Melanosis thyroidi

Despite many studies on melanosis thyroidi, there is no consensus concerning the nature of the pigment or its pathogenesis. Here the past literature on this subject is reviewed and results are reported on a case of melanosis thyroidi that provides fresh insight into the nature of the pigment and its genesis. Briefly, the conclusion is that while a few of the pigment granules are classic lipofuscin granules (i.e., single-membrane-bound structures containing electron-dense material in the form of particles, granules and masses, and electron-lucent lipid droplets), a majority of the pigment granules also contain colloid, which is easily identified by its particulate substructure similar to the colloid in the lumen of follicles. Thus, these pigment granules are, in fact, ambilysosomes, where degradation of both endogenous (organelles) and exogenous (colloid) material occurs. Several phagosomes containing colloid were also seen in the thyroid epithelial cells.