent research has shown that a lobectomy—surgery to remove only one lobe of the thyroid glandis the preferred surgical treatment for papillary thyroid cancer, the most common form of thyroid cancer.

    Second, many experts believe that there is no benefit to aggressive TSH suppression. The current guidelines from the American Thyroid Association and the European Thyroid Association recommend TSH suppression only when a patient “has active tumors or has a very aggressive tumor that has been treated with surgery and radioactive iodine.” According to both groups:

    About 85% of patients can be shown to be free of disease after initial tumor treatment by testing the patient' serum thyroglobulin levels and performing neck ultrasonography. When the patient is felt to be free of tumor on this basis, the ATA and ETA guidelines suggest maintaining the blood TSH in the low normal level.

    Note that despite the guidelines and recommendations, many physicians still routinely perform total thyroidectomies for all thyroid cancers, and prescribe aggressively suppressive doses for all their thyroid cancer patients.

    Third, we are also seeing significant increases in the rates of thyroid cancer diagnosis. While some experts believe that there is an actual increase in the incidence of thyroid cancer, this is a controversial issue, as some believe that the increased rates are mainly due to overdiagnosis of "incidental" cancerous thyroid nodules. Because of increased use of imaging tests, greater numbers of small, cancerous, but very slow-growing "incidental" thyroid nodules are being detected. There is controversy over whether those nodules warrant observation, rather than the aggressive cancer treatment that we now know may increase heart risks.

    Finally, if you have a history of thyroid cancer, you should make sure that your doctor periodically evaluates and monitors your heart health.

    Sources:

    Klein Hesselink EN, Klein Hesselink MS, de Bock GH, et al. Long-term cardiovascular mortality in patients with differentiated thyroid carcinoma: an observational study. J Clin Oncol. 2013;31(32):4046-4053. doi: 10.1200/JCO.2013.49.1043.

    Shin DW, Suh B, Yoon JM, et al. Risk of coronary heart disease and ischemic stroke in thyroid cancer patients taking levothyroxine. J Clin Oncol. 2017;35(suppl 5S; abstr 105).

    “Thyroid Stimulating Hormone (TSH) Suppression.” Thyroid Cancer Survivors Association. http://thyca.org/pap-fol/more/tsh-suppression/

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