Perhaps you're much more tired than usual. You're chilly even in warm weather. Your skin is dry and globs of moisturizer aren't helping. You're grouchier than normal. All of these problems could be caused by stress or aging. But if you've been feeling bad for more than a couple of weeks, the culprit might be your thyroid, a butterfly-shape gland in the lower front of your neck. Most of the time, you don't know your thyroid is there--and that's as it should be. But when something goes wrong, you're in trouble, because the thyroid is like an air-traffic controller for your body, helping to regulate metabolism, body weight and temperature. Your brain needs thyroid hormone, called thyroxine or T4, to develop and function properly. Your heart can't beat properly without it. And your liver needs thyroid hormone to process cholesterol.

For reasons doctors still don't understand, younger women are much more likely to have thyroid disorders than men. However, as men get older, they are more prone to thyroid problems. The most common problem is hypothyroidism--too little thyroid hormone. It affects about one in 10 people and causes symptoms such as fatigue or feeling cold a lot. Women are particularly vulnerable to hypothyroidism during pregnancy, after delivery and around the time of menopause. The other extreme, hyperthyroidism, or too much thyroid hormone, can also make its first appearance in women around pregnancy. It can cause a racing heartbeat, unexplained weight loss, insomnia and anxiety. And then there's another problem: growths in the thyroid gland called nodules can sometimes be cancerous.

Of these three, hypothyroidism can be the most difficult to detect without a blood test, says Paul Ladenson, director of the Division of Endocrinology and Metabolism at Johns Hopkins. Consider some of the symptoms--fatigue, unexplained weight gain, dry skin, hair loss, slower thinking and depressed mood. All could have many causes, and if your doctor isn't thinking about your thyroid, he or she could easily miss the real explanation. In recent years, doctors have begun to suspect that even mild hypothyroidism may increase a woman's risk for heart disease, which makes diagnosis more important. Recently, Ladenson had a patient who entered the hospital for a non-thyroid-related issue. A routine blood test before surgery showed she had severe hypothyroidism, he says. "And this was a woman who was out on a bikeathon last week," he adds. Now she'll get treatment to help prevent problems from a disease that might have gone undetected and untreated for years because of the lack of obvious symptoms.

Screening consists of a simple, inexpensive blood test of the amount of thyroid-stimulating hormone (TSH) released by your pituitary gland. The test takes advantage of the delicate relationship between the thyroid and the pituitary gland. When your thyroid isn't producing as much thyroxine as your body needs, the pituitary cracks the whip by releasing TSH. (It works the opposite way when the thyroid is cranking out too much--the TSH drops.)

When the TSH level is very high, it's probable that a woman should be treated with synthetic thyroid hormone to replace what her body is missing. But what about mild hypothyroidism? In the past, if your TSH was somewhat high but you still had normal levels of thyroid hormone, doctors might have just monitored the situation with follow-up tests. Some doctors still think that's the best course, but now, with evidence that even a mild problem could lead to increased risk for heart disease, others are arguing for more aggressive intervention. "If hypothyroidism is silently harming women, and some of that harm is not going to be reversible by treating them 10 years from now, when they really get overtly hypothyroid, then you should act," says Ladenson. He helped write American Thyroid Association guidelines that recommend testing the TSH of both men and women after age 35. Since other major medical organizations disagree, your best course is to talk to your doctor if you think your symptoms might be caused by thyroid disease.

In this country, the most common cause of hypothyroidism is a condition called Hashimoto's thyroiditis. It's an autoimmune disease--your immune system is basically turning on you and destroying your thyroid. Autoimmune diseases run in families and are more common in women than men, another reason why thyroid disease may be more of a women's problem. Other examples of autoimmune diseases are rheumatoid arthritis and lupus. If members of your family have any of these conditions, you are more likely to have a thyroid disease, which is another reason to push for a blood test.