Last reviewed 01/2018

Classically, the gland is symmetrically enlarged with the capsule intact and rarely adherent to the surrounding structures. Microscopically, there is extensive replacement of the gland by lymphocytes, plasma cells and macrophages with the formation of lymphoid germinal centres. Fibrosis is rare and confined to the interlobula septa.

The less common fibrosing variant shows less thyroid enlargement, less lymphoid infiltration and more fibrosis. Unlike Riedel's thyroiditis, however, the fibrosis does not extend beyond the capsule.