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Your thyroid is a small, butterfly-shaped gland located at the base of your neck, just below your Adam's apple. Hormones produced by the thyroid gland have an enormous impact on your health, affecting all aspects of your metabolism — from the rate at which your heart beats to how quickly you burn calories.

As long as your thyroid releases the proper amounts of these hormones, your system functions normally. But sometimes your thyroid doesn't produce enough hormones, upsetting the balance of chemical reactions in your body. This condition is known as hypothyroidism, or underactive thyroid disease.

Women, especially those older than 50, are more likely to have hypothyroidism than men are. Hypothyroidism seldom causes symptoms in the early stages, but over time, untreated hypothyroidism can cause a number of health problems, such as obesity, joint pain, infertility and heart disease.

The good news is that accurate thyroid function tests are available to diagnose hypothyroidism, and treatment of hypothyroidism with synthetic thyroid hormone is usually simple and effective once the proper dosage is established.

The signs and symptoms of hypothyroidism vary widely, depending on the severity of the hormone deficiency. But in general, any problems you do have tend to develop slowly, often over a number of years.

At first, you may barely notice symptoms such as fatigue and sluggishness, or you may simply attribute them to getting older. But as your metabolism continues to slow, you may develop more obvious signs and symptoms, including:

• Increased sensitivity to cold
• Constipation
• Pale, dry skin
• A puffy face
• Hoarse voice
• An elevated blood cholesterol level
• Unexplained weight gain
• Muscle aches, tenderness and stiffness
• Pain, stiffness or swelling in your joints
• Muscle weakness
• Heavier than normal menstrual periods
• Depression

When hypothyroidism isn't treated, signs and symptoms can gradually become more severe. Constant stimulation of your thyroid to release more hormones may lead to an enlarged thyroid (goiter). In addition, you may become more forgetful, your thought processes may slow or you may feel depressed.

Advanced hypothyroidism, known as myxedema, is rare, but when it occurs it can be life-threatening. Signs and symptoms include low blood pressure, decreased breathing, decreased body temperature, unresponsiveness and even coma. In some cases, myxedema can be fatal.

Hypothyroidism in children and teens
Although hypothyroidism most often affects middle-aged and older women, anyone can develop the condition, including infants and teenagers. Initially, babies born without a thyroid gland or with a gland that doesn't work properly may have few signs and symptoms. When newborns do have problems with hypothyroidism, they may include:

• Yellowing of the skin and whites of the eyes (jaundice). In most cases, this occurs when a baby's liver can't metabolize a molecule called bilirubin, which normally forms when the body recycles old or damaged red blood cells.
• Frequent choking.
• A large, protruding tongue.

As the disease progresses, infants are likely to have trouble feeding and may fail to grow and develop normally. They may also have:

• Constipation
• Poor muscle tone
• Excessive sleepiness

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

In general, children and teens who develop hypothyroidism have the same signs and symptoms as adults do, but they may also experience:

• Poor growth, resulting in short stature
• Delayed development of permanent teeth
• Delayed puberty
• Poor mental development

Your thyroid gland produces two main hormones, thyroxine (T-4) and triiodothyronine (T-3), that influence every cell in your body. They maintain the rate at which your body uses fats and carbohydrates, help control your body temperature, influence your heart rate and help regulate the production of protein. Your thyroid gland also produces calcitonin, a hormone that regulates the amount of calcium in your blood.

The rate at which thyroxine and triiodothyronine are released is controlled by your pituitary gland and your hypothalamus — an area at the base of your brain that acts as a thermostat for your whole system. The hypothalamus signals your pituitary gland to make a hormone called thyroid-stimulating hormone (TSH). Your pituitary gland then releases TSH — the amount depends on how much thyroxine and triiodothyronine are in your blood. Finally, your thyroid gland regulates its production of hormones based on the amount of TSH it receives.

