Microscopic Structure of the Thyroid Gland
Overview
The thyroid gland is unique among endocrine glands in that it stores its secretory product extracellularly in the form of colloid within follicles, rather than storing hormones intracellularly in secretory granules.
Capsule and Stroma
- Enclosed by a thin fibrous capsule (true capsule)
- Septa extend inward, dividing the gland into lobules
- Each lobule contains 20–40 follicles
- Stroma contains blood vessels, lymphatics, nerves, and parafollicular (C) cells
Follicles — The Functional Units
The follicle is the structural and functional unit of the thyroid gland.
Structure:
- Spherical or ovoid, closed sac
- Size: Varies — resting follicles are larger; active follicles are smaller
- Wall: Single layer of follicular epithelial cells (follicular cells / thyrocytes)
- Lumen: Filled with colloid — a homogeneous, eosinophilic, PAS-positive material
Follicular Cells (Thyrocytes)
| Feature | Active (Stimulated) | Resting (Inactive) |
|---|---|---|
| Shape | Columnar / cuboidal | Flat / squamous |
| Nucleus | Round, basal | Flat |
| Colloid | Less; scalloped margins (resorption lacunae) | Abundant; smooth margins |
| Appearance | Tall cells, small lumen | Flat cells, large colloid-filled lumen |
Function of follicular cells:
- Synthesise thyroglobulin (a glycoprotein)
- Iodinate thyroglobulin within the colloid to form T3 and T4 precursors
- Reabsorb colloid by endocytosis
- Cleave T3 and T4 from thyroglobulin using proteolytic enzymes
- Secrete T3 (triiodothyronine) and T4 (thyroxine/tetraiodothyronine) into the bloodstream
Colloid
- Homogeneous, eosinophilic, PAS-positive material
- Composed mainly of thyroglobulin
- In hyperactive glands (e.g., Graves’ disease): colloid is reduced, scalloped resorption lacunae at margins
- In hypothyroid states: colloid accumulates, follicles enlarge
Parafollicular Cells (C Cells / Light Cells)
| Feature | Detail |
|---|---|
| Location | In the wall of follicles or in interfollicular stroma (not lining the lumen) |
| Origin | Neural crest (ultimobranchial body — 4th pharyngeal pouch derivative) |
| Appearance | Large, pale (clear), polygonal cells — hence “C cells” (C = clear, or calcitonin) |
| Secretion | Calcitonin — lowers blood calcium |
| Stain | Argyrophilic (silver staining); immunohistochemistry for calcitonin positive |
Clinical significance: Parafollicular C cells give rise to medullary carcinoma of thyroid — secretes calcitonin (used as a tumour marker). Associated with MEN 2A and 2B syndromes.
Histological Appearance Summary
| Feature | Appearance |
|---|---|
| Follicles | Round/oval spaces lined by follicular cells |
| Colloid | Pink (eosinophilic), homogeneous, PAS-positive |
| Active gland | Columnar cells, scalloped colloid, small follicles |
| Resting gland | Flat cells, smooth abundant colloid, large follicles |
| C cells | Pale, large cells in interfollicular spaces |