Intact PTH is a blood test that measures the full, active form of parathyroid hormone, a molecule made up of 84 amino acids. The normal range is typically 15 to 65 pg/mL. When your lab report says “intact PTH,” it’s distinguishing this complete hormone from smaller, broken-down fragments that also circulate in your blood but don’t do the same job.
Why the Word “Intact” Matters
Your parathyroid glands release PTH as a complete 84-amino-acid chain. Once in the bloodstream, the hormone gets broken down into smaller pieces, particularly large fragments from one end of the molecule (called C-terminal fragments). These fragments can linger in the blood, especially in people with kidney problems, but they don’t regulate calcium the way the whole molecule does.
The intact PTH assay was designed to measure the full molecule so doctors get a more accurate picture of how actively your parathyroid glands are working. It uses two antibodies that grab onto different parts of the hormone, catching only the larger, functional forms. It’s worth knowing that “intact” assays also pick up some of those truncated fragments, not just the perfect 1-84 chain. A newer test, sometimes called a “whole PTH” or “bio-intact PTH” assay, uses an antibody that binds to the very first amino acid, making it specific to the complete molecule. Most routine labs still use the intact version, and for the majority of patients the distinction doesn’t change clinical decisions.
What PTH Does in Your Body
PTH is the main regulator of calcium in your blood. When calcium drops, your parathyroid glands release more PTH. When calcium rises, they pull back. The hormone acts on two main targets: your kidneys and your bones.
In the kidneys, PTH tells the body to hold onto calcium by increasing how much gets reabsorbed back into the blood rather than lost in urine. At the same time, it does the opposite with phosphorus, pushing the kidneys to excrete more of it. PTH also triggers the kidneys to activate vitamin D, converting it into the form that helps your intestines absorb calcium from food.
In bone, PTH stimulates cells that break down bone tissue, releasing stored calcium and phosphorus into the bloodstream. This sounds destructive, but in healthy amounts it’s part of normal bone remodeling. The system stays in balance as long as PTH levels remain within the normal range. Problems emerge when the hormone stays too high or drops too low for extended periods.
What a Normal Result Looks Like
Most labs report a normal intact PTH range of roughly 15 to 65 pg/mL, though the exact numbers can vary slightly between laboratories. Your doctor will interpret the result alongside your blood calcium, phosphorus, and vitamin D levels, because PTH doesn’t mean much in isolation. A PTH of 55 pg/mL with perfectly normal calcium is a very different situation than a PTH of 55 with elevated calcium.
What High Intact PTH Means
Elevated PTH generally falls into two categories depending on what’s happening with calcium.
Primary hyperparathyroidism means one or more of the parathyroid glands are overproducing hormone on their own, usually because of a benign growth. The hallmark is high PTH paired with high blood calcium. Over time, this combination can weaken bones and cause kidney stones.
Secondary hyperparathyroidism means the parathyroid glands are ramping up production in response to something else, most commonly chronic kidney disease. As kidney function declines, the body has trouble excreting phosphorus, activating vitamin D, and maintaining calcium levels. The parathyroid glands compensate by pumping out more PTH. Data from a large study of CKD patients shows this clearly: median PTH in people with early kidney disease (stage 1) was about 35 pg/mL, while those with stage 4 disease had a median of 72 pg/mL and stage 5 patients reached 144 pg/mL. Even correcting vitamin D deficiency in advanced kidney disease didn’t bring PTH back to normal, because declining kidney function itself is the strongest driver.
For people on dialysis, the target PTH is intentionally set much higher than the general population’s normal range. KDIGO guidelines suggest maintaining levels at roughly two to nine times the upper limit of the lab’s normal range, which works out to approximately 130 to 585 pg/mL depending on the assay. This reflects the reality that some degree of elevated PTH is expected and even necessary to maintain bone turnover in dialysis patients.
Vitamin D deficiency, low dietary calcium, and certain medications can also push PTH upward without involving the parathyroid glands or kidneys directly.
What Low Intact PTH Means
Low PTH, called hypoparathyroidism, leaves the body unable to maintain adequate calcium levels. Phosphorus tends to rise because the kidneys are no longer being told to excrete it. Symptoms are mostly tied to low calcium: tingling in the fingers and around the mouth, muscle cramps, and in severe cases, spasms or seizures.
The most common cause is accidental damage to the parathyroid glands during neck surgery, particularly thyroid surgery. The glands are small (about the size of a grain of rice) and sit right behind the thyroid, making them vulnerable during any procedure in that area. Other causes include autoimmune conditions that attack the glands, radiation therapy to the head or neck, and low magnesium levels. Magnesium is essential for the parathyroid glands to function properly, so a deficiency can suppress PTH production even when the glands themselves are healthy.
Preparing for the Test
An intact PTH blood draw doesn’t require fasting in most cases. One thing worth flagging: if you take biotin supplements (also sold as vitamin B7 or marketed for hair and nail health), they can interfere with the assay and produce inaccurate results. Many labs recommend stopping biotin at least 48 hours before the blood draw. This isn’t unique to PTH testing. Biotin interferes with a wide range of hormone and vitamin tests, including thyroid panels, cortisol, and vitamin B12.
PTH levels fluctuate throughout the day, peaking in the early morning hours. Some doctors prefer morning draws for consistency, but there’s no strict requirement. The blood sample is typically processed quickly because the hormone degrades at room temperature.