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Thyroid Gland

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The thyroid gland is a butterfly-shaped endocrine organ located in the neck, just below the larynx (voice box). The anterior pituitary gland controls the thyroid gland. It secretes the thyroid hormone. This hormone affects all cells in the body to make more ATP and increase metabolism.

Microscopic view of thyroid tissue highlighting the colloid-filled follicles, C cells, and parafollicular cells. The cells, stained in shades of pink and purple, are arranged around several follicles forming a network. Annotations indicate 'Colloid' in follicles, and 'parafollicular cells' and 'C cells' within.
Figure 1: Thyroid histology showing colloid, C cells, and parafollicular cells with annotations.
Illustration showing the anatomy of the trachea and surrounding structures. Visible and labeled are the hyoid bone at the top, followed by the thyroid cartilage, cricoid cartilage, a tan-colored thyroid gland illustrating right and left lobes, and the isthmus that connects them. The trachea is shown at the bottom.
Figure 2: Labeled illustration of trachea structures, including thyroid gland, hyoid bone, and trachea.

Location

The thyroid organ consists of a left and right lobe. These lobes are connected by an anterior strip of tissue, the isthmus, that wraps around the trachea or windpipe. The left and right lobe can be seen from a posterior view. They have the 4 parathyroid glands on the posterior face.

Colloid

The thyroid gland is primarily composed of spherical structures called thyroid follicles. Colloid is a non-cellular, protein-rich substance. It fills the lumen (central cavity) of thyroid follicles. This substance serves as a reservoir for thyroid hormone storage. Circular and oval areas of colloid can be visualized from low magnifications of the thyroid gland. They appear in the picture to the right as lightly stained pink bubbles.

Colloid contains thyroglobulin, a large glycoprotein precursor of thyroid hormones thyroxine (T4) and triiodothyronine (T3). When these hormones are low in systemic circulation, the edges of the colloid become wavy. These hormones are then released into circulation.

Follicular Cells

Follicular cells form a ring of simple cuboidal cells that surround and physically touch the colloid. Follicular cells synthesize and secrete thyroglobulin molecules into the colloid. There, they undergo iodination, which is the addition of iodine atoms. They are then cleaved to release thyroid hormones (T3 and T4) when needed.

C Cells

Scattered among the follicular cells are clusters of parafollicular cells (para- means “to the side”), also known as C cells. These cells do not touch the colloid and have no role in thyroid homeostasis. C cells produce and secrete calcitonin, a hormone involved in regulating calcium and phosphate levels in the blood.

Histological section of the thyroid gland, centrally composed, taken in brightfield microscopy. The image displays pink-stained thyroid follicles against a white background. Follicular size varies across the tissue, displaying a diverse network; some follicles are small and densely packed, while others are larger.
Figure 3: Thyroid tissue histology showing follicles stained in pink.
Histological section of the thyroid gland, centrally composed, taken in brightfield microscopy. The image displays pink-stained thyroid follicles against a white background. Follicular size varies across the tissue, displaying a diverse network; some follicles are small and densely packed, while others are larger.
Figure 4: Thyroid tissue histology showing follicles stained in pink.
The image presents three distinct visual elements arranged horizontally. On the left is an illustration of the thyroid gland, a bi-lobed, tan structure. An orange gradient extends from this illustration toward a square that contains a schematic illustration of thyroid tissue including, light-tan follicular cells, a pink colloid, and pink and red C-cells. Black lines extend from the square, labeling these different cells. The rightmost portion of the image displays a histology image of thyroid tissue stained pink and purple. The arrangement compares the structure of the thyroid gland from a macroscopic to microscopic view, using both diagrammatic and realistic representations.
Figure 6: Thyroid gland anatomy: Illustration and histology showing follicles, follicular cells, colloid, and C-cells.

Thyroid Hormone Homeostasis

When thyroid hormone (TH) levels are low, the hypothalamus (HT) is able to detect this in blood.  In response, the hypothalamus releases thyroid releasing hormone (TRH) into the hypophyseal portal system in the infundibulum. 

TRH then travels to the anterior pituitary gland (APG) where it triggers the release of thyroid stimulating hormone (TSH). TSH is released into the systemic circuit and targets receptors on the follicular cells of the thyroid organ. 

The follicular cells in the thyroid begin to secrete the thyroid hormone, which is also released in the systemic circuit. 

Anatomical diagram showing the thyroid hormone feedback loop. Red arrows indicate the release of Thyroid Releasing Hormone from the hypothalamus. A green arrow displays Thyroid Stimulation Hormone being released from the Anterior Pituitary Gland. A blue arrow displays Thyroid Hormone at the Thyroid Gland. Also shown is a sagittal view of the brain with the hypothalamus circled. The bottom shows a person from the clavicles up with a depiction of thyroid.
Figure 7: Thyroid hormone regulation, showing hypothalamus, pituitary gland, and thyroid gland interacting.

Calcium Homeostasis

Calcitonin inhibits osteolysis by osteoclasts and promotes calcium deposition by osteoblasts, thereby reducing it in the blood plasma. Calcitonin encourages the small intestine to reduce calcium absorption from food. It also causes the kidney to secrete more calcium into the urine. These actions reduce the intake of calcium and increase the removal of calcium.

All Figures: Thyroid gland and parathyroid, H&E, 40X Slide 217
All by University of Michigan Histology, licensed under CC BY-NC-SA 3.0.

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Thyroid Gland

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