Black thyroid associated with minocycline therapy: histochemical and ultrastructural studies on the brown pigment.

The brown pigment found in the black thyroid, associated with minocycline therapy, was studied histochemically and ultrastructurally. The brown pigment was Fontana-Masson silver-positive, and was negative for iron, for autofluorescence under uv illumination, for periodic acid Schiff (PAS) and for acid-fast iodine peroxidase (AFIP) lipofuscin, and was bleached with potassium permanganate. The light and electron microscopic studies suggested that the pigment resembled melanin. Although the mechanism of the pigment deposition is not clear, a possible explanation may be that the pigment is related to direct oxidative degradation products of minocycline.