The black thyroid. Its relation to minocycline use in man.

We studied a patient with a grossly black thyroid gland considered to be related to minocycline therapy. Microscopically, a brown granular pigment was localized in follicular cells and colloid, which histochemically was melanin or a melaninlike substance. Ultrastructurally, the pigment is confined to lysosomes. The pigment seems to be either an oxidative degradation product of the drug itself, or the results of an as yet unknown alteration of tyrosine metabolism by the drug. Although minocycline is known to interfere with thyroid function in animals, no evidence has been presented for such an effect in humans. The occurrence of a black thyroid gland in humans seems to be virtually pathognomonic for long-term minocycline therapy.