In this note, we will discuss the anatomy and function of the thyroid gland (glandula thyroidea).

It is an endocrine gland that produces thyroid hormones and calcitonin, both of which affect almost all the metabolic processes in our body.

Anatomy of the thyroid gland

The thyroid gland is a butterfly-shaped structure, located in the neck area, on the anterior surface, along the midline, within the boundaries between the vertebrae С5 and T1. The organ is covered anteriorly by the sternothyroid and sternohyoid muscles.

The gland consists of two lobes situated on either side of the trachea:

• Left lobe (lobus sinister)

• Right lobe (lobus dexter)

These lobes are connected by the isthmus, which is located at the level of the second and third tracheal rings.

Some people have an additional lobe, known as the pyramidal lobe (lobus pyramidalis).

Externally, the thyroid gland is covered by a fibrous capsule (capsula fibrosa), which divides the gland into smaller lobules by invading the so-called septa into the parenchyma.

Within these lobules we can find the follicles, the storage sites for hormones that are synthesized by specialized cells known as thyrocytes.

The gland’s capsule is firmly attached to the cricoid cartilage of the larynx (cartilago cricoidea) and the superior rings of the trachea.

Externally to the capsule, the gland and adjacent structures are covered by the pretracheal layer of the deep cervical fascia.

Thyroid hormones

Thyroid hormones are synthesized in our body under control of the hypothalamic-pituitary system.

When the level of thyroid hormones in the blood decreases, the hypothalamus starts producing thyrotropin-releasing hormone (TRH), or thyroliberin. It reaches the adenohypophysis via the hypophyseal portal system and initiates the synthesis of thyroid-stimulating hormone or thyrotropin (TSH).

TSH travels to the thyroid gland and regulates the synthesis of thyroglobulin, a protein that serves as the precursor to the main thyroid hormones – thyroxine (T4) and triiodothyronine (T3).

These hormones have an impact on almost all systems of our body. You can see their effects in the diagram.

Another hormone produced by the thyroid gland is thyrocalcitonin, commonly known as calcitonin. It is synthesized by parafollicular C-cells and plays a role in regulating phosphorus-calcium metabolism. Calcitonin lowers the levels of calcium and phosphates in the blood, increases the activity of osteoblasts, and decreases the activity of osteoclasts. It acts as an antagonist to parathyroid hormone.

Blood supply and innervation of the thyroid gland

Blood supply to the thyroid gland is provided by two main arteries: the superior thyroid artery (a. thyroidea superior),

which is a branch of the external carotid artery (a. carotis externa),

and the inferior thyroid artery (a. thyroidea inferior),

a branch of the thyrocervical trunk (truncus thyrocervicalis).

In some individuals, there is an additional tiny artery, the thyroid ima artery (a. thyreoidea ima), which can originate from various vessels.

It’s important to remember that the external laryngeal nerve runs close to the superior thyroid artery, while the recurrent laryngeal nerve is located near the inferior thyroid artery.

Venous drainage from the thyroid gland is provided by three pairs of veins:

The superior thyroid vein (v. thyroidea superior)

and the middle thyroid vein (v. thyroidea media)

drain into the internal jugular vein (v. jugularis interna),

and the inferior thyroid vein (v. thyroidea inferior)

drains into the brachiocephalic vein (v. brachiocephalica).

These three pairs of veins form the thyroid venous plexus (plexus thyroideus).

The innervation of the thyroid gland is primarily provided by the sympathetic nervous system, specifically nerves originating from the superior, middle, and inferior cervical ganglia. The gland also receives a certain degree of parasympathetic innervation from the external and recurrent laryngeal nerves.

The lymphatic vessels of the thyroid gland drain into the thyroid, prelaryngeal, pretracheal, and paratracheal lymph nodes.

Glossary

Thyroid gland

thyroid gland

glandula thyroidea

left lobe

lobus sinister

right lobe

lobus dexter

isthmus

isthmus

pyramidal lobe

lobus pyramidalis

fibrous capsule

capsula fibrosa

cricoid cartilage

cartilago cricoidea

superior thyroid artery

a. thyroidea superior

inferior thyroid artery

a. thyroidea inferior

thyroid ima artery

a. thyreoidea ima

superior thyroid vein

v. thyroidea superior

middle thyroid vein

v. thyroidea media

inferior thyroid vein

v. thyroidea inferior

thyroid venous plexus

plexus thyroideus