The parathyroid glands produce one hormone: parathyroid hormone, commonly called PTH. Despite having four separate glands, they all make the same hormone, and its primary job is keeping calcium levels in your blood within a tight range. Normal blood calcium falls between 8.8 and 10.3 mg/dL, and PTH is the body’s main tool for holding that number steady.
What Parathyroid Hormone Does
PTH works on three targets simultaneously to raise blood calcium when levels start to dip. In bone, it triggers the release of small amounts of stored calcium into the bloodstream. In the kidneys, it signals your body to hold onto calcium rather than flushing it out through urine. And the kidneys get a second instruction from PTH: produce the active form of vitamin D, which then helps your intestines absorb more calcium from the food you eat.
PTH also lowers phosphorus levels in your blood. It does this by causing the kidneys to flush out more phosphorus instead of reabsorbing it. This matters because calcium and phosphorus exist in a seesaw relationship. When phosphorus drops, calcium can rise more effectively. So PTH works both sides of the equation at once.
How Your Body Controls PTH Levels
The parathyroid glands don’t release PTH on a set schedule. Instead, they constantly monitor blood calcium through a protein on their surface called the calcium-sensing receptor. When calcium in the blood is high enough, calcium molecules bind to these receptors and shut down PTH production. The receptors also slow the growth of parathyroid cells themselves, keeping the glands from getting larger than they need to be.
When blood calcium drops, fewer calcium molecules bind to these receptors, and the brakes come off. The glands ramp up PTH secretion within minutes. This feedback loop runs continuously, adjusting PTH output in real time to keep calcium at the right level. It’s one of the fastest hormone-response systems in the body.
When Too Much PTH Is Produced
Hyperparathyroidism occurs when one or more parathyroid glands become overactive and pump out excess PTH. It’s often discovered by accident, when a routine blood test shows elevated calcium. The diagnosis is confirmed when blood work reveals both high calcium and high PTH at the same time, since the feedback loop should normally suppress PTH when calcium is already elevated.
The symptoms can be vague enough that people live with them for years without connecting them to a parathyroid problem. Fatigue, depression, difficulty concentrating, forgetfulness, unexplained aches, excessive thirst, and frequent urination are all common complaints. Over time, chronically elevated PTH pulls too much calcium from bones, which can lead to weakened bones and kidney stones from excess calcium in the urine. A 24-hour urine collection can help measure how much calcium your kidneys are filtering.
When Too Little PTH Is Produced
Hypoparathyroidism, a deficiency of PTH, is less common but can cause serious problems because blood calcium drops too low. The most frequent cause is accidental damage to the parathyroid glands during thyroid surgery or parathyroid surgery. Temporary drops in PTH after thyroid removal are actually quite common, with symptoms of low calcium typically appearing 24 to 48 hours after the operation. Permanent hypoparathyroidism occurs in fewer than 3% of thyroidectomies performed by experienced surgeons, though in some cases symptoms don’t show up for months or even years.
Less commonly, hypoparathyroidism results from autoimmune conditions where the body attacks its own parathyroid tissue, or from rare inherited disorders. DiGeorge syndrome, a genetic condition present from birth, can cause the parathyroid glands to be absent entirely. There is also a condition called pseudohypoparathyroidism, where the glands produce PTH normally but the body’s tissues don’t respond to it, creating the same effect as a true deficiency.
Low calcium from insufficient PTH causes muscle cramps, tingling in the fingers and around the mouth, and in severe cases, seizures. These symptoms reflect calcium’s essential role in nerve and muscle function, a role that PTH normally protects by keeping blood levels stable.
PTH and Vitamin D Are Closely Linked
One of PTH’s most important but often overlooked functions is activating vitamin D. The vitamin D you get from sunlight or supplements is in an inactive form. Your kidneys convert it into its active form, and PTH is what drives that conversion. Without enough PTH, even people who get plenty of vitamin D from their diet can’t use it effectively. And without active vitamin D, the intestines absorb far less calcium from food, which forces the body to rely even more heavily on pulling calcium from bones.
This is why vitamin D deficiency and parathyroid problems often overlap. When vitamin D is chronically low, the body compensates by increasing PTH to squeeze more calcium from other sources. Over time, this can push PTH levels high enough to mimic hyperparathyroidism, a condition sometimes called secondary hyperparathyroidism, where the glands themselves are healthy but overworked.