Thyroiditis is an inflammation of the thyroid gland, the butterfly-shaped gland that sits just below the area where an Adam's apple might be located at the front of the neck. The thyroid gland produces hormones that regulate a person's metabolism.

Any damage to the thyroid gland, such as inflammation, can disrupt its hormone production and affect body functioning. For example, if the thyroid gland fails to produce adequate levels of hormones for the body's needs, metabolism may slow, causing symptoms such as fatigue, constipation and unexplained weight gain. In this case, the thyroid gland is considered to be underactive, causing a condition known as hypothyroidism. This is commonly associated with slow, chronic damage to cells of the thyroid gland caused by thyroiditis.

On the other hand, rapid, acute damage to the thyroid gland caused by thyroiditis may cause hormones to leak out into the bloodstream. This causes a condition known as transient thyrotoxicosis (temporarily high levels of thyroid hormones in the blood). Symptoms may be identical to those of other forms of hyperthyroidism (e.g., rapid heartbeat, insomnia, unexplained weight loss), which involves increased metabolism. However, it is important to note that transient thyrotoxicosis caused by thyroiditis results from hormones stored in the thyroid leaking into the bloodstream. This differs from true hyperthyroidism, which is caused by an overproduction of hormone by the thyroid gland.

According to the American Association of Clinical Endocrinologists, 27 million Americans have thyroid problems that involve either underactive or overactive thyroid glands. Many cases are believed to remain undiagnosed.

Thyroiditis can cause serious damage to the thyroid, leading to various health problems. The Centers for Disease Control and Prevention (CDC) lists thyroiditis as a cause of death in a couple of cases per year.