Thyroid gland

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The thyroid gland is a bilobular endocrine gland that is found in the neck, anterior and inferior to the larynx. The main purpose of this organ is to produce, store and secrete the iodine-based hormones triiodothyronine (T3) and thyroxine (T4). 

The thyroid gland is encased by a thin connective tissue capsule that enters the substance of the lobes to further subdivide the gland into irregular lobular units. Each lobule contains a cluster of follicles, which are the structural and functional units of the thyroid gland.

Each follicle is surrounded by thin connective tissue stroma rich in fenestrated capillaries. These follicles are lined with follicular epithelium which is simple epithelium consisting of low columnar, cuboidal or squamous cells depending on the level of activity of the follicle. When the follicles are active, they appear cuboidal to low columnar, but when they are inactive the cells are squamous.

These follicular (principal) cells take up the necessary amino acid precursors and iodine at its basolateral surface and release the final product into the blood stream at its basal end. Follicular cells are responsible for producing thyroglobulin (an iodine rich, inactive form of the thyroid hormones), which is then stored as a semi-solid substance (colloid) in the lumen of the follicles.

Another cell type that can be found here is parafollicular cells, also known as C (clear) cells. They can be found within the basal lamina of the thyroid follicles without extending into the follicular lumen or between thyroid follicles in the interfollicular space, either singly or in the form of groups.

Watch the following video to learn more about the histological appearance of the thyroid gland:

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