Adult male zebrafish; H&E staining Thyroid follicles are found in the pharyngeal region, dispersed around the truncus arteriosus, the jugular veins, and the afferent branchial arteries, which feed the gills. These follicles vary in size, and are not, as in mammals, arranged in a circumscribed organ.
The pharyngeal region further contains gill musculature (for protraction / retraction of the gill arches), which are attached to the hyoid.
The truncus arteriosus arises with its proximal part, the bulbus arteriosus, from the ventricle of the heart, which in turn receives the blood from the atrium.
Adult male zebrafish; H&E staining large follicles
This specimen shows large thyroid follicles around the truncus arteriosus and the branching afferent branchial arteries.
Note the attachment of branchial muscles to the gill arch.