You can’t see it, you can’t feel it and most people have no idea what it does. In fact, most people don’t know about their thyroid unless they’ve been affected by the often-elusive symptoms of thyroid disease.

The thyroid is a 2-inch-long, butterfly-shaped gland in the front of your neck weighing less than an ounce. It’s one of the glands that make up your endocrine system. These glands produce, store and release hormones that travel through the bloodstream and direct the activity of the body’s cells.

Thyroid hormones regulate metabolism—the way the body uses energy—and affect nearly every organ in the body. They influence brain development, breathing, heart and nervous system functions, body temperature, muscle strength, skin dryness, menstrual cycles, weight and cholesterol levels.

When the thyroid gland produces more thyroid hormone than the body needs, it can cause many of the body’s functions to speed up. This problem is called hyperthyroidism. Too little thyroid hormone, called hypothyroidism, causes many of the body’s functions to slow down.

The symptoms of thyroid disease can vary from person to person. Common symptoms of hyperthyroidism are nervousness, irritability, fatigue, muscle weakness, trouble sleeping, heat intolerance, hand tremors, rapid and irregular heartbeat, frequent bowel movements or diarrhea, weight loss, mood swings and goiter, which is an enlarged thyroid that may cause your neck to look swollen.

Common symptoms of hypothyroidism are fatigue, weight gain, a puffy face, cold intolerance, joint and muscle pain, constipation, dry and thinning hair, decreased sweating, heavy or irregular menstrual periods, impaired fertility, depression and a slowed heart rate.

Women are much more likely than men to develop thyroid disease. It’s also more common among people older than age 60.

It’s particularly important if you’re a woman with hyperthyroidism to discuss your condition with your doctor before becoming pregnant. Uncontrolled hyperthyroidism raises the chance of miscarriage, preterm delivery and preeclampsia, a potentially serious complication that increases blood pressure.

Thyroid treatments aim to bring thyroid hormone levels back to normal. Treatment depends on the type of thyroid disease and its cause.

If you suspect you might have thyroid disease, talk to a health care professional. Several tests are available to help confirm a diagnosis and find its cause. The American Thyroid Association (which isn’t affiliated with NIH) recommends that adults, particularly women, have a blood test every 5 years, starting at age 35, to detect thyroid problems.