Video: Thyroid and parathyroid glands

This is the thyroid gland. Yes, that well-known, butterfly-shaped gland found in the neck region. Although comparatively tiny, it controls many vital processes in our body like metabolism, cardiovascular function, and even growth and development. But did you know that an estimated 12% of the population have some form of thyroid disease? Or that women are five to eight times more likely to develop a thyroid disorder than men? Or that thyroid disease, if left untreated, can put patients at risk of developing cardiovascular disease, infertility, osteoporosis, and more? Sounds like some important things to be aware of, huh? So, if you want to learn more about this little, yet mighty control center of the body, stay with us while we explore the thyroid and the parathyroid glands.

Before we jump into the details, here's a quick overview of what we'll cover today. We'll begin by looking at the gross anatomy of the thyroid gland including its lobes, then we'll identify the parathyroid glands and where they are. Next, we'll look at the cartilaginous and ligamentous structures of the larynx and trachea upon which the thyroid and parathyroid glands are found. We'll then move into the vascular supply and innervation of the gland, and finally, we'll explore some clinical notes to wrap up.

Are you ready? Let's get to it!

First of all, what is the thyroid gland? The thyroid gland is an organ that is part of the endocrine system. Its main function is to regulate metabolism throughout the body by producing the hormones triiodothyronine and thyroxine. It also helps to regulate calcium levels in the blood by producing the hormone, calcitonin. The thyroid gland is located in the base of the neck, just anterior and lateral to the trachea and inferior portion of the larynx. It spans from approximately the level of C5 to T1.

The thyroid gland is generally butterfly or eight-shaped in appearance and is composed of two primary lobes, each with a defined superior and inferior pole. These two main lobes are simply known as the right lobe and the left lobe of the thyroid gland and are roughly similar in size and shape. Connecting the two lobes is a region of the thyroid gland called the isthmus, which we can see from both an anterior and posterior view.

In many people, there can be a smaller third lobe, which is only present in about forty percent of the population. This is the pyramidal lobe of the thyroid gland and can vary in size and appearance. It is thought to be a remnant of the thyroglossal duct which is present during development. Functionally, however, it is the same as the other lobes of the thyroid gland.

The other glands that feature in this tutorial are the parathyroid glands, which we can now see highlighted in green. In this image, we're looking at the thyroid gland from a posterior view, the left lobe being here on the left of the screen and the right lobe being here. Usually, there are four parathyroid glands in total – two on the posterior aspect of each lobe of the thyroid gland. Each parathyroid gland is about the size and shape of a pea at roughly six millimeters long, three to four millimeters wide, one to two millimeters deep, and weighing on average fifty to sixty milligrams. That makes it ten times lighter than a tic-tac.

The number, location, and size of the parathyroid glands can vary, however, which is important to keep in mind for events such as surgery or cancer metastasis. The parathyroid glands are also endocrine glands, producing hormones that play an essential role in the regulation of calcium levels. The hormone they produce is called parathyroid hormone, which raises calcium levels in the blood.

Now that we've covered the glands themselves, let's have a look at the structures around them.

In this image, we can see the thyroid gland highlighted in its anatomical position within the neck. As we've already seen, it sits along the anterior and lateral aspects of the cervical part of the trachea and the inferior portion of the larynx. In this zoomed-in image, we can see the thyroid cartilage highlighted in green, superior to the thyroid gland. This structure is a major cartilage of the larynx. The apices of the right and left lobes of the thyroid gland extend to the oblique lines on the thyroid cartilage.

Inferior to the thyroid cartilage is the cricoid cartilage, now highlighted in green. This cartilage is the only full ring cartilage in the windpipe and is just visible superior to the thyroid gland. Inferior to the cricoid cartilage is where the trachea begins. Anterior to the superior part of the trachea is where the isthmus of the thyroid gland lies. Between these pieces of cartilage are ligaments holding more rigid structures together. Highlighted now between the cricoid cartilage and the thyroid cartilage is the cricothyroid ligament. Between the cricoid cartilage and the first tracheal ring is the cricotracheal ligament. Pretty handy that the names of these ligaments tell you the structures they connect.

Located anterior to the thyroid gland are the infrahyoid muscles, also known as the strap muscles of the neck. In this cross section of the neck, we can see the sternothyroid, the sternohyoid, and the omohyoid muscles here. The thyroid gland is an especially well-vascularized structure, so let's take a look at how the body supplies both the thyroid and parathyroid glands.

The brownish-red, highly vascular thyroid gland receives blood from two pairs of arteries – the superior thyroid arteries and the inferior thyroid arteries. In this image, we can see the larynx and trachea within the neck. Remember our thyroid gland is anterior to these structures so would sit just about here.

The superior thyroid artery which we can now see highlighted in green arises from the external carotid artery. It splits into an anterior branch, a posterior branch and a lateral branch. From this lateral view of the head and neck, we can see the superior thyroid artery arising from the external carotid artery and traveling along the thyroid gland. The anterior branch supplies the anterior portion of the gland and travels towards the isthmus. The posterior branch supplies the superior and posterior portion of the thyroid gland. Finally, the lateral branch supplies the lateral portion of the thyroid gland.

Moving down to the inferior thyroid artery, it often arises from the thyrocervical trunk which comes off the subclavian artery. The inferior thyroid artery also has two branches – this time into an ascending or superior branch and an inferior branch – which supply the inferior and posterior aspects of the thyroid gland.

Back to this lateral view of the head and neck, we can see the subclavian artery here and the thyrocervical trunk arising from it. The inferior thyroid artery is highlighted in green branching from the thyrocervical trunk and traveling towards the thyroid gland.

