It produces two types of hormones; the iodine-containing hormones tri-iodothyronine or T3, thyroxine or T4, and calcitonin.
They are responsible for secreting parathyroid hormone, or PTH for short.
Let’s first look at the thyroid gland, which has a thin capsule of connective tissue surrounding it that can be seen in this low power image, mostly on the right side.
The connective tissue extends inward along with blood vessels, lymphatics, and nerves.
Although there aren’t any nerves clearly visible in this image.
The thyroid gland is unique because it’s the only endocrine gland that stores its inactive hormones extracellularly in follicles.
The follicles can vary in size quite a lot, from 50 to 500 um.
Each follicle is filled with a gel-like mass called colloid, which stains dark pink with H&E.
The colloid consists mostly of thyroglobulin and thyroid hormones that are bound to the colloid.
We can see that some follicles have colloid with a “scalloped” pale edge.
This is where the follicle has extracted some of the stored colloid in preparation for hormone activation and secretion.
On the other hand, the clear space around the colloid is just an artifact that was caused by the colloid shrinking in comparison to the follicular wall.
The follicular epithelium is made of a single layer of simple cuboidal cells with round central nuclei and dark chromatin.
These cells are responsible for secreting thyroid hormones when needed.
The nuclear features of these cells are particularly important because they are the main feature looked at when diagnosing papillary thyroid carcinoma.
In papillary thyroid carcinoma, the nuclei will be large and irregular with light chromatin, similar to this image, which has pale intranuclear inclusions.
Still at 40x magnification, this image has an example of a parafollicular cell, also called a C cells, near the center.
These pale-stained cells have a granular cytoplasm and are responsible for secreting calcitonin.
They are larger than follicular cells, but there are fewer of them.
They can usually be found in the periphery within the follicular lining, or in the interstitial space between follicles.