Most types of thyroiditis are more common in women than men and also tend to more commonly affect people over the age of 40. A family history of thyroid disease, or personal history of thyroiditis or immune system disorders, may also increase a person's risk of thyroiditis.

Depending on the type of thyroiditis involved, additional potential risk factors include:

• Pregnancy
• Viral or bacterial illnesses (e.g., the flu, mumps)
• Use of certain medications (e.g., certain anti-inflammatory and antiarrhythmic drugs)
• Radiation therapy

An autoimmune disorder causes most cases of thyroiditis. These occur when the immune system mistakenly produces antibodies that attack healthy body tissue. In some cases, autoimmune disorders can attack cells and enzymes of the thyroid gland, reducing the thyroid gland's ability to produce adequate levels of hormones.

Hashimoto's thyroiditis, the most common thyroid disorder in the United States, is an autoimmune disorder. Postpartum thyroiditis and painless (silent) thyroiditis are also thought to be autoimmune conditions, according to the American Association of Clinical Endocrinologists. Additional autoimmune disorders that are associated with thyroiditis include:

• Type 1 diabetes
• Rheumatoid arthritis
• Autoimmune polyglandular syndrome

Viral and bacterial infections cause some types of thyroiditis. Use of certain medications or radiation also can cause thyroiditis. In addition, disorders such as Hashimoto's thyroiditis appear to have a genetic component. The cause of some types of thyroiditis remains unknown (idiopathic).