Many people expect weight gain and fatigue as they age, but how do you know when it could be something more serious?

More than 20 million Americans suffer from thyroid disorders, according to the American Thyroid Association. An estimated 60 percent of cases go undiagnosed, which may put patients at risk for serious conditions, such as cardiovascular diseases, osteoporosis and infertility.

The thyroid is a butterfly-shaped gland in the front of the neck that produces hormones to control the speed of your metabolism. The most common types of thyroid disorders include:

Hypothyroidism, or underactive thyroid, in which the thyroid gland doesn't produce enough hormones.

Hyperthyroidism, or overactive thyroid, in which the thyroid gland produces too much hormone.

Thyroid cancer, in which cancer develops in the thyroid gland.

Dr. Alyson Myers, an endocrinologist at North Shore-LIJ Health System in New York City, told FoxNews.com when it comes to thyroid disorders, it’s important to know what symptoms may be of concern.

Weight gain and fatigue

One of the most common and noticeable symptoms of hypothyroidism is weight gain. Myers said patients can be quick to blame an expanding waistline on an underactive thyroid, but it’s the type of weight gain that can help determine if there is a hormone problem.

“The biggest misconception about hypothyroidism is when people gain weight, they assume it’s their thyroid. But with hypothyroidism, it usually causes water weight, not stomach fat,” Myers said.

Feeling tired and sluggish can also be a sign of an underactive thyroid. Experts recommend keeping a journal of your changes in mood and energy levels to discuss with your doctor.

Weight loss

Sudden or extreme weight loss can be a sign of many serious health problems, including hyperthyroidism. When the thyroid produces too many hormones, the body’s metabolic rate increases. Myers said that leads to additional symptoms.

“Besides weight loss, hyperthyroidism can cause daily episodes of the sweats, shakes and heart racing or elevated heart rate,” she said.

According to Myers, sometimes symptoms are delayed and patients can go months before they develop physical signs of hyperthyroidism.

Neck pain

Swelling, pain, or a lump in the neck is a red flag something could be wrong with your thyroid, and could even be a sign of cancer.

“Most cases of thyroid cancer are very slow, usually subtle,” Myers said. “But in more severe cases, there is rapid growth in their neck that the patient can see.”

The five-year survival rate of people with thyroid cancer is about 98 percent.

Neck pain and swelling can also occur because of a number of other thyroid issues. Myers said if you experience neck pain or swelling, you should see your doctor, and noted sometimes these symptoms could be the result of conditions totally unrelated to the thyroid.

Being proactive about thyroid disorders

Myers said getting tested for a thyroid disorder isn’t difficult or painful—it can all be done with a blood test called TSH, or thyroid-stimulating hormone blood test. But if left untreated, thyroid issues can become fatal.

“If it is a severe case, you could die from it,” she said. “It could be life-threatening because over production or under production of thyroid hormones could cause severe effects to the heart.”

According to Myers, knowing your risk factors is crucial to avoiding and understanding thyroid complications.

“The key risk factor is family history,” she said. “Also people who have had radiation specifically to their neck from conditions like esophageal cancer, and those with autoimmune diseases like lupus are also at a higher risk.”

Concerns about thyroid disorders should be discussed with your doctor.