High cholesterol is usually tied to lifestyle, genetic factors, or a combination of the two. But there are other causes. One explanation for high cholesterol that isn’t attributable to lifestyle is hypothyroidism, or an underactive thyroid gland. Now comes research that even a slightly underactive thyroid, not bad enough for the problem to be called hypothyroidism, also poses a major heart disease risk. This condition is known as subclinical hypothyroidism.

Between 1990 and 1993, Dutch researchers evaluated women to see if they had blockages in their aortas (the large blood vessel leading from the heart to the rest of the body) or any evidence of a past heart attack. At the same time they collected blood tests of thyroid function. The study found that almost 11 percent of the women had subclinical hypothyroidism when they began the study, and those women were almost twice as likely as the women with no thyroid problems to have blockages in their aortas. They were also twice as likely to have had heart attacks.

The connection is probably related to the role the thyroid plays in controlling

metabolism. If the gland doesn’t produce enough thyroid hormone, metabolism can slow, reducing the body’s ability to clear cholesterol from the bloodstream. (If your metabolism goes into slow motion, cholesterol hangs around longer, so it’s more likely to oxidize and cause damage to your arteries.) 

New research finds that even a slightly under active thyroid poses a major heart disease risk.

There may also be a connection between hypothyroidism and homo cysteine levels; recent studies have found elevated levels of homocysteine in people with hypothyroidism. While hypothyroidism has a fairly recognizable constellation of symptoms—weight gain, fatigue, depression, hair loss, muscle and joint pain–subclinical hypothyroidism is more insidious. Symptoms are rarely obvious, and doctors often don’t pick up on it because you may have normal test results.

By the Numbers

The gold-standard test for thyroid conditions measures a hormone made in the pituitary gland called thyrotropin-stimulating hormone (TSH). TSH regulates the amount of thyroid hormone made and released by the thyroid gland. The pituitary is in charge of this system, so when the TSH level is normal, the pituitary is satisfied with the thyroid’s function. Other tests, called free T4 and T3, measure levels of the hormone itself. Typically, an elevated TSH level would lead your doctor to test your free T4 and T3 levels. Although there is controversy as to what is “normal” TSH and what is subclinical hypothyroidism, “it’s generally accepted that subclinical hypothyroidism is a TSH greater than 5, with normal free T4 tests and T3,” says Hossein Gharib, M.D., president of the American Association of Clinical Endocrinologists.

Some researchers believe that a TSH greater than 3, Dr. Gharib says, is already on the high side. A TSH greater than 5 plus a low free T4 is hypothyroidism.

Subclinical hypothyroidism occurs in about 15 percent of women over the age of 60 and in about 8 percent of elderly men (although since the condition is less studied in men, this latter figure may actually be higher).

What You Can Do

Before your doctor starts you on any cholesterol-lowering drug, ask about being tested for hypothyroidism. Once the condition is treated and TSH levels return to normal, the majority of patients show an estimated 20 to 30 percent reduction in cholesterol levels. The recommended treatment, even for subclinical hypothyroidism, is the synthetic thyroid hormone levothyroxine, sold under various brand names, including Synthroid.

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