The symptoms of hypothyroidism depend on the severity of the condition. Problems tend to develop slowly, often over several years.

At first, you may barely notice the symptoms of hypothyroidism, such as fatigue and weight gain. Or you may think they are just part of getting older. But as your metabolism continues to slow, you may develop more-obvious problems.

Hypothyroidism symptoms may include:

• Tiredness.
• More sensitivity to cold.
• Constipation.
• Dry skin.
• Weight gain.
• Puffy face.
• Hoarse voice.
• Coarse hair and skin.
• Muscle weakness.
• Muscle aches, tenderness and stiffness.
• Menstrual cycles that are heavier than usual or irregular.
• Thinning hair.
• Slowed heart rate, also called bradycardia.
• Depression.
• Memory problems.

Hypothyroidism in infants

Anyone can get hypothyroidism, including infants. Most babies born without a thyroid gland or with a gland that doesn't work correctly don't have symptoms right away. But if hypothyroidism isn't diagnosed and treated, symptoms start to appear. They may include:

• Feeding problems.
• Poor growth.
• Poor weight gain.
• Yellowing of the skin and the whites of the eyes, a condition called jaundice.
• Constipation.
• Poor muscle tone.
• Dry skin.
• Hoarse crying.
• Enlarged tongue.
• A soft swelling or bulge near the belly button, a condition called umbilical hernia.

When hypothyroidism in infants isn't treated, even mild cases can lead to severe physical and mental development problems.

Hypothyroidism in children and teens

In general, children and teens with hypothyroidism have symptoms similar to those in adults. But they also may have:

• Poor growth that leads to short stature.
• Delayed development of permanent teeth.
• Delayed puberty.
• Poor mental development.

When to see a doctor

See your health care provider if you're feeling tired for no reason or if you have other symptoms of hypothyroidism.

If you're taking thyroid hormone medicine for hypothyroidism, follow your health care provider's advice on how often you need medical appointments. At first, you may need regular appointments to make sure you're receiving the right dose of medicine. Over time, you may need checkups so that your health care provider can monitor your condition and medicine.

The thyroid is a small, butterfly-shaped gland located at the base of the neck, just below the Adam's apple. The thyroid gland makes two main hormones: thyroxine (T-4) and triiodothyronine (T-3). These hormones affect every cell in the body. They support the rate at which the body uses fats and carbohydrates. They help control body temperature. They have an effect on heart rate. And they help control how much protein the body makes.

Hypothyroidism happens when the thyroid gland doesn't make enough hormones. Conditions or problems that can lead to hypothyroidism include:

• Autoimmune disease. The most common cause of hypothyroidism is an autoimmune disease called Hashimoto's disease. Autoimmune diseases happen when the immune system makes antibodies that attack healthy tissues. Sometimes that process involves the thyroid gland and affects its ability to make hormones.
• Thyroid surgery. Surgery to remove all or part of the thyroid gland can lower the gland's ability to make thyroid hormones or stop it completely.
• Radiation therapy. Radiation used to treat cancers of the head and neck can affect the thyroid gland and lead to hypothyroidism.
• Thyroiditis. Thyroiditis happens when the thyroid gland becomes inflamed. This may be due to an infection. Or it can result from an autoimmune disorder or another medical condition affecting the thyroid. Thyroiditis can trigger the thyroid to release all of its stored thyroid hormone at once. That causes a spike in thyroid activity, a condition called hyperthyroidism. Afterward, the thyroid becomes underactive.
• Medicine. A number of medicines may lead to hypothyroidism. One such medicine is lithium, which is used to treat some psychiatric disorders. If you're taking medicine, ask your heath care provider about its effect on the thyroid gland.

