By Judy Foreman, Globe Staff, 03/14/00

Most Americans know by now that eating a diet high in saturated fat can raise cholesterol, a major risk factor for heart disease, which kills nearly 500,000 people a year and is the leading cause of death for both men and women.

But what many people don't know is that an underactive thyroid - the butterfly-shaped gland in the neck that produces a crucial hormone that regulates metabolism - may also contribute to high cholesterol.

When thyroid hormone levels are too low, the liver makes fewer molecules called LDL receptors, whose job is to pull LDL, or "bad" cholesterol out of the blood. The result is an increase in cholesterol levels. A low thyroid level can also mean high levels of triglycerides - fatty acids that also contribute to heart disease.

Some data suggest that only 5 percent of people with high cholesterol have an underactive thyroid but several studies put the figure as high as 14 percent, said Dr. Lewis Braverman, chief of the endocrine, diabetes and nutrition section at Boston Medical Center. An underactive thyroid is easily treatable with supplemental thyroid hormone, and when it is, cholesterol levels often return to normal.

But, while doctors routinely screen for cholesterol, many don't do simple blood tests for thyroid levels, even when an elevated cholesterol level should be a signal to do so. Nearly 100 million Americans have cholesterol levels that are either high (240 milligrams per deciliter and up) or borderline (200 to 239 mg per dl), according to the American Heart Association.

There's a strong economic argument for being more aggressive about detecting and treating potential thyroid problems in people with elevated cholesterol. Thyroid drugs such as Synthroid or Levoxyl cost less than $100 a year. By contrast, the class of cholesterol-lowering drugs called "statins," which includes Mevacor and Pravachol, can cost many times that.

But there's an even more compelling medical argument. Better detection and treatment of hypothyroidism, or underactive thyroid gland, could not only lower cholesterol and thereby reduce the risk of future heart disease, but make millions of people feel better.

An estimated 13 million Americans, many of them women over 50, suffer from hypothyroidism, though many don't know it because symptoms can be mild at first and are often chalked up to aging or "the blues." The symptoms include fatigue, forgetfulness, depression, chilliness, weight gain, and goiter (an enlarged thyroid gland), as well as elevated cholesterol.

Another 2 million Americans, many of them women aged 20 to 40, have the opposite problem - an overactive thyroid gland, which causes nervousness, weight loss, intolerance to heat and goiter.

Both hypothyroidism and hyperthyroidism can result from a misguided attack by the body's immune cells and antibodies on the thyroid gland - in the first case, inhibiting or destroying the gland; in the second, forcing it into high gear.

Testing for thyroid problems is fairly straightforward - a blood test for TSH, or thyroid stimulating hormone, which is made by the pituitary gland. When the pituitary gland senses that the body is not making enough thyroid hormone, TSH levels rise, signaling the thyroid gland to make more.

If a TSH test comes back abnormal, the doctor usually does another test for blood levels of thyroid hormone itself. If TSH is high and thyroid hormone levels are low, it's a clear sign that supplemental hormones are needed to get things back in balance. A person also clearly needs treatment if the reverse is true - low TSH and high thyroid levels.

But many people, especially those with "subclinical" hypothyroidism, have more ambiguous test results - typically an elevated TSH but normal levels of thyroid hormone itself. Some doctors advise treating this form of mild disease, while others hold off, arguing that when the disease is still too mild to cause symptoms, treatment may not help.

If your cholesterol is high, ask your doctor for a thyroid test if he or she hasn't suggested it already.

You may have to fight for the thyroid test, though, because some insurers balk at paying for routine thyroid screening, noted Dr. Richard A. Dickey, an endocrinologist at Wake Foreest University in Winston-Salem, N.C, and president of the American Association of Clinical Endocrinologists.

Still, it's important to find out because doctors need to "eliminate secondary causes of high cholesterol, which include low thyroid function," says Dr. David Gordon, a heart researcher at the National Heart, Lung and Blood Institute.

It also works the other way around, said Dr. Peter Wilson, an endocrinologist at the Framingham Heart Study. If you are tested for thyroid function and that is low, ask to be tested for cholesterol, too.

All content herein is © Globe Newspaper Company and may not be republished without permission. If you have questions or comments about the archives, please contact us at any time.