Hashimoto's thyroiditis, or inflammation of the thyroid gland, is an autoimmune disorder. That means it is caused by a malfunction in your immune system. Instead of protecting your thyroid tissue, your immune cells attack it. These immune cells can cause hypothyroidism (underactive thyroid), a goiter (enlarged thyroid), or both. Eventually, the thyroiditis process can even destroy your entire thyroid, if left undetected or untreated.
In Hashimoto's thyroiditis, large amounts of damaged immune cells invade the thyroid. These immune cells are called lymphocytes; this is where Hashimoto's other name—chronic lymphocytic thyroiditis—is derived from.
When these lymphocytes enter the thyroid, they destroy the cells, tissue, and blood vessels within the gland. The process of destroying the thyroid gland is a slow one, which is why many people who have Hashimoto's thyroiditis go many years without any noticeable symptoms. You can read more about this in our article about the symptoms of Hashimoto's thyroiditis.
Because the thyroid is essentially coming under attack from invading cells, it isn't able to produce as much thyroid hormone as it normally would. Eventually, this causes hypothyroidism. And in extreme cases, the immune cells can cause the thyroid to become enlarged and inflamed to the point that it produces a visible mass in the neck—a goiter.
Doctors aren't entirely sure why the immune system, which is supposed to defend the body from harmful viruses and bacteria, sometimes turns against the body's healthy tissues. But what scientists do understand is that there are some factors that may make you more susceptible to this disease, and you can read about them in are article about risk factors for Hashimoto's thyroiditis.