Most people have four parathyroid glands. These small, grain-of-rice-sized glands sit behind your thyroid gland in your neck, with two embedded in each side of the thyroid. Despite their tiny size and close proximity to the thyroid, parathyroid glands have a completely different job: they control the calcium levels in your blood.
Where Exactly Are They?
About 80% of people have four parathyroid glands in the standard position, tucked into the back surface of the thyroid. The thyroid is a butterfly-shaped gland at the front of your neck, and each “wing” typically holds two parathyroid glands, one upper and one lower.
Not everyone’s glands are in this textbook location, though. Around 16% of people have at least one ectopic parathyroid gland, meaning it developed in an unusual spot. These can show up along the esophagus in the neck or deeper in the chest. This happens because of how the glands form during fetal development. The upper and lower pairs originate from different structures in the embryo (the third and fourth pharyngeal pouches), and during that migration process, a gland can end up somewhere unexpected.
Can You Have More or Fewer Than Four?
Yes. While four is the standard number, some people have extra (supernumerary) parathyroid glands, and rarely, some have only three. In studies of patients undergoing parathyroid surgery, roughly 13 to 16% were found to have supernumerary glands. These extras can sit in a normal neck position or in ectopic locations like the chest. Most people with an extra gland never know it exists unless imaging or surgery reveals it.
What Parathyroid Glands Do
Your parathyroid glands have one primary job: keeping blood calcium in a tight, healthy range (roughly 8.6 to 10.3 mg/dL in most labs). They do this by releasing parathyroid hormone, or PTH, which acts on three parts of your body simultaneously.
- Bones: PTH triggers the release of small amounts of stored calcium from your bones into your bloodstream.
- Kidneys: PTH tells your kidneys to hold onto calcium instead of flushing it out in urine. It also activates vitamin D in the kidneys, which helps calcium absorption elsewhere.
- Small intestine: Through that activated vitamin D, PTH increases how much calcium your gut absorbs from food.
This system works like a thermostat. When blood calcium dips, the parathyroid glands sense it and release more PTH. When calcium rises back to normal, PTH production slows down. A healthy PTH level falls between about 11 and 51 pg/mL.
When a Gland Becomes Overactive
The most common parathyroid problem is primary hyperparathyroidism, where one or more glands produce too much PTH. This drives blood calcium above normal levels. In most cases, a single gland has developed a benign growth called an adenoma. Less often, multiple glands become enlarged.
Doctors diagnose this condition by checking blood calcium and PTH levels on at least two separate occasions, spaced at least two weeks apart. The hallmark pattern is elevated calcium alongside PTH that is either high or “inappropriately normal,” meaning the body should have dialed PTH down in response to the high calcium but didn’t. There’s also a subtler form called normocalcemic hyperparathyroidism, where calcium levels stay within range but PTH is persistently elevated for no other identifiable reason.
Symptoms can be vague: fatigue, brain fog, bone thinning, kidney stones, frequent urination, and muscle weakness. Some people have no obvious symptoms at all and only discover the problem through routine bloodwork.
Why the Number of Glands Matters for Surgery
When surgery is needed to remove an overactive parathyroid gland, knowing how many glands a person has and where they’re located is critical. Surgeons rely on imaging to map things out before the operation. The two most common initial tools are ultrasound, which correctly identifies a single overactive gland about 70 to 80% of the time, and sestamibi scanning, a nuclear medicine technique where a mildly radioactive tracer is taken up by overactive parathyroid tissue. Combining the sestamibi scan with 3D imaging raises accuracy to roughly 79% sensitivity with a positive predictive value above 90%.
A newer approach called 4D-CT uses multiple passes of contrast-enhanced CT scanning to catch the rapid blood flow pattern unique to parathyroid adenomas. It correctly pinpoints the right area of the neck in about 86% of cases. MRI plays a smaller role, mostly reserved for complex repeat surgeries, because it’s expensive and sensitive to motion during the long scan times.
When someone has a supernumerary or ectopic gland that wasn’t spotted on initial imaging, it can be the reason hyperparathyroidism persists after surgery. This is why preoperative localization is so important, and why surgeons sometimes explore all four (or more) gland sites during an operation rather than removing just the one that lit up on a scan.