The thyroid gland is a bilobed endocrine gland. It is unique in that it stores its hormones bound to an extracellular pool of protein (colloid).
Capsule - enclosed by a thin layer of connective tissue.
Trabeculae - connective tissue extends inwards from the capsule to partially outline irregular lobes and lobules.
Parenchyma - secretory follicles constitute the functional units of the gland.
Thyroid Follicles - spherical follicles of varying size (50 to 500 µm) in which thyroid hormones are stored.
Colloid - lumen of each follicle is filled with the gel-like mass called colloid. It is mostly the protein thyroglobulin (pink) and bound thyroid hormones (triiodothyronine and tetraiodothyronine (or thyroxin)). The clear space around the colloid is a shrinkage artifact.
Follicular Cells - follicles are lined by a simple cuboidal to columnar epithelium depending on functional activity. Secrete thyroid hormones when active.