It is an essential organ whose functions include growth development, increased muscle gain, decreased fat storage, and production of hormones.
The thyroid gland is essential to maintain the body’s production and use of other hormones and chemicals.
Also, the increase of the metabolism and 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 body’s correct amount of thyroid hormone.
If the body needs more thyroid hormone, the pituitary will increase the production of thyrotropin, stimulating thyroid hormone production.
If the body has enough thyroid hormone or produces too much, the pituitary will 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 an H to a U shape and is formed by two 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, covering 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 to circulate throughout the body.
It weighs more in women and increases in size during menstruation and pregnancy.
Other important anatomical structures surround the thyroid gland in the neck.
In the anterior part, towards the front of the thyroid gland, the gland is covered by the neck muscles, 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 contact the sides of the esophagus.
Several vital 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.
The neck vertebrae, other nerves, arteries, and veins are more posterior to the carotid sheaths.
There are also neck muscles, such as the scalene muscles, lying behind the thyroid gland.
The thyroid gland has an abundant blood supply, ensuring 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 and 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 an anterior division, towards the front and a posterior division, toward the back that irrigates the upper and anterior part of the gland.
The inferior thyroid arteries branch out from the subclavian arteries, 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 ones drain the lower part.
The veins form a plexus or network of tributaries connecting the central 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 essential to remember 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.
The nerves of the thyroid gland travel there through different plexuses, including the cardiac plexus and the superior and inferior thyroid plexuses.
In general, nerves often accompany the inferior and superior thyroid arteries.
Variations in the anatomy of the thyroid gland:
There is another lobe of the thyroid gland in some people, often called a pyramidal lobe.
When present, the pyramidal lobe usually branches into the isthmus and ascends upward toward the hyoid bone.
The pyramidal lobe can be attached to a fibrous tissue band, 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 thyroglossal duct.
The pyramidal lobe is believed to be the remnant of the lower part of the duct that contains thyroid tissue.
Typically 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 thyroglossal duct remnants may contain thyroid tissue outside the gland itself, 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.
Some people have an additional artery called the ima thyroid artery, which usually branches off the brachiocephalic trunk. However, 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 widespread.
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 a thyroid nodule is found, it must be evaluated to ensure 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, and changes in skin, hair, or weight.
Left untreated can cause heart, bone, and other problems.
Graves’ disease is one of the leading causes of hyperthyroidism.
It is an autoimmune disease, more common in women than men, and can be associated with eye problems such as Graves’ eye disease.
Hypothyroidism results from inadequate thyroid hormone production, can cause hair, skin, or weight changes 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.
- Goiter: This refers to the inflammation of the thyroid. Goiters are usually harmless, but they can cause a deficiency of IodineIodine or develop a condition 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 is caused by the autoimmune disease that is the most common cause of hypothyroidism.
- Graves’ disease is an autoimmune condition in which the thyroid is overstimulated, and hyperthyroidism is caused.
- Thyroid cancer 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 is a small mass or abnormal bulk formed in the thyroid gland. Thyroid nodules are widespread and usually are not cancerous. They may secrete an excess of hormones, causing hyperthyroidism, or not cause any problems.
- A thyroid storm is a strange form of hyperthyroidism in which extremely high thyroid hormone levels cause a severe illness.
Proper evaluation of thyroid disease begins with a complete history and physical examination.
A blood test is obtained, and a thyroid ultrasound can be performed depending on the specific problem to look for nodules or other abnormalities.
Other imaging tests can be ordered 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 anti-peroxidase antibodies: When autoimmune thyroid disease occurs, the proteins mistakenly attack the thyroid peroxidase enzyme, which produces thyroid hormones.
- Thyroid ultrasound: A probe is placed on the skin of the neck, and the reflected sound waves can detect abnormal areas of thyroid tissue.
- Thyroid test: A small amount of radioactive IodineIodine is given orally to obtain images of the gland. As reflected in the image, the radioactive IodineIodine is concentrated inside the thyroid gland.
- Thyroid biopsy: A small amount of thyroid tissue is removed in the biopsy. Usually, this procedure is used to detect thyroid cancer.
- Thyroid-stimulating hormone: It is secreted by the brain, is responsible for releasing thyroid hormone, and is known as thyrotropin. A blood test that shows high levels of thyrotropin indicates low thyroid hormone levels (hypothyroidism), and a low concentration of thyrotropin suggests hyperthyroidism.
- T3 and T4 (thyroxine): Thyroxine is one of the primary forms of thyroid hormone, which is verified through a blood test.
- Thyroglobulins: They are only substances secreted by the thyroid gland and which, in turn, are 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, and 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.
Thyroid surgery may be recommended if you suspect the nodule may be cancerous or large enough to cause breathing or swallowing problems.
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 a 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 IodineIodine 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.