The small butterfly-shaped endocrine gland at the bottom of your throat, otherwise known as the thyroid, is responsible for more of your body's functioning than you may think, starting with your metabolism. When your thyroid doesn't function properly it can cause a range of symptoms such as irritability, dry skin, insomnia and even forgetfulness -- making it tricky to diagnose.

Thyroid disease encompasses several disorders, but hypothyroidism and hyperthyroidism are the two most common. Hypothyroidism means you have an underactive thyroid gland that can't produce enough hormones to keep the body running properly, while hyperthyroidism means you have an overactive thyroid with too much thyroid hormone, sending certain bodily functions into overdrive.

We talked with Khalil Afsh, MD, internist at Orange Park Medical Center in Jacksonville, Florida to learn more about this important gland and its function.

Thyroid disease is more common than you think

Thyroid disease affects some 20 million Americans, and 60 percent of those affected don't even know they have it. If thyroid disease isn't diagnosed, it can lead to more serious problems such as cardiovascular disease, osteoporosis and infertility. While thyroid conditions are normally life-long issues, medical treatments can help you manage the symptoms so they don't develop into something more serious.

All symptoms are not the same

While all hypothyroidism and hyperthyroidism involve the same endocrine gland, their symptoms are completely different.

Afsh says hypothyroidism is by far the most common thyroid disorder in the adult population, and most common in older women. "When we talk about hypothyroidism, production of the hormone has been halted," he says. "Patients become fatigued to the point of sleeping a lot." Weight gain is a classic and vexing  problem. As the body's metabolic function slows down, other symptoms such as getting cold more easily, drier skin, becoming forgetful, depressed or constipated may occur.

Hyperthyroidism symptoms are the direct opposite, and sometimes they come on quickly. "Something abrupt happens in the gland, releasing a lot of the hormone into the bloodstream, which is a reflection of a high metabolic rate," says Afsh. "Patients come in with unusual irritability and heart palpitations, insomnia, losing weight, diarrhea."

The causes vary

Hashimoto's disease, a type of inflammation of the thyroid gland, is the most common cause of hypothyroidism in the US. This autoimmune disorder attacks the thyroid specifically, causing inflammation and interference with thyroid hormone production.

"Graves' disease is the most common cause of hyperthyroidism," says Afsh. Graves' disease, another autoimmune disorder, develops when antibodies called thyroid-stimulating immunoglobulin (TSI) attach to thyroid cells and stimulate them to produce too much thyroid hormone

See your doctor if you have symptoms

Who doesn't feel fatigued or irritable at times, or suffer from a bout of constipation or diarrhea? This is where diagnosis can become tricky. "All these symptoms can be so many different things," says Afsh. "People don't even think of the thyroid as a potential cause." When you do experience symptoms, "see your doctor right away," says Afsh. "You should always be reminding your physician to check your thyroid."

This content originally appeared on Sharecare.com.