ATA: Mildly Underactive Thyroid Increases Heart Failure Risk

NEW YORK, Oct. 8 -- Subclinical hypothyroidism may modestly increase the risk of heart failure, researchers reported here.Asymptomatic patients with thyroid stimulating hormone levels of 10 mU/L or higher were at almost twice the risk of heart failure as patients without subclinical hypothyroidism, found a large cohort study presented at the American Thyroid Association meeting.
This risk is comparable to that from diabetes and hypertension, both known heart failure risk factors, said Doug Bauer, M.D., of the University of California at San Francisco.
The study excluded patients who were taking thyroid hormones, but Dr. Bauer said the results suggested that treating subclinical hypothyroidism could reduce heart failure risk.
"When we included people treated during follow-up," he said, "the risk of heart failure was attenuated compared with those not treated."
However, he cautioned that the study was not designed to answer that question, and further study is needed.
"People who have heart failure should be evaluated to make sure that their thyroid tests, specifically their TSH, are normal," Dr. Bauer added.
Previous studies linked overt hyperthyroidism and hypothyroidism to heart problems. The Health Aging and Body Composition (Health ABC) study had shown a similar relationship for subclinical hypothyroidism, but without confirmation of the heart abnormalities on echocardiogram.
So Dr. Bauer's group analyzed echocardiographic heart function changes among 3,065 patients 65 and older in the Cardiovascular Health Study who were initially free of heart failure.
During 12 years of follow-up, 660 participants (22%) had adjudicated heart failure events, 16% had subclinical hypothyroidism (TSH levels 4.5 mU/L or higher but normal free thyroxine levels), and 1.4% had subclinical hyperthyroidism (TSH less than 0.45 mU/L).
After controlling for cardiovascular risk factors, including hypertension and diabetes, subclinical hypothyroid patients with TSH levels in the higher range (10 mU/L or greater) were at a modestly elevated risk of heart failure compared with participants with normal thyroid function (hazard ratio 1.88, 95% confidence interval 1.05 to 3.34, P=0.003).
However, subclinical hypothyroid patients with TSH levels in the lower range (4.5 to 9.9 mU/L) were no more likely than euthyroid participants to develop heart failure in the multivariate analysis (HR 0.92, 95% CI 0.73 to 1.17).
Subclinical hyperthyroidism was not associated with more heart failure events than normal thyroid function (HR 0.94, 95% CI 0.48 to 1.83).
Although most baseline echocardiographic parameters were not correlated with thyroid status, patients with a TSH of at least 10.0 mU/L had a higher peak E velocity than those without subclinical hypothyroidism (0.80 versus 0.72 m/sec, P=0.002), which suggested decreased left ventricular compliance, Dr. Bauer said. This was associated with incident heart failure in the overall cohort (P<0.001).
However, changes in echocardiographic parameters during six years of follow-up for this measure were not significantly associated with thyroid status.
The mechanism for an association between heart failure and thyroid function has been fairly well studied and it is likely that "subclinical hypothyroid state results in a less efficient contraction of the heart," Dr. Bauer said. Another possible explanation could include fluid retention, he said.
If the findings are confirmed, "it also suggests the potential that we might screen for abnormal thyroid tests to prevent people from developing heart failure in the future," he concluded.
Dr. Bauer and colleagues reported no conflicts of interest. Primary source: American Thyroid Association meetingSource reference: Rodondi N, et al "Subclinical Thyroid Dysfunction, Cardiac Function and the Risk of Congestive Heart Failure: The Cardiovascular Health Study" ATA meeting 2007; Program number 92.