USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

What type of hypersensitivity reaction is most commonly associated with Hashimoto thyroiditis?

1. Type I

2. Type II

3. Type III

4. Type IV

The correct answer is: Type IV

Hashimoto thyroiditis is classified as a Type IV hypersensitivity reaction, which is a cell-mediated immune response. This type of hypersensitivity involves T lymphocytes and does not primarily rely on antibodies. In Hashimoto thyroiditis, autoreactive T cells attack the thyroid gland, leading to chronic inflammation and subsequent hypothyroidism. The hallmark of Type IV hypersensitivity is delayed-type hypersensitivity, often manifesting several hours to days after exposure to the antigen. In this context, the antigens are thyroid-specific proteins, and the immune response involves activated CD4+ T-helper cells and often CD8+ cytotoxic T cells, which directly mediate damage to thyroid follicular cells. This immune-mediated destruction of the thyroid tissue contributes to the clinical manifestations of the disease, including goiter, fatigue, weight gain, and other symptoms associated with hypothyroidism. This mechanism distinguishes Hashimoto thyroiditis from other forms of hypersensitivity reactions, which may involve different pathophysiological processes such as antibody-mediated damage or immune complex formation. To summarize, the association of Hashimoto thyroiditis with Type IV hypersensitivity is due to the involvement of cell-mediated immune responses that target the thyroid gland directly, leading to inflammation and dysfunction of thyroid hormone production.