Parathyroid hormone (PTH) is a chemical messenger that controls the level of calcium in your blood. It’s produced by four tiny glands, each about the size of a grain of rice, embedded in the back of your thyroid gland in your neck. When blood calcium dips even slightly, these glands release PTH to bring it back up. Normal levels typically fall between about 15 and 65 pg/mL on a standard blood test.
How PTH Keeps Calcium in Balance
Your body needs calcium in the blood to stay within a very narrow range. Too little and your muscles and nerves can’t function properly. Too much and you risk kidney stones, bone loss, and other problems. PTH is the minute-to-minute controller of that balance, and it works through three main levers.
First, PTH signals your bones to release stored calcium into the bloodstream. Bone is a massive calcium reservoir, and PTH can tap into it quickly when levels fall. Second, it tells your kidneys to hold onto calcium that would otherwise be flushed out in urine, reclaiming it back into the blood. Third, PTH triggers your kidneys to convert vitamin D into its active form, which then boosts calcium absorption from the food in your intestines. That third mechanism works on a slower, more long-term basis, while the bone and kidney effects are nearly immediate.
PTH also has an opposite effect on phosphorus. It causes the kidneys to excrete more phosphorus into the urine. This matters because calcium and phosphorus levels are closely linked: when one goes up, the other tends to go down. By lowering phosphorus, PTH indirectly helps calcium rise further.
The Feedback Loop That Controls PTH
The parathyroid glands don’t just pump out hormone constantly. They have built-in sensors, proteins on their surface called calcium-sensing receptors, that continuously monitor how much calcium is floating in the blood. When calcium is high enough, calcium molecules bind to these receptors and effectively shut off PTH production. When calcium drops, the receptors stop being activated, and the glands ramp PTH back up.
This creates a tight feedback loop. In a healthy person, PTH levels rise and fall throughout the day in response to meals, hydration, and other factors, always nudging calcium back toward the normal range. The system is remarkably precise, keeping blood calcium stable even when your dietary intake varies widely from day to day.
What Happens When PTH Is Too High
When the parathyroid glands produce too much hormone, it’s called hyperparathyroidism. There are two main forms, and they have different causes.
In primary hyperparathyroidism, the problem starts in the glands themselves. Usually a noncancerous growth (adenoma) develops on one of the four glands, causing it to churn out PTH regardless of how much calcium is already in the blood. The result is persistently elevated calcium. Most people with this condition actually have no obvious symptoms, especially early on. When symptoms do appear, they tend to be vague: fatigue, muscle weakness, depression, and aches in bones and joints. More advanced cases can cause loss of appetite, nausea, constipation, confusion, and noticeably increased thirst and urination.
In secondary hyperparathyroidism, the glands are responding appropriately to a real calcium shortage caused by something else, most often chronic kidney disease. When the kidneys can’t activate vitamin D properly or retain enough calcium, blood calcium stays low, and the parathyroid glands work overtime trying to compensate. People with intestinal conditions that impair nutrient absorption or those with severe vitamin D deficiency can also develop secondary hyperparathyroidism.
What Happens When PTH Is Too Low
The opposite condition, hypoparathyroidism, is much less common. When PTH levels are too low, your body can’t pull calcium from bone or reclaim it in the kidneys effectively. The result is low blood calcium and high phosphorus. This increased “neuromuscular irritability,” as clinicians call it, means your muscles and nerves become overly excitable. You might feel tingling in your fingertips, lips, or around your mouth. Muscle cramps and spasms are common, and severe cases can cause seizures.
About 75% of hypoparathyroidism cases are caused by accidental damage to the parathyroid glands during thyroid or other neck surgery. The glands are tiny and sit right on the thyroid, making them vulnerable. Other causes include autoimmune destruction of the glands, certain genetic conditions, and extreme magnesium deficiency (your parathyroid glands need magnesium to secrete PTH properly). The prevalence in the United States is estimated at roughly 6 to 37 per 100,000 people.
How PTH Levels Are Tested
A PTH blood test is straightforward. A healthcare provider draws a standard blood sample, typically from your arm. Some providers ask you to fast beforehand or schedule the draw at a specific time of day, since PTH can fluctuate. The test is almost always ordered alongside a calcium level, because interpreting one without the other doesn’t tell the full story.
What matters most is the relationship between the two numbers. High PTH paired with high calcium suggests primary hyperparathyroidism. High PTH with low calcium points toward secondary hyperparathyroidism, where the glands are reacting to a genuine calcium deficit. Low PTH with low calcium indicates hypoparathyroidism, meaning the glands aren’t doing their job. A PTH level that falls within the normal range of roughly 15 to 65 pg/mL, combined with normal calcium, generally means the system is working as it should.
PTH and Bone Health
Because PTH pulls calcium from bone to raise blood levels, chronically elevated PTH can weaken your skeleton over time. The bone keeps getting asked to give up calcium without adequate replacement, leading to thinning and increased fracture risk. This is one of the main long-term concerns with untreated primary hyperparathyroidism, and it’s why doctors sometimes recommend surgery to remove the overactive gland even in people who feel fine.
Interestingly, PTH in short, intermittent pulses actually stimulates new bone formation rather than breakdown. This property has been harnessed in certain osteoporosis treatments that use a synthetic form of PTH given as a daily injection. The key difference is timing: constant, unrelenting PTH exposure breaks bone down, while brief spikes can build it up.