Although this process usually works well, the thyroid sometimes fails to produce enough hormones. This may be due to a number of different factors, including:

• Autoimmune disease (Hashimoto's thyroiditis). Autoimmune disorders occur when your immune system produces antibodies that attack your own tissues. Sometimes this process occurs within the thyroid gland. Scientists aren't sure why the body produces antibodies against itself. Some think a virus or bacteria might trigger the response, while others believe a genetic flaw may be involved. Most likely, autoimmune diseases result from more than one factor. But however it happens, these antibodies affect the thyroid's ability to produce hormones.
• Treatment for hyperthyroidism. People who produce too much thyroid hormone (hyperthyroidism) are often treated with radioactive iodine or anti-thyroid medications to reduce their thyroid function. However, function can be reduced too much, resulting in hypothyroidism.
• Radiation therapy. Radiation used to treat cancers of the head and neck can affect your thyroid gland and may lead to hypothyroidism.
• Thyroid surgery. Removing all or a large portion of your thyroid can diminish or halt hormone production. In that case, you'll need to take thyroid hormones for life.
• Medications. A number of medications can contribute to hypothyroidism. One such medication is lithium, which is used to treat certain psychiatric disorders. If you're taking medication, ask your doctor about its effect on your thyroid gland.

Less often, hypothyroidism may result from one of the following:

• Congenital disease. Approximately one in 3,000 babies in the United States is born with a defective thyroid gland or no thyroid gland at all. In most cases, the thyroid gland didn't develop normally for unknown reasons, but some children have an inherited form of the disorder. Often, infants with congenital hypothyroidism appear normal at birth. That's one reason why most states now require newborn thyroid screening.
• Pituitary disorder. A relatively rare cause of hypothyroidism is the failure of the pituitary gland to produce enough TSH — usually due to a benign tumor of the pituitary gland.
• Pregnancy. Some women develop hypothyroidism during or after pregnancy (postpartum hypothyroidism), often because they produce antibodies to their own thyroid gland. Left untreated, hypothyroidism increases the risk of miscarriage, premature delivery and preeclampsia — a condition that causes a significant rise in a woman's blood pressure during the last three months of pregnancy. It can also seriously affect the developing fetus.
• Iodine deficiency. The trace mineral iodine — found primarily in seafood, seaweed, plants grown in iodine-rich soil and iodized salt — is essential for the production of thyroid hormones. In some parts of the world, iodine deficiency is common, but the addition of iodine to table salt has virtually eliminated this problem in the United States.

Although anyone can develop hypothyroidism, it occurs mainly in women older than 50, and the risk of developing the disorder increases with age. You also have an increased risk if you:

• Have a close relative, such as a parent or grandparent, with an autoimmune disease
• Have been treated with radioactive iodine or anti-thyroid medications
• Received radiation to your neck or upper chest
• Have had thyroid surgery (partial thyroidectomy)

See your doctor if you're feeling tired for no reason or have any of the other symptoms of hypothyroidism, such as dry skin, a pale, puffy face, constipation or a hoarse voice.

You'll also need to see your doctor for periodic testing of your thyroid function if you've had previous thyroid surgery, treatment with radioactive iodine or anti-thyroid medications, or radiation therapy to your head, neck or upper chest. However, it may take years or even decades before any of these therapies or procedures result in hypothyroidism.

If you have high blood cholesterol, talk to your doctor about whether hypothyroidism may be a cause. And if you're receiving hormone therapy for hypothyroidism, schedule follow-up visits as often as your doctor recommends. Initially, it's important to make sure you're receiving the correct dose of medicine. And over time, the dose you need to keep your thyroid functioning normally may change.

Because hypothyroidism is more prevalent in older women, some doctors recommend that older women be screened for the disorder during routine annual physical examinations. Some doctors also recommend that pregnant women or women thinking about becoming pregnant be tested for hypothyroidism.

In general, your doctor may test for an underactive thyroid if you're feeling increasingly tired or sluggish, have dry skin, constipation and a hoarse voice, or have had previous thyroid problems or goiter.

Blood tests
Diagnosis of hypothyroidism is based on your symptoms and the results of blood tests that measure the level of TSH and sometimes the level of the thyroid hormone thyroxine. A low level of thyroxine and high level of TSH indicate an underactive thyroid. That's because your pituitary produces more TSH in an effort to stimulate your thyroid gland into producing more thyroid hormone.

In the past, doctors weren't able to detect hypothyroidism until symptoms were fairly advanced. But by using the sensitive TSH test, doctors are able to diagnose thyroid disorders much earlier — often before you ever experience symptoms. Because the TSH test is the best screening test, your doctor will likely check TSH first and follow with a thyroid hormone test if needed. TSH tests also play an important role in managing hypothyroidism. They help your doctor determine the right dosage of medication, both initially and over time.

