The parathyroid glands control calcium levels in your blood. They do this by producing parathyroid hormone (PTH), which acts on your bones, kidneys, and intestines to keep blood calcium within a tight range of roughly 8.5 to 10.5 mg/dL. Most people have four parathyroid glands, each about the size of a grain of rice, sitting behind the thyroid gland in the neck. Despite the similar name, they have nothing to do with the thyroid’s job of regulating metabolism.
How Parathyroid Hormone Regulates Calcium
Specialized cells in the parathyroid glands constantly monitor your blood calcium through calcium-sensing receptors on their surface. When calcium drops even slightly, these receptors trigger the release of PTH. The hormone then works on three targets simultaneously to bring calcium back up.
In bone, PTH stimulates cells that break down bone tissue, releasing stored calcium and phosphate into the bloodstream. It does this by signaling bone-building cells to shift their behavior and recruit bone-resorbing cells. In the kidneys, PTH tells the filtering system to hold onto calcium that would otherwise leave the body in urine, pulling it back into the blood through specialized calcium channels in the kidney tubules. And in the gut, PTH works indirectly by triggering the kidneys to convert vitamin D into its active form, which then increases calcium absorption from the food you eat.
When blood calcium rises back to normal, the parathyroid glands sense the change and dial back PTH production. Vitamin D also signals the glands to slow down. This feedback loop runs continuously, adjusting PTH output minute by minute to keep calcium stable.
The Parathyroid-Phosphate Connection
While calcium gets most of the attention, the parathyroid glands also regulate phosphate levels. PTH causes the kidneys to flush out more phosphate in urine. This matters because calcium and phosphate have an inverse relationship in the body. If both rise at the same time, they can combine and form deposits in soft tissues. By lowering phosphate while raising calcium, PTH prevents this harmful buildup.
The Vitamin D Partnership
PTH and vitamin D work as a tightly controlled team. PTH is one of the strongest triggers for vitamin D activation in the kidneys, converting the storage form of vitamin D into its biologically active form. That active vitamin D then boosts calcium absorption from food, which is essential because without it, your intestines absorb only a fraction of the calcium you consume. Once vitamin D levels climb high enough, it feeds back to the parathyroid glands and suppresses further PTH release. This is one reason vitamin D deficiency often shows up alongside elevated PTH: without enough vitamin D to keep calcium levels adequate, the parathyroid glands compensate by pumping out more hormone.
What Happens When PTH Is Too High
When the parathyroid glands produce too much hormone, the condition is called hyperparathyroidism. The most common form, primary hyperparathyroidism, usually results from a benign growth on one of the glands that keeps secreting PTH regardless of how high calcium gets. This pulls too much calcium from bones and reclaims too much from the kidneys, driving blood calcium above normal.
Symptoms of excess PTH and high calcium span the entire body. You might notice increased thirst and frequent urination as the kidneys try to manage the calcium overload. Constipation, nausea, and stomach pain are common. Fatigue, depression, forgetfulness, and difficulty concentrating show up frequently because calcium levels affect nerve signaling. Over time, kidney stones can form from the excess calcium filtered through the kidneys, and bones may weaken.
The bone effects are particularly notable. Hyperparathyroidism tends to cause bone loss at sites rich in compact, dense bone, like the forearm and hip. In a prospective study following 121 people with primary hyperparathyroidism over 10 years, bone density remained stable in most patients without surgery, but about 25% showed meaningful decreases. After surgical removal of the overactive gland, bone density at the spine and hip typically improves.
What Happens When PTH Is Too Low
Hypoparathyroidism, where PTH levels are insufficient, most commonly occurs after thyroid or neck surgery, when the parathyroid glands are accidentally damaged or removed. Without enough PTH, blood calcium drops while phosphate rises.
Low calcium affects muscles and nerves in distinct ways. Tingling and numbness around the mouth and in the fingertips are often the first signs. Muscle cramps, twitching, and spasms follow as calcium drops further, because nerve cells become overly excitable without adequate calcium to stabilize their signaling. Severe cases can cause seizures. Headaches, impaired vision, anxiety, and depression also occur. Over the long term, low calcium can lead to movement disorders and cognitive problems.
How Parathyroid Problems Are Diagnosed
Parathyroid disorders are usually caught through blood tests, sometimes before symptoms become obvious. A routine blood panel showing elevated calcium (above roughly 10.5 mg/dL in adults) is often the first clue. From there, PTH is measured. In primary hyperparathyroidism, both calcium and PTH are elevated, which is abnormal because high calcium should suppress PTH production. A gland that ignores that feedback signal is almost certainly the problem.
Testing also includes vitamin D levels, since deficiency can mimic or complicate the picture. Kidney function and 24-hour urine calcium measurements help assess whether the excess calcium is affecting the kidneys. If surgery is being considered, imaging with ultrasound or a specialized nuclear scan can locate which gland is overactive.
Treatment for Parathyroid Disorders
For primary hyperparathyroidism with symptoms like kidney stones, osteoporosis, or significant bone density loss, surgery to remove the overactive gland is the standard treatment. It’s also recommended for people under 50 or those whose calcium is more than 1 mg/dL above the normal upper limit, even without obvious symptoms. The surgery has a high success rate and typically resolves the calcium imbalance.
People with mild, asymptomatic hyperparathyroidism who don’t meet these thresholds can be monitored with regular blood tests and bone density scans. Not everyone progresses, and many remain stable for years.
Hypoparathyroidism is managed with calcium and active vitamin D supplements to replace what the missing hormone would normally provide. This requires ongoing monitoring to keep calcium in a safe range without overshooting.