It's important to note that the superior and inferior thyroid arteries anastomose with each other both from the same and the opposite side. About ten percent of the population have an additional non-paired artery that supplies the thyroid gland called the thyroid ima artery. If present, this artery most often arises directly from the brachiocephalic trunk, but can occasionally come directly from the arch of the aorta or the right common carotid artery.

Draining the thyroid gland, we have three pairs of veins – the superior, the middle, and inferior thyroid veins. All these veins emerge from a thyroid venous plexus within the gland. The superior thyroid veins which are now highlighted in green drain the superior aspect of the thyroid gland and they travel very near the superior thyroid arteries. These veins travel towards the carotid sheath and drain into the internal jugular vein. The middle thyroid veins exit the lateral side of the thyroid gland and also drain into the internal jugular vein. The inferior thyroid veins are formed from the pretracheal plexus. In this image, we can see the plexus draining into one common inferior thyroid vein, which can sometimes happen.

This single inferior thyroid vein will either drain into the left or right brachiocephalic vein or occasionally directly into the superior vena cava. When the inferior thyroid veins do not form a single trunk, the left drains into the left brachiocephalic vein and the right usually drains into the right brachiocephalic vein or sometimes into the superior vena cava.

We’ll now explore the vascular supply to the parathyroid glands, starting with the arteries.

Luckily, a lot of it is quite similar to the thyroid gland. In this image, we're looking at the pharynx and the thyroid gland from a posterior view. The arteries we can see highlighted in green are the inferior thyroid arteries arising from the thyrocervical trunk. This artery supplies the posterior aspect of the thyroid gland, and since the parathyroid glands are located on the posterior aspect, it makes sense that they get their blood supply from the inferior thyroid artery too.

The venous drainage of the parathyroid glands is also shared with the thyroid gland. Blood from the parathyroid glands drains into the thyroid venous plexus and then follows the paths of the thyroid veins.

The last topic we’ll tackle with these glands is their innervation. The thyroid gland is innervated by both the sympathetic and parasympathetic nervous systems. The nerves from these systems acts on the blood vessels within the gland to constrict or dilate them as needed. Nerves from all three cervical sympathetic ganglia innervate the thyroid gland. Here we can see the superior, middle, and inferior cervical ganglia in the neck.

The parasympathetic nervous system gets to the thyroid gland in two ways. Both of these nerves, however, arise from the vagus nerve. The first is the external branch of the superior laryngeal nerve which we can see approaching the area of the thyroid gland from the superior aspect. The second is the recurrent laryngeal nerve, which comes towards the thyroid gland from an inferior angle. Remember, both are branches of the vagus nerve – cranial nerve ten.

Innervation to the parathyroid glands is nice and straightforward. It only receives sympathetic innervation which comes from the superior and middle cervical ganglia.

Alright, that's the anatomy part of the tutorial finished, but let's take a look at the significance of the thyroid gland in a clinical setting.

Normally, our thyroid gland is a small and not very noticeable structure in our neck, but just like any organ, things can go wrong – like this. Can you guess what's going on here? This is a picture of a goiter. A goiter is an abnormal enlargement of the thyroid gland, most commonly caused by a lack of iodine in the diet. Other causes can include autoimmune diseases such as Graves’ disease or Hashimoto's disease. Symptoms include swelling in the base of the neck. It's usually painless, but it can cause coughing, difficulty swallowing and breathing, hoarseness, and a tight feeling in your throat. Treatment depends on the size, symptoms, and underlying cause of the goiter and can include medications particularly if thyroid hormones need to be replaced or surgery.

And now you're an expert on the thyroid and parathyroid glands.

Before I let you go, let's have a quick review of what we covered today. We first had a look at the thyroid gland itself which is located in the neck anterior to the trachea and the inferior portion of the larynx. We identified the two main lobes of the thyroid gland – the right and the left lobe. Between the two main lobes is where we identified the isthmus and we also saw the variable, but relatively common pyramidal lobe, which is a remnant of the thyroglossal duct.

From the posterior aspect of the thyroid gland, we identified four parathyroid glands – two superior and two inferior – also seen as two on the left and two on the right.

Next up we whizzed through some surrounding structures such as the thyroid cartilage which we now can see highlighted in green. Inferior to that is the cricoid cartilage – the only complete ring cartilage in the airway – and inferior to that, we identified the trachea. Between these structures, we identified a couple of ligaments such as the cricothyroid ligament between the cricoid and thyroid cartilages, and the cricotracheal ligament between the cricoid cartilage and the trachea.

We moved on to the vascular supply of these glands starting with the two arteries that supply the thyroid gland. First up was the superior thyroid artery branching from the external carotid artery, and then from the thyrocervical trunk, we found the inferior thyroid artery. The three pairs of thyroid veins we found were the superior thyroid veins, the middle thyroid veins, and the inferior thyroid veins, which sometimes merge to form one common trunk. We saw that the blood supply to the parathyroid glands is well connected to the thyroid gland, and they receive arterial blood from branches of the inferior thyroid arteries. Venous blood from the parathyroid glands drains into the thyroid venous plexus.

The final anatomical structures we looked at were related to the innervation of the thyroid and the parathyroid glands which are similar for each; therefore, we saw sympathetic innervation coming from the superior and middle cervical ganglia for both and the inferior cervical ganglia contributes to innervation of the thyroid gland as well. Parasympathetic innervation for the thyroid gland comes from branches of the vagus nerve, the external branch of the superior laryngeal nerve, and the recurrent laryngeal nerve.

To finish up our tutorial, we looked at an abnormal enlargement of the thyroid gland called a goiter which is most commonly caused by an iodine deficiency in the diet.

And that brings us to the end of our tutorial on the thyroid and the parathyroid glands. I hope you enjoyed it. Thanks for joining me.