In addition, TSH tests are used to help diagnose a condition called subclinical hypothyroidism, which usually causes no outward signs or symptoms. In this condition, you have normal blood levels of T-3 and T-4, but higher than normal levels of TSH.

Untreated hypothyroidism can lead to a number of health problems:

• Goiter. Constant stimulation of your thyroid to release more hormones may cause the gland to become larger — a condition known as goiter. Hashimoto's thyroiditis is one of the most common causes of a goiter. Although generally not uncomfortable, a large goiter can affect your appearance and may interfere with swallowing or breathing.
• Heart problems. Hypothyroidism may also be associated with an increased risk of heart disease, primarily because high levels of low-density lipoprotein (LDL) cholesterol — the "bad" cholesterol — can occur in people with an underactive thyroid. Even subclinical hypothyroidism, a more benign condition than true hypothyroidism, can cause an increase in total cholesterol levels and impair the pumping ability of your heart. Hypothyroidism can also lead to an enlarged heart and heart failure.
• Mental health issues. Depression may occur early in hypothyroidism and may become more severe over time. Hypothyroidism can also cause slowed mental functioning.
• Myxedema. This rare, life-threatening condition is the result of long-term, undiagnosed hypothyroidism. Its symptoms include intense cold intolerance and drowsiness followed by profound lethargy and unconsciousness. A myxedema coma may be triggered by sedatives, infection or other stress on your body. If you have symptoms of myxedema, you need immediate emergency medical treatment.

• Birth defects. Babies born to women with untreated thyroid disease may have a higher risk of birth defects than do babies born to healthy mothers. These children are more prone to serious intellectual and developmental problems.

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

Standard treatment for an underactive thyroid involves daily use of the synthetic thyroid hormone levothyroxine (Levothroid, Levoxyl, Synthroid, Unithroid). The oral medication restores adequate hormone levels, shifting your body back into normal gear.

Soon after starting treatment, you'll notice that you're feeling less fatigued. The medication also gradually lowers cholesterol levels elevated by the disease and may reverse any weight gain. Treatment with levothyroxine is usually lifelong, but because the dosage you need may change, your doctor is likely to check your TSH level every year or so.

To determine the right dosage of levothyroxine initially, your doctor generally checks your level of TSH after two to three months. Excessive amounts of the hormone can cause side effects, such as increased appetite, insomnia, heart palpitations and shakiness.

If you have coronary artery disease or severe hypothyroidism, your doctor may start treatment with a smaller amount of medication and gradually increase the dosage. Progressive hormone replacement allows your heart to adjust to the increase in metabolism.

Levothyroxine causes virtually no side effects when used in the appropriate dose and is relatively inexpensive. If you change brands, let your doctor know to ensure you're still receiving the right dosage. Also, don't skip doses or stop taking the drug because you're feeling better. If you do, the symptoms of hypothyroidism will gradually return. People with hypothyroidism need to take medication for the rest of their lives.

Proper absorption of levothyroxine
Certain medications, supplements and even some foods may affect your ability to absorb levothyroxine. Talk to your doctor if you eat large amounts of soy products or a high-fiber diet or you take other medications, such as:

• Iron supplements
• Cholestyramine (Questran)
• Aluminum hydroxide, which is found in some antacids

If you have subclinical hypothyroidism, discuss treatment with your doctor. For a relatively low level of TSH, you probably won't benefit from thyroid hormone therapy, and treatment could even be harmful. On the other hand, for a higher TSH level, thyroid hormones may improve your cholesterol level, the pumping ability of your heart or your energy level.

Although most doctors recommend synthetic thyroxine, natural extracts containing thyroid hormone derived from the thyroid glands of pigs are available. These products contain both thyroxine and triiodothyronine. Synthetic thyroid medications contain thyroxine only, and the triiodothyronine your body needs is derived from the thyroxine.

Extracts are available by prescription only and shouldn't be confused with the glandular concentrates sold in natural foods stores. These products aren't regulated by the Food and Drug Administration, and their potency isn't guaranteed.