Adult male zebrafish; H&E The thyroid follicles are lined with epithelial cells, which are cuboidal to flat (insert), depending on the level of activity.
The thyroid epithelium produces thyroid hormones, which are stored bound to thyroglobulin in the follicle lumen, as colloid. This colloid contains resorption vacuoles, visible adjacent to the epithelial lining.

Adult male zebrafish; PAS The colloid shows an intense reaction with periodic acid-Schiff (PAS), indicating the glycoprotein nature of the thyroglobulin (note, that there also intracellular PAS reactive inclusions).

Adult male zebrafish; H&E Interspersed between the thyroid parenchymal cell another cell type can be found, presumably rodlet cells (see the page on rodlet cells).