For people with almost any type of heart disease, disorders of the thyroid gland can worsen old cardiac symptoms or contribute to new ones. New research now strengthens the evidence that thyroid disorders and heart disease may also be a deadly combination.

In the June issue of the Journal of Clinical Endocrinology & Metabolism (JCEM), a large study revealed that among participants with congestive heart failure, having hypothyroidism — even the mild form — significantly increases the risk of death compared to people with normal thyroid function.

“Hypothyroidism has known effects on multiple cardiovascular pathways, including adverse effects on systolic and diastolic function, endothelial function, and lipid levels, and our study suggests that if someone has higher underlying cardiovascular risk, they may be more vulnerable to the effects of mild hypothyroidism,” says Connie Rhee, MD, of Brigham and Women’s Hospital in Boston, and lead author of the study.

In hypothyroidism, an underperforming thyroid gland makes insufficient thyroid hormone, which may affect nearly every organ in the body, including the heart. According to the NIH’s National Institute of Diabetes and Digestive and Kidney Diseases, nearly 5% of the U.S. population over the age of 12 (more than 9.5 million people) has hypothyroidism, which is diagnosed by measuring the levels of thyroid stimulating hormone (TSH) and thyroxine (T4) in the blood.

In the JCEM study, researchers analyzed data from 14,879 participants from the “Third National Health and Nutrition Examination Survey”. Almost 750 of the participants had hypothyroidism, and 691 had a mild (“subclinical”) form of the condition. Patients were divided into two groups: those with heart failure and those without heart failure. In the group with heart failure, when researchers compared subclinical hypothyroidism to those patients with normal thyroid function, there was an increased death risk for subclinical hypothyroidism patients. But there was no greater risk for those in the non-heart failure group.

“In heart failure patients, we found that both hypothyroidism overall and subclinical hypothyroidism increased the risk of death,” Rhee says.

Rhee and her team considered the normal reference ranges for TSH at 0.39 to 4.6 mIU/L and for total T4 (TT4 ) at 4.5 to 13.2 μg/dL. Subclinical hypothyroidism was defined as an elevated TSH (>4.6 mIU/L) and a normal TT4 level in the primary analyses, and as a TSH between 0.39–10 mIU/L in the secondary analyses.

The JCEM study joins a number of previous studies that have established correlations between thyroid problems and an increased risk of advanced heart disease. A recent study in Circulation reported that serious health risks appeared at both ends of the thyroid disorder spectrum. Both higher and lower TSH levels, particularly for TSH ≥10 and <0.10 mIU/L, were found to cause increased heart failure events.A 2010 Th e Journal of the American Medical Association study of more than 55,000 participants also found subclinical hypothyroidism was linked to an increased risk of heart disease events and death in patients with higher TSH levels, especially those with a TSH of 10 mIU/L or greater.So, why are heart patients so susceptible to such grave outcomes?

Heart Failure caused By Hypothyroidism

“Heart failure is the final common pathway of all different kinds of negative events that happen to the cardiovascular system, such as untreated hypertension and diabetes, and heart failure is the state you end up in when these types of stresses have negatively impacted heart function,” explains Ann Bolger, MD, American Heart Association spokesperson and William Watt Kerr Professor of Medicine at the University of California, San Francisco.

“And no matter how they got there, this population of patients is very vulnerable.”

Hypothyroidism is a disorder that interferes with the efficiency of body functions, such as muscular function and aerobic capacity, and heart patients are the ones who can least afford any additional negative effects, continues Bolger.

“I don’t think hypothyroidism would be well-tolerated by almost anyone, but most of us are in the happy circumstance where nature gives us so many different layers of coping mechanisms that we can defend ourselves in other ways,” she adds. “Hypothyroidism, however, is a total body challenge. It’s not hard to understand how it can have a negative impact on heart patients.”

Treatment for Hypothyroidism

When treating patients with heart disease, thyroid functioning may not always be a normal part of the health assessment for cardiologists. Bolger says the treatment plan often depends on the patient’s actual heart condition.

“Heart patients are not all the same,” she says. “We monitor 100% of the patients with arrhythmia, for example, because we know that thyroid levels could be the reason for the heart problems. But with other types of heart problems, such as acute heart attack, thyroid function might not be as tightly correlated, so doctors may not be paying as much attention.

“And when people get into more serious stages of heart disease and then heart failure, we do look for anything that can help them feel better so we look at thyroid function,” Bolger continues.

Rhee has also advised a more individualized approach. She says health care providers should not use a blanket approach in assessing people’s risk for mild hypothyroidism and determining whether they require treatment.

On the contrary, thyroid expert E. Chester Ridgway, MD, executive vice chair of medicine at the University of Colorado Denver School of Medicine, says a blanket treatment plan for those with mild hypothyroidism is seldom used.

“I think thyroid experts are taking a more personalized approach and are very interested in defining which patients with mild thyroid failure should in fact be treated,” he says.

“If the patient with a high TSH has valid hypothyroid symptoms and elevated lipids, most would recommend treatment,” Ridgway continues. “In contrast, a person with no symptoms and normal lipids might just be observed over time to see if symptoms develop.”

Although the JCEM study might suggest that patients with heart failure and high TSH values should be appropriately treated, prospective randomized studies will need to be done to determine whether there is benefit from the treatment, he adds.

Rhee agrees more research is needed. “Our study was an observational study that found an association between mild hypothyroidism and increased death risk in heart failure patients, but it doesn’t tell us if it causes it,” she says.

Rhee adds that there is a need for more studies that determine the mechanisms that drive the increased death risk in these patients.

Her team has received much interest in their study and is planning follow-up research.

— Fauntleroy is a freelance writer in Carmel, Ind., and a regular contributor to Endocrine News.

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