Fine structural aspects on the fate of rat black thyroids induced by minocycline.

The fate of the black thyroid induced by minocycline treatment (100 mg/kg daily for 21 days) in the rat was studied by light and electron microscopy after 6 months. The black discoloration of the thyroid gland remained and numerous dense bodies containing highly electron-dense deposits were seen in most of the follicular epithelial cells. It appears that the turnover rate of follicular epithelial cells is very low and the electron-dense deposits are largely retained, though debris derived from degenerate follicular epithelial cells containing the dense deposits may be phagocytosed by macrophage-like cells in the interfollicular connective tissue. Brown-black granules are also found in the extremely attenuated follicular epithelial cells of cold follicles.