MH 151 Thyroid

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Thyroid

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).

Stroma
- - enclosed by a thin layer of connective tissue.
- - connective tissue extends inwards from the capsule to partially outline irregular lobes and lobules.
Parenchyma - secretory follicles constitute the functional units of the gland.
- - spherical follicles of varying size (50 to 500 µm) in which thyroid hormones are stored.
  - - 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.
  - - follicles are lined by a simple cuboidal to columnar epithelium depending on functional activity. Secrete thyroid hormones when active.
  - - a rich network surrounds each follicle.
- Parafollicular Cells (,)- small numbers of larger cells located at the periphery of follicles that secrete calcitonin. They stain poorly with H&E making identification difficult.