Thyroid Gland: Definition, Anatomy, Common Disorders, Conditions, Diagnosis and Treatment

It is an essential organ whose functions include the development of growth, increased muscle gain, decreased fat storage and production of hormones.

The thyroid gland is important to maintain the production and use of other hormones and chemicals in the body.

Also the increase of the metabolism and increase of the effect of the catecholamine , controls the general temperature of the body, among others.

The thyroid gland produces hormones (T3 and T4) that help regulate the function of other organs.

The production of thyroid hormone by the thyroid gland is also regulated by another hormone called thyroid stimulating hormone, which is produced by the pituitary gland (a small gland located in the brain).

The pituitary gland and the thyroid work together to produce the correct amount of thyroid hormone for the body.

If the body needs more thyroid hormone, the pituitary will increase the production of thyrotropin, which in turn will stimulate the production of thyroid hormone.

If the body has enough thyroid hormone or produces too much, the pituitary will then produce less thyrotropin.

Anatomy of the thyroid gland

The thyroid is a highly vascular, reddish-brown gland located in the lower part of the neck.

The gland varies from a H to a U shape and is formed by 2 elongated lateral lobes with upper and lower poles connected by a median isthmus.

These reproduce the image of a small butterfly, with an average height of 12-15 mm, which covers the second to fourth tracheal rings.

The isthmus is found during routine tracheotomy and must be retracted (superior or inferior) or divided.

Occasionally, the isthmus is absent and the gland exists as two distinct lobes.

Inside the neck, the thyroid gland lies between the vertebrae C5 and T1, and overlaps the second, third and fourth rings of cartilage that support the trachea.

The thyroid gland has an abundant blood supply to ensure that the hormones produced in the gland have immediate access to the bloodstream so that they can circulate throughout the body.

It tends to weigh more in women and increases in size during menstruation and pregnancy.

Surrounding structures:

In the neck, the thyroid gland is surrounded by other important anatomical structures.

In the anterior part, towards the front of the thyroid gland, the gland is covered by the muscles of the neck, called sternohyoid and sternothyroid muscles.

The parathyroid glands, which are small, related endocrine glands that produce parathyroid hormone, are often found inside the connective tissue capsule on the posterior surface of the gland.

However, the position of the parathyroid glands is variable.

The isthmus of the thyroid gland lies on the trachea, and the left and right lobes come into contact with the sides of the trachea.

The esophagus is located behind the trachea, and the back of the right and left lobes can come into contact with the sides of the esophagus.

A number of important structures are located behind the thyroid gland.

The carotid sheath contains the common carotid artery , the internal jugular vein, the vagus nerve, and the deep lymph nodes of the neck.

The carotid sheaths are bilateral, there is a carotid sheath on the left and right sides of the neck.

The carotid sheaths are on the back of the left and right lobes of the thyroid gland.

More posterior to the carotid sheaths are the vertebrae of the neck and other nerves, arteries and veins.

There are also neck muscles, such as the scalene muscles that lie behind the thyroid gland.

Blood supply:

The thyroid gland has an abundant supply of blood, which ensures that the hormones it produces can enter the bloodstream and reach other organs and tissues.

The main arteries that supply the thyroid gland are the superior thyroid arteries and the inferior thyroid arteries.

The superior thyroid arteries are the first branches of the external carotid arteries.

They travel down to the upper part of the thyroid gland and then branch into a anterior division, towards the front and a posterior division, toward the back that irrigate the upper and anterior part of the gland.

The inferior thyroid arteries branch out from the subclavian arteries and run behind the carotid sheath and reach the posterior aspect of the gland.

The superior thyroid veins drain the upper part of the thyroid gland, the central thyroid veins drain the central parts of the thyroid gland and the inferior thyroid veins drain the lower part of the thyroid gland.

The veins form a plexus or network of tributaries connecting the main veins, on the anterior surface of the gland.

The superior and middle thyroid veins drain blood from the thyroid gland into the internal jugular veins, and the inferior thyroid veins drain into the brachiocephalic veins.

Nerves in the thyroid gland:

The thyroid gland is innervated by the nerves of the autonomic nervous system, arising from the sympathetic trunk in the neck, specifically, the nerves of the upper, middle and lower cervical sympathetic ganglia.

