Answer: Granulomatous thyroiditis (subacute thyroiditis)

Histology: At low power, there is nodular accentuation of the thyroid lobules with bands of interlobular fibrosis. There is a patchy infiltrate comprised of lymphocytes and histiocytes that is centered on individual thyroid follicles. Destruction of the follicular epithelium results in a rim of histiocytes and giant cells surrounding residual droplets of colloid. The process spills into the surrounding thyroid parenchyma where it is associated with progressive fibrosis.

Discussion: Subacute thyroiditis (also called granulomatous, giant-cell, and De Quervain’s thyroiditis), is a painful condition that is probably caused by a viral infection, although no specific virus has been isolated from affected patients. It is rarely encountered as a surgical pathology specimen owing to the facts that it is usually recognized clinically (patients typically present with malaise, fever, and thyroidal pain that extends to the ears or down to the anterior chest wall; and the thyroid is classically firm and very painful to palpation) and it is a self limiting process that is usually treated conservatively (e.g. salicylates or other non-steroidal anti-inflammatory drugs). Once the inflammatory process resolves, permanent thyroid damage is rare, as are recurrences.

Relative to palpation granulomas where the granulomatous reaction is limited to a few scattered follicles, the process is more widespread in subacute thyroiditis with extension of the inflammatory infiltrate into the surrounding parenchyma. The granulomas of mycobacterial infections are not centered around the follicles, and are more likely to have more prominent zones of central (caseous) necrosis. Although a histiocytic infiltrate is commonly encountered in multinodular hyperplasia, it usually occurs with other secondary degenerative changes including hemorrhage, hemosiderin deposition, and calcification.