You might not notice signs or symptoms of Hashimoto's disease at first, or you may notice a swelling at the front of your throat (goiter). Hashimoto's disease typically progresses slowly over years and causes chronic thyroid damage, leading to a drop in thyroid hormone levels in your blood. The signs and symptoms are mainly those of an underactive thyroid gland (hypothyroidism).

Signs and symptoms of hypothyroidism include:

• Fatigue and sluggishness

• Increased sensitivity to cold

• Constipation

• Pale, dry skin

• A puffy face

• Brittle nails

• Hair loss

• Enlargement of the tongue

• Unexplained weight gain

• Muscle aches, tenderness and stiffness

• Joint pain and stiffness

• Muscle weakness

• Excessive or prolonged menstrual bleeding (menorrhagia)

• Depression

• Memory lapses

Hashimoto's disease is an autoimmune disorder in which your immune system creates antibodies that damage your thyroid gland. Doctors don't know what causes your immune system to attack your thyroid gland. Some scientists think a virus or bacterium might trigger the response, while others believe a genetic flaw may be involved.

A combination of factors — including heredity, sex and age — may determine your likelihood of developing the disorder.

These factors may contribute to your risk of developing Hashimoto's disease:

• Sex. Women are much more likely to get Hashimoto's disease.

• Age. Hashimoto's disease can occur at any age but more commonly occurs during middle age.

• Heredity. You're at higher risk for Hashimoto's disease if others in your family have thyroid or other autoimmune diseases.

• Other autoimmune disease. Having another autoimmune disease — such as rheumatoid arthritis, type 1 diabetes or lupus — increases your risk of developing Hashimoto's disease.

• Radiation exposure. People exposed to excessive levels of environmental radiation are more prone to Hashimoto's disease.

Left untreated, an underactive thyroid gland (hypothyroidism) caused by Hashimoto's disease can lead to a number of health problems:

• Goiter. Constant stimulation of your thyroid to release more hormones may cause the gland to become enlarged, a condition known as goiter. Hypothyroidism is one of the most common causes of goiters. Although generally not uncomfortable, a large goiter can affect your appearance and may interfere with swallowing or breathing.

• Heart problems. Hashimoto's disease also may 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 gland (hypothyroidism). If left untreated, hypothyroidism can lead to an enlarged heart and, possibly, heart failure.

• Mental health issues. Depression may occur early in Hashimoto's disease and may become more severe over time. Hashimoto's disease can also cause sexual desire (libido) to decrease in both men and women and can lead to slowed mental functioning.

• Myxedema (miks-uh-DEE-muh). This rare, life-threatening condition can develop due to long-term hypothyroidism as a result of untreated Hashimoto's disease. Its signs and symptoms include drowsiness followed by profound lethargy and unconsciousness.

A myxedema coma may be triggered by exposure to cold, sedatives, infection or other stress on your body. Myxedema requires immediate emergency medical treatment.

• Birth defects. Babies born to women with untreated hypothyroidism due to Hashimoto's disease may have a higher risk of birth defects than do babies born to healthy mothers. Doctors have long known that these children are more prone to intellectual and developmental problems. There may be a link between hypothyroid pregnancies and birth defects, such as a cleft palate.

A connection also exists between hypothyroid pregnancies and heart, brain and kidney problems in infants. If you're planning to get pregnant or if you're in early pregnancy, be sure to have your thyroid level checked.