Risk of death from a sudden loss of heart function has been found to be significantly greater in patients free thyroxine (FT4) levels at the higher end of normal range, compared to patients with levels at the lower end, according to new research published in Circulation.
According to a 2012 Circulation study, more than half of all cardiovascular deaths stem from sudden cardiac death, and in many instances no previous symptoms of heart disease were apparent.
“Currently, we do not have a good way to predict sudden cardiac death in the general population,” says Layal Chaker, study lead author and doctoral candidate and research fellow in endocrinology and epidemiology at Erasmus University Medical Center Rotterdam (Rotterdam, The Netherlands). “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.”
Researchers analysed 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’ average age was 65, more than half were women, and nearly all were caucasian.
Researchers linked the association of FT4 hormone levels in blood samples with sudden cardiac deaths listed on medical records and death certificates. During an average follow-up of nine years they found:
- Participants with FT4 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 FT4 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.
- 261 cases of sudden cardiac death occurred.
“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,” Chaker says. “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.”
Researchers suggest further research is warranted because factors other than thyroid hormone levels could have contributed to the sudden cardiac deaths. The study also relied on a one-time test, and thyroid hormone levels could have changed throughout the course of the study. And, since most participants were caucasian, the findings also may not apply to people of other races.