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 Alan Farwell, MD, chairman of patient education and advocacy for the American Thyroid Association. The result: decreased endurance and a sluggish mind.
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.