(OPRAH.com) -- The bounce in your step has become a plod. Climbing stairs feels like summiting Mount Everest. Your brain's mired in fog. Whatever your personal energy crisis might be, it's time to act.

"Fatigue that's new or dramatic, prolonged or unexplained can signal a serious medical problem like heart disease or anemia," says Dr. Lori Mosca, M.D., director of preventive cardiology at New York-Presbyterian Hospital. "A lot of women hesitate to tell their doctors about low energy. But if you're feeling any new kind of tired, speak up."

Your physician should check for the usual suspects, including insomnia, stress, sleep apnea, depression, and diabetes. If she rules those out, ask her to probe deeper for these often overlooked causes.

Located at the base of your neck -- and barely larger than the knot in a bow tie -- the thyroid gland controls your body's metabolic speed by producing the hormones T4 and T3. If it churns out too little -- as may happen in 12 to 15 percent of women at midlife -- "all the processes in your body slow down," says Dr. Alan Farwell, M.D., chairman of patient education and advocacy for the American Thyroid Association. The result: decreased endurance and a sluggish mind.

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Why It's Overlooked: "Some people are very sensitive to small changes in thyroid hormones," Farwell says, "even when their numbers aren't low enough to qualify for treatment. After age 70, 25 percent of women may have subclinical or mild hypothyroidism."

Other Symptoms: Weight gain. Feeling cold. Constipation. Dry skin and hair. Depression.

Tests: You'll get blood checks for levels of T4 and thyroid-stimulating hormone (TSH). High TSH plus low T4 is a sign of full-blown hypothyroidism, but pay attention if your TSH is high and your T4 is normal -- you may have mild hypothyroidism, which should still be treated.

Treatment: Synthetic thyroxine pills.

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Fatigue is a distinct characteristic of cardiovascular disease in women, according to recent research. In one study of 515 female heart attack survivors, 70 percent reported unusual fatigue in the weeks before; just 57 percent had acute chest pain. In another study, fatigue was a symptom for women with dangerously clogged arteries that escaped notice on heart scans.

Why It's Overlooked: Only one in 10 women realizes that heart disease is her biggest health threat. And emergency room doctors are six times more likely to give women with serious heart problems (as opposed to men) a clean bill of health.

Other Symptoms: Shortness of breath. Indigestion. Pain in your shoulder, arm, or jaw. But for many women, nothing at all.

Tests: Your doctor will order an exercise stress test or angiogram if she suspects clogged arteries in your heart. Because that test isn't always accurate in women, she may order a CT scan or echocardiogram as well. She'll also test your cholesterol, blood pressure, and blood sugar -- diabetes can quadruple a woman's heart risk.

Treatment: You may get a cholesterol-lowering statin and medicines to treat blood pressure, such as diuretics. You'll also be advised to follow a heart-healthy diet and get regular exercise.

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Heavy menstrual periods, pregnancy, avoidance of red meat, or being a vegetarian or a serious distance runner can all drain your body's stores of energizing iron, leaving you anemic.

That means underpowered red blood cells can't deliver enough oxygen to every cell in your body, says iron researcher Rebecca J. Stoltzfus, Ph.D., professor of nutrition at Cornell University. Your muscles may feel weak, your thinking gets cloudy. One in five women and half of all pregnant women may have low iron.

Why It's Overlooked: In a blood test, having numbers at the low end of normal can mean you won't get treatment even though it could help. Studies show that treating anemia with iron supplements under a doctor's care can relieve fatigue. (Never take iron supplements on your own -- about one in 200 people have a genetic condition in which their bodies can't process iron, leaving them vulnerable to overdose.)

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Other Symptoms: Abnormally pale skin. A fast heartbeat. Irritability.

Tests: Hemoglobin and hematocrit screens will reveal the levels of iron-rich, oxygen-carrying hemoglobin in your blood and the number of red blood cells. Your doctor should also look at the size and color of your red blood cells (small and pale could mean trouble). Ask about a serum ferritin test, too -- it measures a protein that helps store iron.

Treatment: Expect to get a short course of iron supplements and advice to follow an iron-rich diet. "Your doctor will retest your blood in one to two months. If your iron levels are rising, you'll continue on the same plan for a few more months to rebuild your reserves," says Dr. Matthew Heeney, M.D., director of the Hematology Clinic, Children's Hospital Boston. "If levels are still low, your doctor will look for other causes." These can include mild but chronic internal bleeding or compromised blood cell production.

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If you ever experimented with IV drugs (even once), snorted cocaine through a straw, had a blood transfusion before 1992, or have ever received abnormal results on a liver function test, you need a hepatitis C test -- whether or not you're feeling tired.

"Most of my patients are pillars of the community today -- the things they did in their youth are well behind them," says liver disease expert Dr. Michael W. Fried, M.D., director of hepatology at the University of North Carolina at Chapel Hill. "Once you're feeling persistently tired, liver damage may have begun."

Why It's Overlooked: "Many family doctors won't ask about risky behavior in your distant past -- especially if you don't look the part now," Fried says. "If you're feeling tired and know you have risk factors, tell your doctor so you can be tested."

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Other Symptoms: Jaundice, diminished appetite, fever, aches, and other flulike symptoms may occur soon after infection. But often the only symptom is fatigue 20 or 30 years later.

Tests: First, a blood check for hepatitis C antibodies. If results are positive, further testing includes a liver enzyme check and possibly a liver biopsy to assess liver health.

Treatment: Weekly injections of peginterferon plus daily ribavirin capsules. This combination can cure about 50 percent of hepatitis C, Fried says.

As a reminder, always consult your doctor for medical advice and treatment before starting any program.

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