Hashimoto’s thyroiditis is the most common type of thyroiditis, an inflammatory disease of the thyroid gland. Antibodies that the body normally produces against substances foreign to the body, together with the immune system, attack the thyroid cells and cause inflammation and damage to the thyroid gland. Thus, the thyroid gland cannot fulfill its function, and the level of hormones it secretes decreases over time. It is familial and is mostly seen in women between the ages of 30–50. Hashimoto’s thyroid is one of the causes of “hypothyroidism”, a condition in which the thyroid gland is underactive. The thyroid hormone, which regulates the body’s metabolic rate and thus affects all tissues, decreases over time in these patients, and the risk of hypothyroidism increases with age. Another problem that Hashimoto’s thyroid can cause in the thyroid gland is nodule formation. In general, enlargement of the thyroid gland is common, especially in the beginning, and this may be accompanied by nodule formation in the process. Hashimoto’s thyroid should be detected early, and thyroid hormone levels should be monitored at regular intervals and replaced with medication if necessary. With close follow-up and treatment, patients do not experience adverse effects related to Hashimoto’s thyroid.