Less often, hypothyroidism may be caused by:

• Problems present at birth. Some babies are born with a thyroid gland that doesn't work correctly. Others are born with no thyroid gland. In most cases, the reason the thyroid gland didn't develop properly is not clear. But some children have an inherited form of a thyroid disorder. Often, infants born with hypothyroidism don't have noticeable symptoms at first. That's one reason why most states require newborn thyroid screening.
• Pituitary disorder. A relatively rare cause of hypothyroidism is the failure of the pituitary gland to make enough thyroid-stimulating hormone (TSH). This is usually because of a noncancerous tumor of the pituitary gland.
• Pregnancy. Some people develop hypothyroidism during or after pregnancy. If hypothyroidism happens during pregnancy and isn't treated, it raises the risk of pregnancy loss, premature delivery and preeclampsia. Preeclampsia causes a significant rise in blood pressure during the last three months of pregnancy. Hypothyroidism also can seriously affect the developing fetus.
• Not enough iodine. The thyroid gland needs the mineral iodine to make thyroid hormones. Iodine is found mainly in seafood, seaweed, plants grown in iodine-rich soil and iodized salt. Too little iodine can lead to hypothyroidism. Too much iodine can make hypothyroidism worse in people who already have the condition. In some parts of the world, it's common for people not to get enough iodine in their diets. The addition of iodine to table salt has almost eliminated this problem in the United States.

Although anyone can develop hypothyroidism, you're at an increased risk if you:

• Are a woman.
• Have a family history of thyroid disease.
• Have an autoimmune disease, such as type 1 diabetes or celiac disease.
• Have received treatment for hyperthyroidism.
• Received radiation to your neck or upper chest.
• Have had thyroid surgery.

Hypothyroidism that isn't treated can lead to other health problems, including:

• Goiter. Hypothyroidism may cause the thyroid gland to become larger. This condition is called a goiter. A large goiter may cause problems with swallowing or breathing.
• Heart problems. Hypothyroidism can lead to a higher risk of heart disease and heart failure. That's mainly because people with an underactive thyroid tend to develop high levels of low-density lipoprotein (LDL) cholesterol — the "bad" cholesterol.
• Peripheral neuropathy. Hypothyroidism that goes without treatment for a long time can damage the peripheral nerves. These are the nerves that carry information from the brain and spinal cord to the rest of the body. Peripheral neuropathy may cause pain, numbness and tingling in the arms and legs.
• Infertility. Low levels of thyroid hormone can interfere with ovulation, which can limit fertility. Some of the causes of hypothyroidism, such as autoimmune disorders, also can harm fertility.

• Birth defects. Babies born to people with untreated thyroid disease may have a higher risk of birth defects compared with babies born to mothers who do not have thyroid disease.

  Infants with hypothyroidism present at birth that goes untreated are at risk of serious physical and mental development problems. But if the condition is diagnosed within the first few months of life, the chances of typical development are excellent.

• Myxedema coma. This rare, life-threatening condition can happen when hypothyroidism goes without treatment for a long time. A myxedema coma may be triggered by sedatives, infection or other stress on the body. Its symptoms include intense cold intolerance and drowsiness, followed by an extreme lack of energy and then unconsciousness. Myxedema coma requires emergency medical treatment.

The symptoms of hypothyroidism can be different from person to person. And they often look like symptoms of other health problems. Because of that, a diagnosis of hypothyroidism doesn't rely on symptoms alone. It's usually based on the results of blood tests.

The first blood test typically done to diagnose hypothyroidism measures the level of thyroid-stimulating hormone (TSH) in the blood. If it's high, the test is done again, along with a blood test for the thyroid hormone T-4. If the results show that TSH is high and T-4 is low, then the diagnosis is hypothyroidism. In some cases, the thyroid hormone T-3 may be measured as well.

If the second test shows high TSH but T-4 and T-3 are in the standard range, then the diagnosis is a condition called subclinical hypothyroidism. It usually doesn't cause any noticeable symptoms.

TSH tests also play an important role in managing hypothyroidism over time. They help your health care provider find and maintain the right dosage of medication for you.

The results of these blood tests can be affected by some medicines or supplements. This includes biotin, a vitamin taken as a stand-alone supplement or as part of a multivitamin. Before you have blood tests done, tell your health care provider about any medicines or supplements you take.

Treatment for hypothyroidism usually includes taking the thyroid hormone medicine levothyroxine (Levo-T, Synthroid, others) every day. This medicine is taken by mouth. It returns hormone levels to a healthy range, eliminating symptoms of hypothyroidism.

