Black thyroid syndrome: exaggeration of a normal process?

Coal-black thyroid discoloration usually is identified in patients receiving chronic minocycline therapy. This report concerns the use of light microscopic, electron microscopic, and energy dispersion spectroscopy of thyroid pigments in three separate situations: minocycline-associated black thyroid; idiopathic black thyroid; and normally pigmented thyroid glands. One of the pigments, which is found in each situation, is best described as neuromelanin. This melanin pigment, like lipofuscin, appears to accumulate with advancing age. Pigment accumulation, therefore, is a normal process in the thyroid gland. Accelerated pigment accumulation occurs with minocycline therapy but can uncommonly be seen without associated minocycline treatment. Possible mechanisms for the development of these pigments in normal and black thyroid glands are discussed. Minocycline-associated pigment is also described in substantia nigra and atherosclerotic plaques.