It is important to keep in mind that nerves do not influence the secretion of hormones from the gland.

This is purely under the control of the pituitary gland, which releases the thyroid stimulating hormone into the bloodstream to signal the thyroid gland to release thyroid hormones into the bloodstream.

The nerves of the thyroid gland travel there through different plexuses, including the cardiac plexus, the thyroid superior and inferior thyroid plexuses.

In general, nerves are often found accompanying the inferior and superior thyroid arteries.

Variations in the anatomy of the thyroid gland:

In some people, there is another lobe of the thyroid gland, which is often called a pyramidal lobe.

When present, the pyramidal lobe usually branches into the isthmus and ascends upward in the direction of the hyoid bone.

The pyramidal lobe can be attached to a fibrous band of tissue, which is sometimes called the thyroid gland elevator.

The pyramidal lobe does not have a real clinical significance and does not cause any problems.

Its presence is linked to the embryology of the thyroid gland, which initially develops at the base of the tongue and descends to the neck through a duct known as the thyroglossal duct.

It is believed that the pyramidal lobe is essentially the remnant of the lower part of the duct that contains thyroid tissue.

Normally the thyroglossal duct recedes, however, in some people, portions of the duct remain and form cysts of the thyroglossal duct.

These cysts can be visible when the person is swallowing, and move upwards when the tongue is pulled out.

They are usually painless, however, they can cause some pain when the person is swallowing, or if they become infected.

The cysts can be removed surgically.

In some people, the remnants of the thyroglossal duct may contain thyroid tissue outside the gland itself; This is known as ectopic thyroid tissue.

Another abnormality related to the embryology of the thyroid gland is called the lingual thyroid.

This is where thyroid tissue is located at the base of the tongue, where the thyroid gland initially develops during fetal life, this region is called blind foramen.

People with lingual thyroid glands often experience pain when swallowing and talking.

There are also variations in the vascular anatomy of the thyroid gland.

In some people, there is an additional artery called the ima thyroid artery , which usually branches off the brachiocephalic trunk, but it can also come from the aorta, the subclavian artery, or the right common carotid artery.

The thyroid artery is directed towards the thyroid gland along the surface of the trachea, essentially in the midline.

When present, it supplies blood to the trachea and isthmus of the thyroid gland.

Common thyroid disorders

Thyroid nodules are very common.

About 95% of the nodules are benign (non-cancerous) and most do not require any treatment or surgery.

While thyroid nodules can be found in up to 60% of patients, especially in older patients, only 5 to 8% can be palpated.

Most thyroid nodules are identified in imaging studies performed for reasons unrelated to the thyroid gland (carotid duplex, CT scans of the neck or chest).

Once the finding of a thyroid nodule is found, it must be evaluated to make sure it is benign.

Even if nodules are found inside the thyroid gland, the gland usually continues to function normally.

Occasionally, some thyroid nodules produce an excess of thyroid hormone that leads to hyperthyroidism .

Hyperthyroidism is the result of the production of excess thyroid hormone .

Hyperthyroidism can lead to weight loss, rapid heartbeat, sweating, nervousness, changes in skin, hair or weight.

If left untreated, it can cause heart, bone and other problems.

Graves’ disease is one of the main causes of hyperthyroidism.

It is an autoimmune disease, more common in women than in men, and can be associated with eye problems such as Graves’ eye diseases.

The hypothyroidism is the result of inadequate production of thyroid hormone can cause changes in hair, skin or weight, and can cause fatigue, weakness and other problems.

Hashimoto’s thyroiditis is an autoimmune disorder that can lead to hypothyroidism.

Thyroid cancer is found in about 5% of thyroid nodules.

The most common types of thyroid cancer are papillary and follicular.

The long-term results of thyroid cancer are generally quite good.

Hurthle cell cancers, medullary thyroid cancers, thyroid lymphoma and anaplastic thyroid cancers are much less common.

