Parathyroid Anatomy & Development
The vast majority of people have four parathyroid glands, and they are normally the size of a grain of rice. However, occasionally, they can be as large as a pea and still be normal. They weigh roughly 50mg and have a yellow tan color. There are generally adjacent to the thyroid gland and are flattened against it. They have the same blood vessels as the thyroid gland not only because they are close to each other but also because they develop at the same stage in the fetus.
During the 6th week of fetal development the parathyroid glands develop as 2 pouches on each side of the throat and then travel down to rest behind the thyroid that is also traveling downwards into the lower neck. (Figure 1) Because of this migration and the fact that all the other organs are also developing at the same time, sometimes the parathyroid glands end up in an unusual area and not behind the thyroid. They may not travel far and stay up further in the neck or overshoot and travel too far and end up in the chest.
The superior parathyroids have a more consistent location because they have to travel less. The inferior parathyroids travel a longer distance, so they can end up in an unusual position more often. The inferior parathyroid glands also develop in the same pouch as the thymus gland which travels along with the parathyroids and ends up finally in the upper middle chest. (Figure 2) So often times when a parathyroid gland cannot be found it maybe inside the thymus or even lower in the chest. (Figure 3)
Adding another twist to the parathyroid travel itinerary is that sometimes the two parathyroid glands traveling on the same side may fuse, and the person may end up having only 3 parathyroid glands. (Figure 4) At other times, as the parathyroid gland travels down, it may leave pieces of itself along the way, so the person may end up have 5 or more parathyroid glands. (Figure 5) Expert parathyroid surgeons keep these possibilities in mind when treating hyperparathyroid patients. This is also why intra-operative PTH testing is so important, because if your surgeon finds 4 glands and remove one that may seem slightly larger, and doesn’t check the rapid intra-operative PTH then they will not know if a 5th large abnormal gland is there that has not been found yet.
Recurrent Laryngeal Nerve Anatomy
The nerve in charge of controlling the vocal cords is called the recurrent laryngeal nerve (RLN). There is one RLN on each side of the neck. They are each a branch of the Vagus Nerve that leaves the brain and goes all the way down into the abdomen giving off branches on it’s way down. The RLN generally travels under (or deep to) the inferior parathyroids and over (or superficial to) the superior parathyroids.
This is, in fact, the most certain way to know if the parathyroid you are removing is a superior one or inferior one, because the two parathyroids can be very close to each, or not in their usual location; their location with respect to the RLN can help the surgeon know which gland they are dealing with. The RLN can usually be easily identified in a person that has NOT had previous neck surgery, because it usually has a consistent location as it comes up from below the thyroid as it moves up towards the voice box. However, in a person who has had previous surgery around the thyroid gland (such as thyroidectomy, parathyroidectomy, or spine surgery from the front of the neck) then scarring around the RLN can make it difficult to identify & protect the nerve. The greater the surgeon’s experience and expertise the more likely they are to be able to safely find and protect the RLN in scar from previous surgery.
Parathyroid Disease
If there is a disturbance in just a single cell on one of the parathyroid glands and the calcium levels are no longer being monitored properly, you will develop hyperparathyroidism. Watch the video below to learn about hyperparathyroid disease.