iPTH stands for intact parathyroid hormone, a blood test that measures the full, active form of parathyroid hormone circulating in your body. The normal range for most adults falls between roughly 15 and 65 pg/mL, though it can vary slightly by lab and age. This test is one of the most common ways doctors assess how well your body is managing calcium, and it shows up frequently on lab panels for bone health, kidney disease, and unexplained calcium abnormalities.
What Parathyroid Hormone Does
Parathyroid hormone is produced by four tiny glands sitting behind your thyroid in your neck. Their job is straightforward: when calcium in your blood drops too low, the parathyroid glands release PTH to bring it back up. When calcium rises high enough, the glands stop releasing it. This feedback loop runs constantly throughout the day.
PTH raises calcium through three different pathways. In your bones, it triggers cells that break down small amounts of bone tissue, releasing stored calcium into the bloodstream. In your kidneys, it tells the body to hold onto calcium instead of excreting it in urine, while simultaneously flushing out more phosphorus (phosphorus binds to calcium and makes it unavailable, so removing phosphorus frees up more usable calcium). The kidneys also get a signal to activate vitamin D, which then travels to the small intestine and helps absorb more calcium from food.
This is why doctors rarely look at iPTH in isolation. Calcium, phosphorus, and vitamin D levels all interact with PTH, and interpreting one without the others can be misleading.
Why It’s Called “Intact”
Parathyroid hormone is a chain of 84 building blocks (amino acids). Your body constantly breaks this chain into fragments, and older lab tests couldn’t distinguish the full working molecule from these inactive pieces. Second-generation assays solved most of this problem by detecting the complete 1-84 chain, though they still pick up some longer fragments. These second-generation tests are what most labs use today, and “intact PTH” or “iPTH” is the standard name for them.
Third-generation assays exist that measure only the true 1-84 molecule with higher specificity. These are sometimes called “bio-intact” or “whole PTH” tests. They’re mostly used in research settings or for dialysis patients, where the distinction between full PTH and its fragments matters more because kidney failure causes inactive fragments to accumulate in the blood.
Normal Ranges by Age
For healthy adults aged 20 to 79, the overall reference range is about 16.5 to 62 pg/mL. The range shifts modestly with age. Adults in their 20s typically fall between 17 and 53 pg/mL, while people in their 70s have a wider range of roughly 16.5 to 82 pg/mL. The commonly used upper limit across most labs is 65 pg/mL.
These numbers assume adequate vitamin D levels. When vitamin D is low, PTH rises as the body compensates for reduced calcium absorption, which can push results above the reference range even without a parathyroid problem.
What High iPTH Levels Mean
Elevated iPTH falls into a few categories depending on what’s driving it.
Primary hyperparathyroidism means one or more of the parathyroid glands is overactive on its own, usually because of a benign growth. Calcium levels are high while PTH stays inappropriately normal or elevated, when it should be suppressed by all that extra calcium. This is the most common cause of high calcium in the general population and is often treated surgically.
Secondary hyperparathyroidism is the body’s appropriate response to something else pulling calcium down. The most common triggers are chronic kidney disease and vitamin D deficiency. Gastrointestinal conditions that impair calcium absorption can also cause it. In these cases, calcium tends to be low or normal while PTH is high, because the glands are working overtime to compensate.
Tertiary hyperparathyroidism develops after years of secondary hyperparathyroidism, most often in people with end-stage kidney disease or after a kidney transplant. The parathyroid glands have been overstimulated for so long that they essentially go rogue, continuing to overproduce PTH even after the original problem is corrected.
What Low iPTH Levels Mean
Abnormally low or undetectable iPTH points to hypoparathyroidism, a condition where the glands aren’t producing enough hormone. The most common cause is damage to the parathyroid glands during thyroid or neck surgery. It can also result from autoimmune conditions or, less commonly, from genetic disorders.
Without enough PTH, calcium drops and phosphorus rises. Symptoms reflect that calcium deficit: muscle cramps, tingling or numbness around the mouth and in the hands and feet, muscle twitching, and in severe cases, seizures. Some people develop heart-related symptoms because calcium is essential for normal heart muscle contraction.
Diagnosis requires low calcium alongside low or inappropriately normal PTH on at least two separate blood draws taken at least two weeks apart. Doctors also check magnesium first, since low magnesium can suppress PTH production and mimic the condition.
iPTH in Chronic Kidney Disease
iPTH monitoring becomes especially important as kidney function declines. Damaged kidneys can’t activate vitamin D or excrete phosphorus efficiently, which drives PTH steadily upward. For people with moderate to advanced kidney disease who aren’t yet on dialysis, international guidelines recommend investigating when iPTH is progressively rising or stays persistently above the upper limit of normal. The focus is on correcting modifiable factors: high phosphorus, low calcium, excessive phosphorus intake, and vitamin D deficiency.
For patients on dialysis, the target range is considerably higher than for healthy adults. Guidelines suggest maintaining iPTH at roughly 2 to 9 times the upper limit of normal for the assay being used. With a typical upper limit of 65 pg/mL, that translates to approximately 130 to 585 pg/mL. Some regional guidelines use narrower windows, such as 150 to 300 pg/mL. The reason for tolerating higher levels is that suppressing PTH too aggressively in dialysis patients can cause a different bone disease from excessively low bone turnover.
How the Test Works
An iPTH test is a standard blood draw. Most people don’t need special preparation, though some providers ask you to fast or come in at a specific time of day since PTH levels fluctuate with circadian rhythms. The blood sample is analyzed using an immunoassay, a technique that uses antibodies to capture and measure the hormone.
One important factor that can throw off results is biotin, also known as vitamin B7. Many people take biotin supplements for hair, skin, and nail health, sometimes at doses of 5 mg or more per day. Biotin interferes with the type of immunoassay used for iPTH and can produce falsely low readings. In one documented case, a patient taking 5 mg of daily biotin showed a PTH of just 4.3 pg/mL, well below normal. After stopping biotin for a month, the repeat test came back at 21 pg/mL. If you take biotin supplements, let your doctor know before the test. Stopping for several days beforehand is generally enough to avoid interference.
How Vitamin D and Calcium Affect Results
Your iPTH result doesn’t exist in a vacuum. Vitamin D and calcium intake both influence where your PTH settles, and doctors interpret all three together. When vitamin D levels fall below about 50 nmol/L (20 ng/mL), the body absorbs less calcium from food, which pushes PTH upward. Research shows that when vitamin D is adequate, calcium intake has minimal effect on PTH. But when vitamin D is low, getting less than 800 mg of calcium per day is associated with noticeably higher PTH compared to higher calcium intakes.
This means a mildly elevated iPTH doesn’t automatically signal a parathyroid problem. It could simply reflect low vitamin D, low dietary calcium, or both. Correcting those deficiencies often brings PTH back into the normal range without further intervention.