Answer: Subacute granulomatous (De Quervain’s) thyroiditis

Histology: The thyroid is involved by a patchy infiltrate comprised of mostly of histiocytes. The histiocytic infiltrate 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 inflammatory process spills into the surrounding thyroid parenchyma resulting in follicular dropout and interstitial fibrosis.

Discussion: De Quervain’s (subacute granulomatous) thyroiditis is a most common cause of a painful thyroid gland. It is a transient inflammatory process with a highly variable clinical course. Most patients have pain in the region of the thyroid, which is usually diffusely tender, and some have systemic symptoms. Hyperthyroidism often occurs initially, sometimes followed by transient hypothyroidism. It is a self-limiting disease, and most patients complete recovery in weeks to months. As this condition is not typically treated surgically, these are not common surgical specimens and most pathologists are not familiar with the histopathologic findings. In this particular case, the thyroid was removed for the presence of a follicular adenoma (not shown).

Unlike the more common palpation granulomas that tend to be very focal and restricted to the thyroid follicles, the inflammatory process encountered in sub acute granulomatous thyroiditis tends to spills into the surrounding thyroid parenchyma causing varying degrees of follicular dropout and interstitial fibrosis. In infectious processes (e.g. fungal or Mycobacterial infections), the granulomas are not consistently centered on the follicles. If there is any difficulty in excluding an infectious process on histologic grounds, special stains for microorganisms are in order.