Hashimoto thyroiditis (HT), also called chronic lymphocytic thyroiditis, is the most common form of acquired hypothyroidism in the pediatric population in iodine-replete areas of the world. This results from an autoimmune attack on the thyroid gland inhibiting the gland's ability to produce adequate amounts of thyroid hormone, leading to hypothyroidism. There are many different anti-thyroid antigen antibodies involved with anti-thyrotropin (anti-TPO), an anti-thyroglobulin (anti-Tg) found in up to 90% of patients with HT.

HT most commonly presents in the teen years and has a variable clinical presentation ranging from asymptomatic (clinical euthyroidism with or without goiter) to overt symptoms of hypothyroidism. Initially, thyrotropin and levorotatory thyroxine (T₄) may be normal, with some patients developing an enlarged thyroid gland resulting from lymphocytic infiltration of the thyroid from thyroid growth-stimulating antibodies. The thyrotropin will usually rise with a normal to low normal T₄. During this phase, patients may develop a goiter but are often clinically euthyroid (no overt hypothyroid symptoms other than a goiter).

As the condition progresses and more of the gland is destroyed, thyrotropin levels continue to rise and free T₄ levels fall below the normal range. Clinical symptoms of hypothyroidism are often present during this stage. Up to 40% of these patients may revert back to a euthyroid state and do not progress to overt hypothyroidism. Sometimes a transient hyperthyroid state called Hashitoxicosis can be encountered. During this period of time, active thyroid hormone is released from destroyed glandular tissue and results in a transient hyperthyroid state, which is usually followed by true hypothyroidism. Symptoms of hypothyroidism include slow growth rate, unexplained weight gain, dry skin, coarse dry hair, goiter, fatigue, constipation, cold intolerance, irregular menses, or delayed puberty.

This condition is 5 times more common in females. Up to 25% of siblings of a patient with HT may have HT. HT can be found more commonly in patients with other autoimmune disorders and in those with Turner syndrome or Down syndrome.