An Underactive Thyroid Carries Cardiac Risks

Summarized by Robert W. Griffith, MD
December 19, 2005

Introduction

If you've got a thyroid gland condition that causes symptoms, you are at greater risk of cardiovascular disease of one sort or another. But many people may have a thyroid gland that is functioning poorly, but not producing symptoms; this is known as subclinical hypothyroidism. Not surprisingly, doctors want to know if subclinical hypothyroidism is also linked to cardiac problems. Two recently published studies - one done in Australia, the other in Europe and the USA - provide answers, of sorts. They are to be found in the Archives of Internal Medicine, and we summarized them here.

The Australian Study

Stored serum samples collected in 1981 from 2,100 people were analyzed for serum thyroxine and serum thyrotropin (or thyroid stimulating hormone, TSH) levels. Based on the results, the subjects were classified as having subclinical hypothyroidism if their TSH was reduced but their thyroxine was normal. There were 119 of the participants (5.5%) who were diagnosed as having subclinical hypothyroidism. These subjects had a significantly higher frequency of coronary heart disease than the people without thyroid problems - 1.8 times more, in fact.

The subjects were followed up to 2001 to determine deaths from cardiovascular disorders, or heart attack, unstable angina, and so on. In those subjects who had subclinical hypothyroidism at baseline (a total of 101) there were 21 cardiovascular deaths, compared with 9.5 that were 'expected', based on known age, sex, and other risk factors. This represents a 1½ times increased likelihood of death. With regard to coronary artery events, there were 33 in the under-active thyroid group, which was more than double the expected number, i.e. an increased likelihood of 1.7 times.

The European-USA Study

There were 2,730 men and women aged 70 to 79 in this study, where TSH levels were measured initially and then the subjects were followed for 4 years. The aim was to see if subclinical hypothyroidism (TSH 4.5 mIU/L or below) was associated with congestive hear failure, coronary heart disease, stroke, peripheral arterial disease, and cardiac or all-cause mortality.

Overall, 12.4% of the subjects had hypothyroidism. They were classified according to their TSH levels: 4.5 to 6.9, 7.0 to 9.9, and 10 mIU/L or above. Congestive heart failure was diagnosed more frequently in those subjects with TSH levels of 7.0 or above; for levels between 7.0 and 9.9 the risk was increased 2½ times, and at levels over 10 mIU/L the risk was 3.25 times greater.

No association between subclinical hypothyroidism and an increased risk for coronary heart disease, stroke, peripheral artery disease, or cardiovascular or total mortality was established in this study.

Reconciling the Studies' Results

The Australian study found that coronary artery disease and mortality was increased in subclinical hypothyroidism, while the European-USA study failed to confirm this; on the other hand, the second study found that congestive heart failure was increased. Several earlier studies have failed to find any link between subclinical hypothyroidism and cardiovascular disease, but overt hypothyroidism does have clear-cut links. So these two large studies are important, even though their results appear to be conflicting.

Taken together, the two results suggest that the frequency of some cardiovascular disorders may be increased in people with subclinical hypothyroidism, especially when the TSH is above 10 mIU/L. This makes it reasonable to treat subclinical cases with thyroxine if the TSH is over 10 mIU/L, whereas subjects with a TSH between 7.0 and 10.0 should be screened carefully for cardiovascular risk factors, and have these addressed appropriately.

Source

• Walsh JP, Bremner AP, Bulsara MK, et al. Subclinical thyroid dysfunction as a risk factor for cardiovascular disease. Arch Intern Med 2005165:2467-2472 Rodondi N, Newman AB, Vittinghoff E, et al. Subclinical hypothyroidism and the risk of heart failur

Related Links
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Subclinical Hypothyroidism: Deciding When to Treat

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