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Recognize this scenario? You diet and exercise, but your waistline stays the same — or worse, it steadily expands. It might not be your fault. You could be suffering from hypothyroidism, also known as underactive thyroid.

Because thyroid hormones regulate many different systems in the body, including growth and metabolism, deficiency causes a broad spectrum of symptoms, most commonly — but not limited to — depression, irregular menstrual cycles and fertility problems, weight gain, fatigue and chronic constipation.

The condition has several causes. Adults most commonly suffer from Hashimoto's thyroiditis, an autoimmune response in which antibodies mistakenly attack thyroid tissue, eventually destroying the butterfly-shaped thyroid gland, which sits at the front of the neck.

Affected Americans

According to the American Thyroid Association, 2 percent to 3 percent of Americans have severe hypothyroidism, and as many as 10 percent to 15 percent have mild hypothyroidism.

Joanna Branham, 54, was diagnosed with the condition when she was 32.

"Before I was diagnosed, I was very tired, and I began to think something very serious was wrong with me, but once I got on medication it made a huge difference," Branham says. "The fatigue went away after about six weeks."

Branham suffered from depression and weight gain.

As with other autoimmune diseases, the condition affects more women (post-menopausal women are most vulnerable) than it does men, although "as more men live into their 70s and 80s, their chance of developing the condition increases," says Dr. Gregory Brent.

Brent is professor of medicine and physiology at the David Geffen School of Medicine, at the University of California, Los Angeles. He attributes the rise in hypothyroidism cases to growing awareness and increased testing. Standard treatment is thyroid hormone replacement therapy with thyroxine (T4).

Cindy Heroux, a registered dietitian and author of "The Manual That Should Have Come with Your Body," advocates replacement therapy but says three dietary modifications are also key.

Keeping cholesterol in check

Because the condition causes cholesterol levels to rise, Heroux recommends a diet that keeps cholesterol in check, cuts back on calories and increases fiber to "combat constipation."

But be careful which fiber you consume. Most grains contain gluten, and research shows a higher incidence of the auto-immune disease Celiac's (also known as gluten intolerance or sensitivity) in patients with Hashimoto's. Brent says Celiac's interferes with T4 absorption. So does the consumption of soy.

Some food tips if you suffer from hypothyroidism: Limit intake of cruciferous vegetables such as broccoli, cauliflower, brussels sprouts and cabbage. They contain goitrogens, which suppress the thyroid. And remember, fruit is your friend.

Certain vitamin and mineral supplements might enhance the results of thyroid-replacement therapy, but consult your dietitian or nutritionist.