In medical terms, the suffix -itis means inflammation; thyroiditis is inflammation of the thyroid gland which may be associated with an underactive thyroid gland or hypothyroidism and in some cases patients have an overactive thyroid gland or hyperthyroidism prior to experiencing hypothyroidism. There are several different kinds of thyroiditis. Although each different type of thyroiditis may cause different symptoms, many times they can be quite similar.

In the case of thyroiditis, hypothyroidism is caused by the destruction of the thyroid gland by an inflammatory process. When thyroid cells are attacked by the inflammation, these cells stop functioning. Without thyroid cells, the thyroid is no longer able to produce enough thyroid hormone to maintain the body's normal metabolism. Hypothyroidism, or an underactive thyroid gland, results.