Parathyroid hormone (PTH) intact is a blood test that measures the amount of parathyroid hormone your parathyroid glands are producing right now. It is the standard test doctors use to check how well your calcium levels are being regulated in your body. Unlike other forms of the hormone that break down quickly, the “intact” version is the full, active molecule that your body actually uses. This test helps diagnose why your calcium levels might be too high or too low and points to whether the problem is coming from your parathyroid glands or from somewhere else.
What Does the PTH Intact Test Measure?
The PTH intact test measures the full-length, biologically active form of parathyroid hormone in your blood. Your parathyroid glands, which are four tiny glands located behind your thyroid in your neck, release this hormone in response to low calcium levels. Once released, PTH works on your bones, kidneys, and intestines to raise calcium levels back to normal.
The “intact” part of the name matters because PTH breaks down into smaller fragments in your bloodstream very quickly. Older tests could not tell the difference between the active whole hormone and these useless fragments. The intact test specifically captures only the full 84-amino acid chain that your body actually uses. This gives doctors a much clearer picture of what your parathyroid glands are doing.
Research published in the Journal of Clinical Endocrinology and Metabolism shows that the intact PTH assay has become the gold standard because it avoids the confusion caused by those fragments. The test is highly reliable when your kidney function is normal, though interpretation gets trickier in people with kidney disease.
Why Would a Doctor Order a PTH Intact Test?
Doctors order this test most often when routine blood work shows abnormal calcium levels. High calcium with a high PTH usually points to primary hyperparathyroidism, which is the most common cause of high calcium in otherwise healthy adults. Low calcium with a low PTH suggests hypoparathyroidism, where the glands are not producing enough hormone.
The test also helps doctors figure out the cause of kidney stones, bone loss, or unexplained fatigue. According to the National Institutes of Health, about 100,000 new cases of primary hyperparathyroidism are diagnosed in the United States each year, and the PTH intact test is central to making that diagnosis. Doctors also use it to monitor people with chronic kidney disease, because the kidneys play a major role in clearing PTH from the blood.
Another common reason for this test is to check if parathyroid surgery was successful. Surgeons often measure PTH during the operation itself. A sharp drop in levels confirms they removed the overactive gland. This approach, called intraoperative PTH monitoring, has made parathyroid surgery much more precise.
What Are Normal PTH Intact Levels?
Normal PTH intact levels typically fall between 10 and 65 picograms per milliliter (pg/mL), though exact ranges vary slightly between laboratories. Your doctor will always interpret your result against the reference range provided by the lab that ran your test, not against a generic number you find online.
The tricky part is that PTH levels must always be read alongside your calcium level. A PTH of 50 pg/mL might be perfectly normal if your calcium is normal, but it is abnormally high if your calcium is already elevated. In a healthy person, high calcium should suppress PTH production. When it does not, that is a red flag.
Age also matters. Some studies suggest that normal PTH levels drift upward slightly as people get older. Vitamin D deficiency can also push PTH levels higher because your body tries to compensate by pulling more calcium from your bones. For this reason, doctors often check vitamin D levels at the same time.
What Do High PTH Intact Levels Mean?
High PTH intact levels with high calcium usually mean primary hyperparathyroidism. This happens when one or more of your parathyroid glands develop a benign growth that produces hormone without listening to the body’s normal feedback signals. The gland keeps pumping out PTH even though calcium is already too high.
Secondary hyperparathyroidism is different. In this case, PTH is high but calcium is normal or low. The parathyroid glands are working correctly — they are responding to a problem elsewhere in the body. Common causes include vitamin D deficiency, chronic kidney disease, or certain medications like lithium or diuretics that disturb calcium balance.
A third condition called tertiary hyperparathyroidism can develop in people with long-standing kidney disease, even after they receive a kidney transplant. The parathyroid glands have been overworking for so long that they keep producing too much hormone even after the original kidney problem is fixed. This is less common but important to recognize.
What Do Low PTH Intact Levels Mean?
