About thyroid gland anatomy
The thyroid gland is located in the neck. It is described as having a butterfly shape, because it has a right lobe and a left lobe that are connected in the centre by a thinner segment of thyroid tissue, called the isthmus. Within the neck, the thyroid gland is located between the C5 and T1 vertebra, and overlies the second, third and fourth cartilage rings supporting the trachea (windpipe). The thyroid gland has a rich blood supply to ensure that the hormones produced in the gland have ready access to the bloodstream so that they can circulate around the body. On average, the thyroid gland weighs approximately 30 grams. It tends to weigh more in women, and increases in size during menstruation and pregnancy.
In the neck, the thyroid gland is surrounded by a number of other important anatomical structures. Anteriorly (towards the front of the thyroid gland), the gland is covered by muscles of the neck, called the sternohyoid and the sternothyroid muscles. The thyroid gland itself is covered by a capsule made of connective tissue. The parathyroid glands, which are small, related endocrine glands that make parathyroid hormone, are often found within the connective tissue capsule on the posterior (back) surface of the gland. However, the position of the parathyroid glands is variable.
The isthmus of the thyroid gland lies over the trachea, and the left and right lobes contact the sides of the trachea. The oesophagus sits behind the trachea, and the very rear portion of the right and left lobes may contact the sides of the oesophagus.
A number of important structures lie posterior to the thyroid gland. The carotid sheath contains the common carotid artery, the interior jugular vein, the vagus nerve and deep lymph nodes of the neck. The carotid sheaths are bilateral; that is, there is a carotid sheath on both the left and the right sides of the neck. The carotid sheaths are posterior relations of both the left and the right lobes of the thyroid gland. Further posterior to the carotid sheaths are the vertebrae of the neck, and other nerves, arteries and veins. There are also muscles of the neck, such as the scalene muscles sitting behind the thyroid gland.
The thyroid gland has a rich blood supply, ensuring that the hormones it produces can enter the bloodstream and reach other organs and tissues. The main arteries supplying 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 downwards to the upper part of the thyroid gland and then branch into an anterior division (towards the front) and a posterior division (towards the back) which supply the upper, anterior part of the gland. The inferior thyroid arteries branch from the subclavian arteries and run behind the carotid sheath and reach the posterior aspect of the gland. Here, the inferior thyroid arteries divide into a number of branches that supply the lower, posterior parts of the gland.
There are 3 main pairs of veins which drain the thyroid gland. The superior thyroid veins drain the upper part of the thyroid gland, the middle 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 (a network of connecting tributaries of the major veins) on the anterior surface of the gland. The superior and middle thyroid veins drain the blood from the thyroid gland into the internal jugular veins, and the inferior thyroid veins drain into the brachiocephalic veins.
Thyroid gland nerve supply
The thyroid gland is innervated by nerves of the autonomic nervous system, arising from the sympathetic trunk in the neck (specifically, nerves from the superior, middle and inferior cervical sympathetic ganglia). It is important to note that the nerves do not influence the secretion of hormones from the gland; this is purely under the control of the pituitary gland, which releases thyroid-stimulating hormone into the bloodstream to signal to the thyroid gland to release thyroid hormones into the bloodstream. The nerves to the thyroid gland travel there via different plexuses, including the cardiac plexus, superior and inferior thyroid peri-arterial plexuses. In general, the nerves are often found accompanying the inferior and superior thyroid arteries.
Variations in anatomy
In some people, there is another lobe of the thyroid gland, which is often called the pyramidal lobe. When present, the pyramidal lobe generally branches off the isthmus and ascends superiorly in the direction of the hyoid bone. The pyramidal lobe may be attached to a fibrous band of tissue, which is sometimes called the levator of the thyroid gland. The pyramidal lobe has no 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 into the neck via a passage known as the thyroglossal duct. It is believed that the pyramidal lobe is essentially the remnant of the bottom part of the duct that contains thyroid tissue.
Normally the thyroglossal duct regresses, however, in some people, portions of the duct remain and form thyroglossal duct cysts. These cysts may be visible when the person is swallowing, and move upwards when the tongue is poked out. They are usually painless; however they may cause some pain when the person is swallowing, or if they become infected. The cysts may be removed surgically. In some people, thyroglossal duct remnants may contain thyroid tissue outside of the gland itself; this is known as ectopic thyroid tissue.
Another abnormality related to the embryology of the thyroid gland is called lingual thyroid. This is where thyroid tissue is found at the base of the tongue, in the location where the thyroid gland develops initially during foetal life – this region is called the foramen caecum. People with lingual thyroid glands often experience pain when swallowing, speaking and talking.
There are also variations to the vascular anatomy of the thyroid gland. In some people, there is an additional artery, called the thyroid ima artery, which usually branches off the brachiocephalic trunk, but can also come from the aorta, subclavian artery or the right common carotid artery. The thyroid ima artery runs upwards towards the thyroid gland along the surface of the trachea, essentially in the midline. When present, it supplies blood to the trachea and the isthmus of the thyroid gland.
If you have questions about the thyroid gland make an appointment to see our thyroid surgeon.