A 28-year-old woman presents with heat intolerance, weight loss despite increased appetite, and a diffusely enlarged thyroid with a bruit. Thyroid function tests show suppressed TSH with elevated free T4. The hypersensitivity mechanism underlying her disease is best described as:

• A Type I hypersensitivity mediated by IgE bound to mast cells
• B Type III hypersensitivity due to circulating immune complex deposition
• C Type II hypersensitivity mediated by an antibody that stimulates a cell surface receptor ✓
• D Type IV hypersensitivity mediated by CD4+ T cells against thyroid follicular cells

Explanation

Graves disease is caused by autoantibodies against the TSH receptor that act as agonists, stimulating hormone synthesis and gland growth. Antibody-mediated stimulation of a cell surface molecule places it in Type II hypersensitivity, the same category as myasthenia gravis where the antibody blocks rather than stimulates its receptor. Type IV mechanisms dominate in Hashimoto thyroiditis, where cytotoxic T cells destroy follicular cells instead of overstimulating them.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.