## Thyroxine and heart: what is the connection? Thyroxine, a thyroid hormone, is produced in the thyroid gland and circulates in the blood, helping to regulate almost all organs, including the heart. Although the link between abnormal thyroid hormone levels and cardiovascular disease has long been established, the hormone's link to sudden death from a heart attack has been unclear. In 2012, scientists noticed that more than half of all cardiovascular deaths came from sudden cardiac arrest, and in many cases, there were no symptoms prior to the attack. Dr. Lyall Chaker of Erasmus Medical Center in the Netherlands noted that most of those who died had endocrine problems. The attempt to establish a link between these diseases was the basis of a large-scale study. ## Evidence base The researchers followed 10,318 middle-aged and elderly patients. Over nine years, they found that participants with free thyroxine levels above normal and at the upper end of the normal range were 4 times more likely to die from sudden cardiac arrest compared to patients with hormone levels at the lower end of the normal range. The increased risk persisted even after excluding other factors such as high cholesterol and high blood pressure. "We know that a significant proportion of patients receiving replacement therapy have high levels of thyroid hormone," says lead researcher Lial Chaker, MD. “Doctors don't think this is dangerous because thyroxine stays at the upper end of the normal range. However, this level of thyroxine has also been shown to increase the risk of sudden death." Currently, there is no reliable way to predict a heart attack. Therefore, identifying additional risk factors is critical. __Sources__: Canadian Medical Association Journal, www.cmaj.ca/lookup/doi/10.1503/cmaj.140688 Arch Intern Med. doi:10.1001/archinternmed.2012.402 Arch Intern Med. doi:10.1001/archinternmed.2012.1114