Answer: Thyroid with minocycline-associated pigment deposition

Histology: The cut surface of the thyroid was jet black throughout. Microscopically, the thyroid parenchyma is peppered by brown-black pigment. The pigment is present as dust-like particles within the apical cytoplasm of the cells lining the follicules; and as larger clumps within the colloid.

Discussion: A number of different types of pigments can deposit in the thyroid, but the gross finding of a diffusely black thyroid is only encountered in the context of therapeutic minocycline use. Despite its remarkable gross appearance, minocycline-associated pigmentation of the thyroid is of no functional relevance. The nature and etiology of the pigment is not completely understood, but it closely resembles lipofuscin. The pigment is diffusely dispersed throughout the thyroid pigment as particles within the follicular epithelium and colloid. In contrast, iron is more focally and haphazardly distributed, usually in areas of scarring and hemorrhage within the context of multinodular hyperplasia. Melanin pigment may be encountered in the context of metastatic melanoma or melanin-producing medullary carcinoma. In these cases, the pigment is confined to the tumor nodules.