Hypothyroidism has been shown to be a risk factor for cardiovascular disease. Now, research appearing in the Annals of Internal Medicine (2000; 132(4):270-8) shows that subclinical hypothyroidism and thyroid autoimmunity can also increase the risk of heart disease. In subclinical hypothyroidism, patients do not have the symptoms of hypothyroidism, but their blood tests indicate an underfunctioning thyroid. Women in the study had a 70% higher chance of having hardening of the aorta (the largest artery in the body), and more than two times the risk for a heart attack than the group with normal thyroid hormone levels. It is estimated that 17% of all older Americans have subclinical hypothyroidism.

In the study, even after statistically adjusting for all the other factors affecting heart disease risk — including weight, smoking, cholesterol levels, and blood pressure — women with hypothyroidism were 70% more likely to have hardened aortas than those with normal hormone activity. They also had more than twice the risk of heart attack. Having autoimmune hypothyroidism increased the risk even further. The researchers concluded that subclinical hypothyroidism is a strong indicator of risk for atherosclerosis and myocardial infarction in elderly women.