Thyroid hormone is made in the thyroid gland and circulates in the blood to help regulate nearly all of the body’s organs, including the heart. Although the link between abnormal levels of thyroid hormone and cardiovascular disease is well established, the hormone’s relationship with sudden cardiac death is unclear.
Thyroid hormone is known to increase heart rate, increase cardiac contractility, alter systolic and diastolic function, and decrease systematic vascular resistance. Thyroid dysfunction, even in the subclinical range, is associated with an increased incidence of cardiovascular risk factors and disease.
Scientists at the Erasmus University Medical Center (Rotterdam, The Netherlands) analyzed 10,318 patients in the Rotterdam Study, which is a long-term investigation of heart and other chronic disease among the middle-aged and elderly in the Netherlands. Participants’ mean age was 65, more than half were women, and nearly all were Caucasian. They assessed the association of thyroid-stimulating hormone and FT4 with the risk of sudden cardiac death (SCD).
Thyroid-stimulating hormone (TSH) or free thyroxine (FT4) measurements were made at the baseline visit of the study cohorts. Serum TSH (TSH LUMItest; Brahms GmbH, Hennigsdorf, Germany) and FT4 levels (FT4; Vitros, ECI Immunodiagnostic System; Ortho-Clinical Diagnostics, Raritan, NJ, USA) were determined in a random subset of the 1,855 baseline serum samples, thyroid function assessment was also performed in baseline serum samples for TSH and FT4 (the electrochemiluminescence immunoassay for thyroxine and thyrotropin, (ECLIA, Roche Diagnostics, Indianapolis, IN, USA).
The investigators found that participants with free thyroxine hormone levels at the high end of the normal range were 2.5 times more likely to die of sudden cardiac death, compared to patients with levels at the lower end. The ten-year risk of sudden cardiac death was four times greater among patients with higher free thyroxine levels compared to those with lower levels, 4% versus 1%. The increased risk persisted even after controlling for other risk factors, such as high cholesterol and high blood pressure and 261 cases of sudden cardiac death occurred.
Layal Chaker, MD, MSc, a doctoral candidate and lead author of the study, “Currently, we do not have a good way to predict sudden cardiac death in the general population. Thus identifying additional risk factors is crucial. Our results indicate that thyroid hormone levels may be useful for assessing risk to prevent sudden cardiac death. We know that a considerable proportion of patients on thyroid hormone replacement therapy are over-treated and so have high blood levels of thyroid hormone. Our study suggests more caution is warranted in the treatment of thyroid hormone replacement. Replacement therapy is often aimed at the high normal range which carries a risk of overtreatment.” The study was published on |September 6, 2016 in the journal Circulation.