Harvard Women's Health Watch

Do you need a thyroid test?

Undetected low levels of thyroid hormone may underlie subtle changes that can increase your risk of cardiovascular disease.

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For a gland only two inches in size, the thyroid has a huge influence on our health. It produces a hormone that is carried in the bloodstream to all parts of the body. Thyroid hormone plays a major role in regulating metabolism—the process by which body cells convert nutrients into energy—and thereby helps regulate body temperature, heart rate, and even brain function. So when thyroid hormone levels fall, the body slows.

You're probably aware of the common symptoms of low thyroid hormone—fatigue, fuzzy-headedness, weight gain, cold hands, and dry skin. But if you're like most of us, you're likely to blame your diet and exercise regimen instead of your thyroid if your cholesterol levels and weight are creeping up, especially if you don't have any of the other symptoms of low thyroid. "Symptoms are often nonspecific, and since women over 60 generally have more of these nonspecific symptoms, their doctors may not think to test for hypothyroidism," says endocrinologist Dr. Jeffrey Garber, associate professor of medicine at Harvard Medical School and author of The Harvard Medical School Guide to Overcoming Thyroid Problems.

How the thyroid works

Just as the thyroid gland communicates with other organs through the hormone it produces, the pituitary gland in the brain communicates with the thyroid through a hormone it makes—thyroid-stimulating hormone, or TSH. When the pituitary senses that thyroid hormone levels are too low, it releases more TSH to coax the thyroid into action. When the thyroid is nudged by TSH, it produces thyroid hormone—a large proportion of which is thyroxine (T4) and a smaller proportion triiodothyronine (T3). The T4 is eventually converted into T3, the "active" form that is taken up by receptors in body cells.

Thyroid blood test results



Thyroid levels

0.4-5–4.12 mIU/L

0.8–2.0 ng/dL


>4.12 mIU/L

0.8–2.0 ng/dL

Subclinical hypothyroidism

>4.12 mIU/L

<0.8 ng/dL


Source: Subclinical Thyroid Disease: Scientific Review and Guidelines for Diagnosis and Management (Consensus Panel Recommendation).

Why you may need a thyroid test

Women of all ages are more likely than men to have low thyroid hormone levels. By some estimates, almost one-fourth of women over 60 have inadequate levels. However, many of their symptoms are attributed to other conditions or written off as a consequence of aging.

A blood test for levels of TSH and of the portion of T4 that is available to the cells (also called "free T4") can determine whether you have clinical or subclinical hypothyroidism.

As many as 60% of people with hypothyroidism or subclinical hypothyroidism aren't aware anything is wrong because they haven't been tested. The U.S. Preventive Services Task Force (USPSTF) doesn't recommend thyroid screening because it hasn't found sufficient evidence that testing thyroid hormone levels in large groups of people without symptoms is cost-effective. Dr. Garber agrees, and suggests a different approach—testing asymptomatic people who are most likely to develop thyroid disease and benefit from treatment. In a report in the Aug. 18, 2015, issue of Annals of Internal Medicine, he and colleagues from the American Association of Clinical Endocrinologists cite evidence that treating even subclinical hypothyroidism with synthetic thyroid hormone reduces the risk of developing more serious problems like cardiovascular disease. They note that low thyroid hormone is often the cause of high cholesterol and suggest that treatment with thyroid hormone may make statin therapy unnecessary.

What you can do

If you have definite symptoms of hypothyroidism, you should talk to your clinician about being tested. If your LDL cholesterol has been advancing or your weight has been creeping up unexplainably, you'll want to discuss getting a test. If you're 60 or older and generally healthy, it's still a good idea to check with your doctor to see whether your medical history suggests you might benefit from testing.