Terms

  • Goiter:  This refers to the inflammation of the thyroid. Goiters are usually harmless, but they can cause a deficiency of Iodine or develop a condition that is associated with thyroid inflammation called chronic thyroiditis or Hashimoto’s disease.
  • Thyroiditis:  Also refers to the inflammation of the thyroid, when it is caused by a viral infection or an autoimmune condition. Thyroiditis may be asymptomatic or, on the contrary, painful.
  • Hyperthyroidism: It is the excessive production of thyroid hormone. Hyperthyroidism is most often caused by the incidence of Graves’ disease or hyperactivity of a thyroid nodule.
  • Hypothyroidism: It is the low production of thyroid hormones. Thyroid damage caused by the autoimmune disease that is the most common cause of hypothyroidism.
  • Graves’ disease: It is an autoimmune condition in which the thyroid is over stimulated and hyperthyroidism is caused.
  • Thyroid cancer: It is an uncommon form of cancer, thyroid cancer is usually a curable disease. Procedures such as surgery, radiation, and hormone treatments can be used to treat thyroid cancer.
  • The thyroid nodule: It is a small mass or abnormal bulk that is formed in the thyroid gland. Thyroid nodules are extremely common and usually are not cancerous. They may secrete an excess of hormones, or cause hyperthyroidism, or not cause any problems.
  • Thyroid Storm: It is a strange form of hyperthyroidism in which extremely high levels of thyroid hormone cause a serious illness.

Diagnosis

Proper evaluation of thyroid disease begins with a complete history and physical examination.

A blood test is obtained, and depending on the specific problem, an ultrasound of the thyroid can be performed to look for nodules or other abnormalities.

Other imaging tests that can be ordered include CT scans, MRIs, or nuclear medicine studies such as radioiodine and positron emission tomography.

If a thyroid nodule or other abnormality is detected, your doctor may recommend a biopsy. Within the tests to detect thyroid disorders we have:

  • Thyroid antiperoxidase antibodies: When autoimmune thyroid disease occurs, the proteins mistakenly attack the enzyme thyroid peroxidase, which is used by the thyroid to produce thyroid hormones.
  • Thyroid ultrasound: A probe is placed in the skin of the neck and the reflected sound waves can detect abnormal areas of thyroid tissue.
  • Thyroid test: In this test, a small amount of radioactive iodine is given orally to obtain images of the gland. The radioactive iodine is concentrated inside the thyroid gland, reflected in the image.
  • Thyroid biopsy: In the biopsy a small amount of thyroid tissue is removed, usually this procedure is used to detect thyroid cancer.
  • Thyroid stimulating hormone:  It is secreted by the brain, is responsible for the release of thyroid hormone and is known as thyrotropin. A blood test that shows high levels of thyrotropin indicates low levels of thyroid hormone (hypothyroidism), and a low concentration of thyrotropin suggests hyperthyroidism.
  • T3 and T4 (thyroxine):  Thyroxine is one of the main forms of thyroid hormone, which are verified through a blood test.
  • Thyroglobulins:  They are only substances secreted by the thyroid gland and which in turn is used as a marker element in thyroid cancer. High levels indicate the recurrence of cancer.
  • Other imaging tests: If thyroid cancer has metastasized, magnetic resonance imaging, CT scans, positron emission tomography scans can help identify the extent of the metastasis.

Treatment for thyroid disorders

To decide the best course of treatment, each patient must be carefully evaluated.

In most cases, the evaluation shows that the nodule is benign (not cancerous) and that the function of the thyroid is normal.

In this circumstance, a specific treatment is usually not needed, and the thyroid nodule can be followed to ensure that there are no disturbing changes over time.

If you suspect that the nodule may be cancerous or large enough to cause problems breathing or swallowing, thyroid surgery may be recommended.

The surgeon will evaluate the appropriate surgical options for each patient.

Surgical options range from removing only the side of the thyroid gland that contains the nodule called thyroid lobectomy or hemithyroidectomy, to extirpating the entire thyroid gland called total thyroidectomy, these procedures are performed in cases of thyroid cancer, goiter or hyperthyroidism.

Treatment may include antithyroid medications such as propylthiouracil and methimazole. These medications can delay the excessive production of thyroid hormone in hyperthyroidism.

The radioactive iodine is used in low doses to destroy an overactive gland and in large doses to eliminate cancerous tissue.

Thyroid hormone pills treat hypothyroidism and are also used to help prevent thyroid cancer from coming back after treatment.

You can also recommend treatments with external radiation.