9 Things You Need to Know About Your Thyroid

Your thyroid pumps out the key hormones triiodothyronine (T3) and thyroxine (T4), which are partially composed of iodine. Here's how the gland affect your well-being, head to toe.

Your heart: Thyroid hormones influence your heart rate and help control blood flow by relaxing the muscles in the walls of your blood vessels.

Your fertility: Thyroid hormones influence your menstrual cycle. When they're out of whack, you might have irregular ovulation and periods.

Your bones: The rate at which old bone is broken down is driven by thyroid hormones; when that process speeds up, bone is destroyed faster than it can be replaced—which can lead to osteoporosis.

Your weight: Because the thyroid regulates your metabolism (how quickly your body burns through fuel), an overactive thyroid—or hyperthyroidism—is linked to weight loss, while hypothyroidism is linked to weight gain.

Your brain: Low thyroid levels can bring on forgetfulness, difficulty concentrating, and depression. Luckily, these symptoms tend to reverse with synthetic hormone treatment.

Your skin: When your thyroid is underactive, your body stops making and shedding skin cells at its normal pace. Cells build up, causing dry, dull-looking skin. (Hair and nail growth slow down as well.)

We are five to eight times as likely to have thyroid problems as men. But why that's the case remains a mystery. "The key suspect is estrogen," says Dr. Nasr. Thyroid cells have a large concentration of estrogen receptors, he explains, which means they're extra sensitive to the effects of the female sex hormone. Another possible reason: Many causes of hypo- and hyperthyroidism are related to autoimmune diseases, and women are generally more prone to those disorders, explains M. Regina Castro, MD, an endocrinologist at the Mayo Clinic. The most common cause of hypothyroidism, for example, is Hashimoto's disease (which is about seven times as prevalent in women), and the most common cause of hyperthyroidism is Graves' disease (which is up to 10 times as prevalent in women).

Both hypothyroidism and hyperthyroidism can be challenging to detect because the symptoms tend to be vague and common to other ailments. But if you notice more than two of the signs below, talk to your doctor about having your thyroid tested.

Hypothyroidism: Dry skin and hair, forgetfulness, constipation, a rundown feeling, muscle cramps, unexplained weight gain, heavier/irregular menstrual flow, swelling in the face, heightened sensitivity to cold.

Hyperthyroidism: Irritability, increased perspiration, racing heartbeat, difficulty sleeping, frequent BMs, unexplained weight loss, less frequent and lighter periods, bulging eyes, shaky hands.

About 30 percent of people taking thyroid medications for hypothyroidism may not need them, suggests a U.K. study. Many of these folks have what's dubbed "subclinical hypothyroidism." It means that their levels of TSH are mildly elevated, but their levels of T3 and T4 are normal, and they have few, if any, symptoms. (TSH is a pituitary hormone; high levels indicate your body is trying to stimulate your thyroid to produce more hormones.) If your TSH test reveals a level of 10 mIU/L or higher, most experts recommend treatment. But if your level is in the range of roughly 4 to 10mIU/L, consider your symptoms. "If you're not having symptoms, thyroid medication won't help, and it could result in overtreatment and increase your risk of heart palpitations and other symptoms of hyperthyroidism," says Dr. Castro. If you are treated, your doctor should monitor you with blood tests after 6 to 12 weeks to make sure the meds don't cause hyperthyroidism.

Since 1975, the incidence of thyroid cancer has nearly tripled, while the death rate has remained low and stable. "There's an overdiagnosis of thyroid cancer going on, and it's a real mess," says Otis Brawley, MD, chief medical officer at the American Cancer Society. These days, more and more women are getting scans of their heads, necks, and chests to diagnose other health problems, and those highly detailed images are revealing small cancers in the thyroid that would have otherwise gone unnoticed. "Most of these cancers don't need treatment, because they grow slowly or not at all," says Dr. Brawley, also pointing out that surgery carries its own risks.

Typically, if your tumor is smaller than 1 centimeter, monitoring it is best, says Michael Tuttle, MD, a thyroid cancer expert at Memorial Slowan Kettering Cancer Center in New York City. Get another opinion if your doc doesn't discuss this option.

"One of the most common questions I hear is 'I'm gaining weight. Could it be my thyroid?'" says endocrinologist Dorothy Fink, MD, assistant professor of medicine at NYU Langone Medical Center. It's true that if you're hypothyroid, you may see a change on the scale. But the condition typically comes with other symptoms, too, such as fatigue, constipation, and irregular periods. And even if you are experiencing multiple signs, your doc will likely also consider a hormone dysfunction (such as polycystic ovary syndrome) or perimenopause, as well as unhealthy habits (such as skimping on exercise or sleep). Ideally, says Dr. Fink, your doc would analyze these factors in addition to ordering a blood test for thyroid disease.

You've probably seen supplements claiming to offer "thyroid support." Don't believe the hype: When Mayo Clinic researchers analyzed popular pills available online, they discovered that even those billed as herb-based contained varying amounts of actual thyroid hormones. Even scarier, the doses of hormones found in some of these products were higher than those in prescription meds. "A large dose can cause dangerous side effects, such as arrhythmias and problems with your bones," says Dr. Fink.

Try this simple test from the American Association of Clinical Endocrinologists to see if you have a telltale sign of thyroid disease.

1. Use a handheld mirror to look at the lower front area of your neck, above your collarbones and below your voice box.

2. Tip your head back, take a sip of water, and watch for protrusions in that area.

3. If you notice a bulge, call your doc. You may have a thyroid nodule or enlarged thyroid (an indication of hypo- or hyperthyroidism).