Thyroid and parathyroid gland histology

Definitions & Key takeaways

The thyroid gland and parathyroid glands are two distinct endocrine glands located in the neck. The thyroid gland is a butterfly-shaped endocrine gland located anterior and inferior to the larynx. It produces hormones like triiodothyronine, thyroxine, and calcitonin. The parathyroid glands consist of four small ovoid glands located on the posterior surface of the thyroid gland. They are responsible for secreting parathyroid hormone (PTH).

The thyroid is composed of numerous spherical sacs called thyroid follicles. Each follicle is lined by a single layer of follicular cells that synthesize, store, and secrete thyroid hormones (T3 and T4). The colloid, which fills the lumen of the follicles, is composed of thyroglobulin, a glycoprotein that serves as a precursor for the synthesis of thyroid hormones. Parafollicular cells, also called C cells, are located between the follicles and produce the hormone calcitonin, which helps regulate blood calcium levels.

The parathyroid glands are composed of chief cells and oxyphil cells, both of which are responsible for the synthesis and secretion of PTH. PTH helps regulate blood calcium levels by promoting the release of calcium from bones and increasing the reabsorption of calcium by the kidneys. The chief cells are the most numerous and are responsible for most hormone production. Oxyphil cells are larger and less numerous and their function is not well understood.

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The thyroid gland is a butterfly-shaped endocrine gland with average weight of 25-30 grams, and located anterior and inferior to the larynx.
It produces two types of hormones; the iodine-containing hormones tri-iodothyronine or T3, thyroxine or T4, and calcitonin.
The parathyroid glands consist of four small ovoid glands weigh 30 milligrams each and are located on the posterior surface of the thyroid gland.
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.
Because they don’t absorb a lot of the H&E stain, they can be difficult to identify. Moving onto the parathyroid glands, this low magnification image shows how different the tissue looks even at first glance.
But the parathyroid does have its own thin fibrous capsule that also extends inward, carrying nerves, lymphatics, and blood vessels.
Although, the nerves and lymphatics are not easily seen in this image. The fibrous connective tissue in the parathyroid also forms irregular lobules.
Up to 40% of the normal parathyroid gland tissue is composed of adipose tissue. Zooming in closer to a different area of the parathyroid, we can see a lot of the glandular cells.
The glandular cells are made of mainly two types of cells, the chief or principal cells and the oxyphil cells. The majority of the parathyroid cell are chief cells, which are smaller in size; have a dark, round, and central nuclei; and a pale cytoplasm.
These are the cells responsible for secreting PTH. If the cytoplasm looks darker, it’s because of an increase in rough endoplasmic reticulum or RER, which stains strongly and is a sign that the cell is actively secreting PTH.
The oxyphil cells are the much larger cells that also have dark nuclei, but they also have a very eosinophilic cytoplasm because of their higher content of mitochondria.
These cells don’t secrete PTH, and they actually increase in number overall with age. Adipocytes can also be seen within the parathyroid in this image, and they increase in number with age as well.
Alright, as a quick recap. The thyroid is unique because it stores inactive hormones extracellularly in colloid follicles that stain pink with H&E.
The follicular epithelial cells are simple cuboidal cells, and are responsible for secreting T3 and T4 when needed. The parafollicular cells or C cells secrete calcitonin and have a larger, pale cytoplasm.
The parathyroid consists mostly of glandular chief cells that have round, central nuclei and a small pale cytoplasm. These are the only cells that secrete PTH.
The oxyphil cells are noticeably bigger due to their larger, eosinophilic cytoplasm.