You'll likely start to feel better one or two weeks after you begin treatment. Treatment with levothyroxine likely will be lifelong. Because the dosage you need may change, your health care provider may check your TSH level every year.

Finding the right dosage

To find the right dosage of levothyroxine for you, your health care provider checks your level of TSH about 6 to 8 weeks after you start taking the medicine. You may need another blood test to check TSH again six months later. Too much levothyroxine can cause side effects, such as:

• Tiredness.
• Increased appetite.
• Sleep problems.
• Shakiness.
• Pounding of the heart, sometimes called heart palpitations.

Levothyroxine typically causes no side effects when used in the correct dose. If you change brands of the medicine, tell your health care provider, as the dosage may need to change.

If you have coronary artery disease or severe hypothyroidism, your health care provider may start treatment with a smaller amount of medicine and then slowly increase the dosage. This allows your heart to adjust to the rise in your body's metabolism.

Taking levothyroxine correctly

Levothyroxine is best taken on an empty stomach at the same time every day. Ideally, you take the hormone in the morning, and then wait 30 to 60 minutes before you eat or take other medicine. If you take the medicine at bedtime, wait to take it until at least four hours after your last meal or snack.

Don't skip doses or stop taking the medicine because you feel better. If you do, it's likely that the symptoms of hypothyroidism will slowly return. If you miss a dose of levothyroxine, take two pills the next day.

Some medicines, supplements and even some foods may affect your body's ability to absorb levothyroxine. Talk to your health care provider if you eat large amounts of soy products, or if you typically eat a high-fiber diet. Also, tell your provider if you take other medicines, especially:

• Iron supplements or multivitamins that contain iron.
• Aluminum hydroxide, which is found in some antacids.
• Calcium supplements.

Subclinical hypothyroidism

If you are diagnosed with subclinical hypothyroidism, talk about treatment with your health care provider. For a mild rise in TSH, thyroid hormone medicine may not be useful. If your TSH level is higher, but still in the subclinical range, thyroid hormones may improve some symptoms.

Most health care providers recommend taking the medicine levothyroxine to treat hypothyroidism. But an extract containing thyroid hormone derived from the thyroid glands of pigs is available. It is sometimes called desiccated thyroid extract. However, this treatment is not recommended because the amount of T-4 and T-3 in it may not be consistent from batch to batch. It is not safe for pregnant people to take desiccated thyroid extract because it can harm a fetus's development.

You'll likely start by seeing your primary health care provider. Or you may be referred to a specialist in hormone disorders, called an endocrinologist. Infants with hypothyroidism need to see a pediatric endocrinologist right away. Children or teens can start with their primary health care provider. But they need to see a pediatric endocrinologist if there are any questions about levothyroxine or about the correct dosage of the medicine.

Here's information to help you get ready for your appointment and know what to expect from your health care provider.

What you can do

• Be aware of any pre-appointment restrictions. When you make the appointment, ask if there's anything you need to do before you arrive.
• Write down any symptoms you're experiencing, including any that may seem unrelated to the reason you scheduled the appointment.
• Write down key personal information, including any major stresses or recent life changes.
• Make a list of all medicines, vitamins or supplements you're taking.
• Take a family member or friend along, if possible. Someone who goes with you may remember something that you missed or forgot.
• Write down questions to ask your health care provider.

Having a list of questions helps you make the most of your time with your health care provider. For hypothyroidism, some basic questions to ask include:

• What's the most likely cause of my symptoms?
• What tests do I need?
• Is my condition likely temporary or long lasting?
• What treatments are available, and which do you recommend?
• I have other health conditions. How can I best manage them together?
• Are there restrictions I need to follow?
• Should I see a specialist?
• Is there a generic alternative to the medicine you're prescribing?
• Are there brochures or other printed material I can take with me? What websites do you recommend?

Don't hesitate to ask other questions.

What to expect from your doctor

Your health care provider is likely to ask you a number of questions, including:

• When did you begin experiencing symptoms?
• Have your symptoms been continuous or occasional?
• How severe are your symptoms?
• What, if anything, seems to improve your symptoms?
• What, if anything, makes your symptoms worse?
• Do you have a family history of thyroid disease?