WEDNESDAY, May 22 (HealthDay News) -- People with heart failure are more likely to experience poorer health from having a thyroid gland that is even mildly underactive, according to a new study.

And among black patients, the researchers found an increased risk of death linked to the condition, which is known as hypothyroidism.

"This study is the first to show that African-Americans who have hypothyroidism face a greater risk of death than patients of other racial and ethnic groups," Dr. Connie Rhee, of Brigham and Women's Hospital in Boston, said in a news release from the Endocrine Society. "This elevated risk exists despite the fact that hypothyroidism is less common in the African-American population compared to other groups."

"More research is needed to confirm these findings and to determine the underlying reasons why hypothyroidism has a differential impact on people of different races and ethnicities," Rhee said.

Hypothyroidism occurs when the thyroid doesn't produce enough hormones. The researchers noted that people with mild forms of the condition have thyroid function at the low end of the normal range.

In conducting the study, Rhee and her colleagues analyzed information on about 750 patients with hypothyroidism, including nearly 700 people with a mild form of the condition.

Although it's unclear why hypothyroidism has a different effect on people of various races and ethnicities, the study authors said doctors should not take a blanket approach to assessing people's risk for the condition and determining whether they require treatment.

"Our data suggest that mild hypothyroidism may, in fact, be harmful in specific populations, including people with heart failure," Rhee said in the news release. "A one-size-fits-all approach may not be appropriate for assessing risk and determining whether treatment is required for [mild] hypothyroidism."

More than 9.5 million people in the United States have hypothyroidism, the study authors said.

The study is scheduled for publication in the June issue of the Journal of Clinical Endocrinology & Metabolism.

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Underactive Thyroid and Heart Failure a Bad Combination: Study