Most common cause of hyperthyroidism.Thyroid-stimulating immunoglobulin (IgG; type II hypersensitivity) stimulates TSH receptors on thyroid (hyperthyroidism, diffuse goiter) and dermal fibroblasts (pretibial myxedema). Infiltration of retroorbital space by activated T-cells → ↑ cytokines (eg, TNF-α, IFN-γ) → ↑ fibroblast secretion of hydrophilic GAGs → ↑ osmotic muscle swelling, muscle inflammation, and adipocyte count → exophthalmos. Often presents during stress (eg, pregnancy).

- Associated with HLA-DR3 and HLA-B8.

Histology: Tall, crowded follicular epithelial cells projecting into the follicular lumen; scalloped colloid.