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Your thyroid is a butterfly-shaped gland located at the base of your neck, just below your Adam's apple. Although it weighs less than an ounce, the thyroid gland has an enormous effect on your health. Your thyroid is part of the endocrine system, which is made up of several glands and tissues that produce hormones. These chemical messengers coordinate many of your body's activities, from digestion to metabolism to reproduction.

Hashimoto's disease, also known as chronic lymphocytic thyroiditis, causes inflammation of your thyroid gland that often leads to underactive thyroid (hypothyroidism). It's an autoimmune disorder in which your immune system inappropriately attacks your thyroid gland, causing damage to your thyroid cells and upsetting the balance of chemical reactions in your body. Hashimoto's disease is the most common cause of hypothyroidism in the United States.

Blood tests of thyroid function are used to detect Hashimoto's disease. Treatment with synthetic thyroid hormone replacement medication usually is simple and effective. Natural treatment options also exist.

Hashimoto's disease does not have unique signs and symptoms. The disease progresses slowly over a number of years and causes chronic thyroid damage, leading to a drop in thyroid hormone levels in your blood. The signs and symptoms, if any, are those of underactive thyroid (hypothyroidism).

The signs and symptoms of hypothyroidism vary widely, depending on the severity of hormone deficiency. At first, you may barely notice symptoms, such as fatigue and sluggishness, or you may simply attribute them to getting older. But as the disease progresses, 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. Many people attribute their weight gain to an underactive thyroid, but this is true only in a few cases. Hypothyroidism rarely causes weight gain of more than 10 to 20 pounds, most of which is fluid.
• Muscle aches, tenderness and stiffness, especially in your shoulders and hips.
• Pain and stiffness in your joints and swelling in your knees or the small joints in your hands and feet.
• Muscle weakness, especially in your lower extremities.
• Excessive or prolonged menstrual bleeding (menorrhagia).
• Depression.

Without treatment, signs and symptoms gradually become more severe and your thyroid may become enlarged (goiter). In addition, you may become more forgetful, your thought processes may slow, or you may feel depressed.

Your thyroid gland produces two main hormones, thyroxine (T-4) and triiodothyronine (T-3). 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.

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.

Normally, your immune system uses naturally occurring proteins (antibodies) and white blood cells (lymphocytes) to help protect against viruses, bacteria and foreign substances (antigens) that invade your body. Hashimoto's disease is an autoimmune disorder in which your immune system creates antibodies that damage your thyroid gland. The disease causes inflammation of your thyroid gland (thyroiditis), which may impair the ability of your thyroid to produce hormones, leading to underactive thyroid (hypothyroidism). Then, your pituitary gland attempts to stimulate your thyroid gland to produce more thyroid hormones, thus causing your thyroid gland to enlarge (goiter).

Doctors don't know what causes your immune system to attack your thyroid gland. Some scientists think a virus or bacteria might trigger the response, while others believe a genetic flaw may be involved. Most likely, Hashimoto's disease results from more than one factor. A combination of factors including heredity, sex and age may determine your likelihood of developing the disorder. Hashimoto's disease is most common in older women and tends to run in families.

See your doctor if you're feeling tired for no reason or have any other signs and 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.

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 caused by Hashimoto's disease, 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 adequately replace your thyroid function may change.

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

Diagnosis of Hashimoto's disease is based on your symptoms and the results of blood tests that measure levels of thyroid hormone and thyroid-stimulating hormone (TSH). These may include:

• A hormone test. Blood tests can determine the amount of hormones produced by your thyroid and pituitary glands. If your thyroid is underactive, the level of thyroid hormone is low. At the same time, the level of TSH is elevated because your pituitary gland tries to stimulate your thyroid gland to produce more thyroid hormone.
• An antibody test. Because Hashimoto's disease is an autoimmune disorder, the cause involves production of abnormal antibodies. A blood test may confirm the presence of such antibodies.

In the past, doctors were unable to detect underactive thyroid (hypothyroidism), the main indicator of Hashimoto's disease, until signs and symptoms were fairly advanced. But by using the sensitive TSH test, doctors are able to diagnose thyroid disorders much earlier, often before you experience any signs and 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. These tests also help your doctor determine the right dosage of medication, both initially and over time.