A disorder that happens when the immune system attacks the thyroid — a butterfly-shaped gland at the base of the neck just below the Adam's apple.

In Hashimoto's disease, the immune system causes the death of the cells in the thyroid that make hormones. The disease usually causes a drop in the amount of hormones the thyroid makes. That condition is called hypothyroidism. Although anyone can develop Hashimoto's disease, it's most common among middle-aged women.

Hashimoto's disease slowly gets worse over time. Symptoms may not be noticeable. Eventually, the drop in thyroid hormone triggers a variety of symptoms. They may include fatigue; extra sensitivity to cold; dry skin; constipation; muscle weakness; thyroid swelling; depression; and muscle and joint aches, tenderness and stiffness, among others.

Most people with Hashimoto's disease take medicine to treat hypothyroidism. If hypothyroidism is mild, though, no treatment may be needed. In that case, regular testing to monitor thyroid hormone levels usually is recommended.