Low PTH intact levels with low calcium usually point to hypoparathyroidism. This is a much rarer condition than hyperparathyroidism. It often happens after thyroid or parathyroid surgery when the glands are accidentally damaged or removed. Autoimmune destruction of the glands is another cause, though less common.
Low PTH with high calcium suggests that the high calcium is coming from somewhere other than the parathyroid glands. Possible causes include cancer that has spread to bone, excessive calcium intake from supplements, or conditions like sarcoidosis that cause the body to absorb too much calcium from food. In these cases, the parathyroid glands are behaving normally — they are appropriately suppressed by the high calcium.
Some people also have low PTH due to genetic conditions or magnesium deficiency. Magnesium is required for the parathyroid glands to release PTH, so severely low magnesium levels can cause a functional hypoparathyroidism that reverses once magnesium is corrected.
How to Prepare for a PTH Intact Test
Most doctors recommend taking the PTH intact test in the morning after an overnight fast. PTH levels follow a natural daily rhythm and tend to be highest in the early morning hours. Fasting also helps avoid interference from recent meals, which can temporarily affect calcium and PTH levels.
You should tell your doctor about all medications and supplements you take. Biotin, which is common in hair and nail supplements, can interfere with some PTH assays and cause falsely low results. Calcium supplements and vitamin D can also affect your levels, so your doctor may ask you to stop them for a few days before the test.
One practical tip: the blood sample needs to be processed quickly. PTH breaks down rapidly once drawn, so the lab must spin the blood down and freeze the plasma within a short window. If the lab is slow, the result may be falsely low. This is why some hospitals run this test on-site rather than sending it to an outside lab.
| PTH Level | Calcium Level | Most Likely Cause |
|---|---|---|
| High | High | Primary hyperparathyroidism |
| High | Normal or Low | Secondary hyperparathyroidism |
| Low | Low | Hypoparathyroidism |
| Low | High | Non-parathyroid cause of high calcium |
Common Misconceptions About PTH Intact
One widespread myth is that a high PTH level always means you have a parathyroid tumor. In reality, secondary hyperparathyroidism from vitamin D deficiency is far more common than a parathyroid adenoma. Many people with high PTH simply need more vitamin D and their levels return to normal without any surgery.
Another misunderstanding is that the PTH intact test alone can diagnose osteoporosis or guide treatment. The test is helpful for understanding bone metabolism, but it is not a direct measure of bone density. Doctors use DXA scans, not PTH levels, to diagnose osteoporosis. The PTH test helps them understand what is driving bone loss, but it does not replace the bone density scan.
Some people also believe that PTH levels should be zero after successful parathyroid surgery. That is not true. Your remaining parathyroid glands should still produce some hormone to keep your calcium balanced. A level that is low but detectable is normal and expected. Only a completely undetectable level with low calcium would suggest permanent hypoparathyroidism.
- Always get your calcium and PTH drawn at the same time — one is meaningless without the other
- Vitamin D deficiency is the most common cause of mildly elevated PTH and is easily treatable
- Kidney disease changes how PTH is cleared and requires special interpretation by a nephrologist
- The test must be processed quickly after the blood draw or the result will be unreliable
Frequently Asked Questions
What is the difference between PTH and PTH intact?
PTH intact measures only the full, active 84-amino acid hormone while older tests also detected inactive fragments. The intact test gives a more accurate picture of what your parathyroid glands are actually doing.
Can I eat before a PTH intact blood test?
Most doctors recommend fasting for 8 to 12 hours before the test. Eating can temporarily affect calcium levels and make the result harder to interpret.
How long does it take to get PTH intact results?
Results typically come back within 24 to 48 hours. Some hospitals can run the test and return results the same day if needed for surgery monitoring.
Does vitamin D affect PTH intact levels?
Yes, low vitamin D causes the parathyroid glands to work harder and raises PTH levels. Correcting vitamin D deficiency often brings PTH back to normal without any other treatment.
