The thyroid gland is a bilobed or "butterfly-shaped" structure located over the upper part of the trachea. It serves to regulate metabolism as well as affects gene expression and blood calcium levels. It is the only endocrine gland that stores its secretions extracellularly.

Identify the capsule, the connective tissue septa that separates the thyroid into lobules and the follicles containing pink-stained colloid (iodinated thyroglobulin).

How would you characterize the secretory state of this gland? What criteria do you use? Do the different diameters of the follicles represent different levels of activity? Generally the taller the follicular cell, the more actively the cells are ingesting colloid to release thyroid hormones into the blood. Squamous follicular cells generally are found in inactive follicles.

The colloid is closely applied to the apical surfaces of the epithelial cells. Separation of colloid from the wall of the follicle is an artifact caused by tissue dehydration, although vacuolated colloid results when follicular cells phagocytose the stored colloid and process it prior to thyroid hormone secretion into the blood stream.

You may see abundant small blood vessels between the follicles if you look carefully. Can you identify any pale-staining parafollicular or "C" cells? (The "C" is for "clear".) These cells are the source of calcitonin, but may not be readily identifiable in your H&E slides. Consult your text for further identification. These cells are located within the basal lamina of the follicular wall, just under the follicular lining cells. They are found in relatively high numbers in the middle third of each lateral lobe and so may not be easily seen in your sections.

Calcitonin helps decreases blood calcium by increasing calcium storage in bone (memory aid: calcitonin = bone in).

REFERENCES: Gardner, p. 208-209, Fig. 1 - 2Ross, p. 628 - 629, Fig. 2 Wheater, p. 309, Fig. 17.6 DiFiore, pp. 268 - 271 p. 310, Fig. 17.7