The thyroid is the butterfly-shaped gland in your neck. This gland produces hormones that regulate your body’s energy use, along with many other important functions. Hypothyroidism is when the thyroid is underactive. When thyroid hormone production drops, your body’s processes slow down and change. Hypothyroidism can affect many different systems in your body.

Hypothyroidism can affect your metabolism, mental functions, energy level, and bowel movements. Depending on how low your thyroid hormone production drops, your symptoms might be mild to severe.

Sometimes symptoms like fatigue, weakness, and constipation can be hard to tell apart from other conditions. To confirm that you have hypothyroidism, your doctor will need to do blood tests to check your thyroid hormone levels.

When you have hypothyroidism, your body makes too little of the thyroid hormones T3 and T4. These hormones control your metabolism. They affect the way your body uses energy. As a result, many of your body’s main functions change and can slow down.

Hypothyroidism slows your heart rate and weakens your heartbeat, making your heart less efficient at pumping blood out to your body. This can make you short of breath when you exercise. By narrowing your arteries, this condition can also raise blood pressure.

Hypothyroidism can lead to high cholesterol. Together, high blood pressure and high cholesterol can increase your risk for heart disease.

Untreated hypothyroidism can change how the nerves carry information to and from your brain, spinal cord, and body. This can cause a condition called peripheral neuropathy. Its symptoms include numbness, tingling, pain, or burning in the affected parts of your body.

Too little thyroid hormone weakens the muscles you use to breathe and makes your lungs work less efficiently. As a result, you may feel short of breath or have trouble exercising.

Hypothyroidism also makes it more likely to develop sleep apnea, which are pauses in breathing that happen while you sleep.

Hypothyroidism slows the movement of food through your stomach and intestines. Slowed digestion can lead to symptoms like heartburn, constipation, and bloating.

Women with hypothyroidism may have irregular periods, heavy periods, or missed periods. They can have trouble getting pregnant or be more likely to have a miscarriage if they do get pregnant.

Because too little thyroid hormone slows your body’s metabolism, it can cause general symptoms like:

  • fatigue
  • weight gain
  • cold intolerance
  • swelling of hands and feet

A lack of thyroid hormone can leave your skin dry and pale. It can also affect how you control your body temperature, making you sweat less than usual. Your hair — including the hair on your scalp and along the outer edges of your eyebrows — can become thin. Your nails may look different and become brittle.

Hypothyroidism can affect every part of your body, from your brain to your skin. Yet the condition is different in everyone. Some people have mild symptoms while others have more severe symptoms. Taking medication your doctor prescribes is important so you can manage this condition and its symptoms and maintain a healthy lifestyle.