the thyroid gland at the histologic level is composed of numerous microscopic, spherical structures called thyroid follicles. the wall of each follicle is formed by simple cuboidal epithelial cells, called follicular cells, which surround a central lumen.
that lumen houses a viscous, protein-rich fluid termed colloid. thus, follices are microscopic fluid-filled spheres.
the follicular cells produce and later release thyroid hormone vy first synthesizing a glycoprotein called thyroglobulin and secreting it by exocytosis into the colloid fluid. In brief, iodine molecules must be combined with the thyroglobulin in the colloid to produce hormone precursors, which are TGB molecules that comtain immature thyroid hormone within their structure. the precursors are stored in the colloid untill the secretion of thyroid hormone is needed.