The parathyroid glands are four tiny glands in your neck whose sole job is to control the level of calcium in your blood. Each one is roughly the size of a grain of rice, and despite their name, they have nothing to do with thyroid function. The thyroid and parathyroid glands just happen to be neighbors.
Location and Anatomy
Most people have four parathyroid glands, two on each side, sitting on the back surface of the thyroid gland in the lower neck. The upper pair tends to be slightly larger than the lower pair. In about 5% of people, a fifth gland exists, and roughly 2% of people have only three. These variations rarely cause problems on their own, but they matter a great deal to surgeons who need to find them.
About 8.5% of parathyroid glands sit in an unexpected location. Some end up embedded within the thyroid itself, others drift elsewhere in the neck, and a significant number settle in the mediastinum, the space in the chest between the lungs. These “ectopic” glands function normally but can make diagnosis and surgery more complicated if disease develops.
How Parathyroid Glands Control Calcium
The parathyroid glands produce parathyroid hormone, commonly called PTH. This hormone’s entire purpose is keeping blood calcium within a very tight range. The glands monitor calcium levels constantly through a molecular sensor on their surface called the calcium-sensing receptor, which detects even tiny shifts in blood calcium concentration. When calcium drops, PTH release increases. When calcium rises, PTH release slows down.
PTH raises blood calcium through three pathways working together:
- Bone: PTH signals cells called osteoclasts to break down small amounts of bone tissue, releasing stored calcium into the bloodstream.
- Kidneys: PTH tells the kidneys to hold onto calcium instead of letting it pass into urine. At the same time, it increases the excretion of phosphate, which helps calcium stay available in the blood. PTH also triggers the kidneys to activate vitamin D.
- Intestines: PTH doesn’t act on the gut directly. Instead, the active vitamin D it stimulates the kidneys to produce travels to the small intestine and increases calcium absorption from food.
The PTH and Vitamin D Feedback Loop
PTH and vitamin D form a tightly controlled feedback cycle. PTH is the main driver of vitamin D activation in the kidneys, converting the inactive form you get from sunlight and food into the active form your body can use (calcitriol). Once active vitamin D levels rise sufficiently, it circles back and suppresses PTH production. This back-and-forth keeps both hormones in balance. When vitamin D levels are chronically low, the brakes on PTH secretion weaken, and the parathyroid glands can start overproducing hormone even when nothing else is wrong.
Overactive Parathyroid Glands
Primary hyperparathyroidism occurs when one or more parathyroid glands produce too much PTH regardless of how much calcium is already in the blood. The most common cause is a benign growth called an adenoma on a single gland. Less often, two or more glands enlarge (a condition called hyperplasia). Parathyroid cancer exists but is very rare.
The excess PTH pushes blood calcium too high, which can affect nearly every system in the body. Classic symptoms include frequent urination, excessive thirst, fatigue, bone and joint pain, constipation, nausea, and difficulty concentrating. Depression and itchy skin are also common. Kidney stones are one of the most frequent complications, forming because the kidneys are filtering far more calcium than normal. The old medical mnemonic for remembering these symptoms is “stones, bones, abdominal groans, and psychic moans.”
Many cases today are caught early through routine blood work before symptoms become obvious. When calcium and PTH levels are both elevated on a blood test, that combination points strongly toward the diagnosis.
Underactive Parathyroid Glands
Hypoparathyroidism, where the glands produce too little PTH, most often happens after neck surgery. The parathyroid glands are so small and so close to the thyroid that they can be accidentally damaged or removed during thyroid operations. Without enough PTH, blood calcium drops. Low calcium makes nerves and muscles overly excitable, leading to tingling around the mouth and fingertips, muscle cramps, and in more severe cases, painful involuntary muscle spasms called tetany.
Two physical signs are associated with this condition. Tapping the facial nerve in front of the ear can cause the facial muscles to twitch. Inflating a blood pressure cuff on the arm can trigger the hand to spasm into a characteristic position. Both indicate that low calcium has made the nervous system hypersensitive.
Secondary Hyperparathyroidism
Sometimes the parathyroid glands become overactive not because of a problem with the glands themselves, but in response to chronically low calcium caused by another condition. The most common culprit is chronic kidney disease. As kidney function declines, the kidneys lose the ability to activate vitamin D and struggle to excrete phosphate. Rising phosphate levels further suppress vitamin D activation, creating a vicious cycle. Blood calcium drops, and the parathyroid glands respond by producing more and more PTH to compensate. Over time, the glands physically enlarge from the constant demand.
This form of hyperparathyroidism is managed by treating the underlying kidney disease and correcting vitamin D and phosphate levels, rather than by removing the glands.
Surgery for Parathyroid Disease
When an overactive gland needs to be removed, the procedure is called a parathyroidectomy. For a single adenoma, surgeons often use a minimally invasive approach, making a small incision and targeting just the affected gland. The main risk is injury to the recurrent laryngeal nerve, which controls the vocal cords and runs very close to the parathyroid glands. In one large study, nerve injury occurred in about 7% of parathyroid surgeries, but the vast majority of those injuries were temporary. Permanent damage, lasting longer than a year, occurred in roughly 1% of cases and sometimes required a follow-up procedure to restore voice function.
After surgery, some patients experience a rapid drop in blood calcium as the bones, starved of calcium during the period of overactivity, rapidly absorb it from the bloodstream. This is known informally as “hungry bone syndrome” and is managed with calcium and vitamin D supplementation until levels stabilize.