What is Hashimoto's Thyroiditis?

It is the most common cause of hypothyroidism in iodine adequate places, usually adults 45-65 yrs old 15: 1 (female to male), may affect children. We see symmetrical enlargement of the gland (diffuse goiter w/nodular reinforcements) and intact capsule. We see infiltration of parenchyma by mononuclear inflammatory infiltrate of small lymphocytes, plasma cells and well developed germinal center, follicles are atrophic lined in many areas by epithelial cells (Oxyphil cells/Hurthle cells high eosinophilic) Fibrosis doesn't extend beyond the capsule and no venulitis (differs from Riedel's Thyroiditis)