ScienceDaily (Oct. 9, 2007) — Individuals with subclinical hypothyroidism—a mildly underactive thyroid only detectable by a blood test—are twice as likely to develop heart failure, compared to those with normal thyroid levels, according to a new study. Heart failure, also called congestive heart failure, is when the heart can’t pump enough blood to the body’s other organs, which can cause fatigue, ankle swelling and shortness of breath.
Although previous studies have shown that hyperthyroidism—an overactive thyroid—and hypothyroidism can cause heart problems, this is the first time that a large study found a negative effect on heart function when the thyroid was only mildly under-active.
“If other studies confirm these findings, then physicians might want to consider treating mild thyroid problems to prevent potential cardiac problems or to avoid increasing the severity of an existing heart condition,” said Doug Bauer, M.D., an author of the study and a Professor of Medicine, Epidemiology and Biostatistics at the University of California at San Francisco School of Medicine in San Francisco.
Subclinical thyroid disorders are detected by a blood test that evaluates the levels of thyroid-stimulating hormone (TSH). Subclinical hypothyroidism is defined by TSH levels greater than 4.5 mU/L and normal free thyroxine levels. Individuals with subclinical hypothyroidism can evolve into overt hypothyroidism, where the free thyroxine levels fall below normal, which always requires thyroid hormone therapy.
The Cardiovascular Health Study involved over 3,000 adults 65 years and older, who were evaluated to determine if those individuals who had subclinical hypothyroidism had an increased risk of developing heart failure over a twelve-year period. The study shows that individuals who had a TSH level equal or greater than 10 mU/L had a two-fold risk of developing heart failure, compared to those who had normal thyroid levels.
This research was presented on Oct. 4, at the 78th Annual Meeting of the American Thyroid Association (ATA) in New York.