The key to these questions is to understand where parathyroids come from. When you understand this, you will understand why parathyroid anatomy is some of the most variable anatomy in the human body--and why it is VERY important to get a very experienced parathyroid surgeon to perform your parathyroid surgery (read more).
Technically speaking, the four parathyroids arise from the third and forth branchial pouches. This means that they are formed early in embryogenesis along with other organs of the neck. The lower 2 parathyroids come from the third brachial pouch which is also responsible for producing the thymus gland (not thyroid) which is important for the development of a normal immune system. The thymus eventually sits behind the breast bone (sternum). The upper 2 parathyroids come from the fourth branchial pouch which also produces the thyroid gland.
Normally, as the larger thymus and thyroid glands migrate from the upper neck (where they are formed) into the lower neck and upper chest, they take the small "passenger" parathyroids with them. The lower parathyroids usually getting stuck in the neck next to the thyroid during this migration and rarely make it all the way into the chest with the thymus. Therefore, under normal circumstances, the lower parathyroids will usually be found just below and behind the bottom of the thyroid. Likewise, the upper parathyroids will be found behind the middle of the thyroid gland. That is to say, normally, all four parathyroids are found around the back side of the thyroid (like in our main parathyroid picture).
BUT, it doesn't always go as planned !
It is this migration of the parathyroid glands through the neck during our gestation in the womb that makes finding them tricky. Occasionally, the upper parathyroid glands are formed with the thyroid to such a high degree that they never become separated. Under these circumstances, an upper parathyroid will actually be found WITHIN the thyroid gland. This picture shows the right half of the thyroid gland of a recently operated patient. I cut the thyroid in half to demonstrate the large parathyroid tumor (adenoma) which was inside the thyroid. This young patient had primary hyperparathyroidism with a very high calcium and parathyroid hormone. The sestamibi scan showed that it was completely buried within the thyroid gland and the only way to adequately get the entire adenoma out and cure her disease was to remove the right lobe of the thyroid. Her remaining thyroid (the isthmus and left lobe) will make enough thyroid hormone so she will not have to take thyroid hormone pills. Keep in mind that this situation is uncommon (about 1 percent of all cases). But your surgeon should have enough experience and knowledge to deal with these difficult circumstances should they arise. This picture was taken after the specimen was in preservatives overnight (prior to being sliced and stained by the pathologist) that is why the colors have all turned brown.
When the lower parathyroids fail to migrate, they can reside very high in the neck, several inches above the top of the thyroid. This is quite rare. A more common migration problem occurs when the lower parathyroids fail to separate from the thymus as the thymus makes its way to the chest (although still uncommon, this happens in about 1% of patients with primary hyperparathyroidism). Instead of the parathyroids ending up just below the lower edge of the thyroid, they will go all the way into the chest and reside next to the heart. This picture shows just this case. There is a very "hot" parathyroid adenoma which shows up on this sestamibi scan just to the right of the patient's heart (outlined in yellow).
Once again, it is the migration of parathyroids which makes operating on them so tricky. This is one of the reasons that a number of surgeons believe that minimally invasive parathyroidectomy can be advantageous. With proper testing, a surgeon can tell before the operation where the overactive adenoma is located so so he/she doesn't have to search all areas of the neck. As discussed on a few of our other pages, sometimes these tests don't give any predictable information.
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