PTH stands for parathyroid hormone, and a PTH blood test measures how much of this hormone is circulating in your bloodstream. It’s one of the key hormones that controls your calcium levels, and doctors typically order this test when a routine calcium test comes back too high or too low. The normal range for an intact PTH test generally falls between 15 and 65 pg/mL, though reference ranges vary slightly between labs.
What Parathyroid Hormone Does
PTH is produced by four tiny glands, each about the size of a grain of rice, sitting behind your thyroid in your neck. Despite the similar name, the parathyroid glands have nothing to do with thyroid function. Their sole job is to keep calcium in your blood at a precise level, because calcium is essential for muscle contraction, nerve signaling, and heart rhythm.
When your blood calcium drops even slightly, the parathyroid glands release PTH, which raises calcium in three ways. First, it pulls calcium out of your bones, both through a rapid process that works within minutes and a slower one over days that activates bone-dissolving cells. Second, it tells your kidneys to hold onto calcium instead of excreting it in urine, while flushing out excess phosphorus. Third, it triggers your kidneys to convert vitamin D into its most active form, which helps your intestines absorb more calcium from the food you eat.
Once calcium levels return to normal, the parathyroid glands sense the change directly and dial back PTH production. This feedback loop keeps your blood calcium remarkably stable throughout the day.
Why Your Doctor Ordered This Test
The most common reason for a PTH test is an abnormal calcium level on previous blood work. Your doctor may also order it if you have symptoms that suggest your calcium is off balance.
Symptoms of too much calcium in the blood include constipation, nausea, belly pain, loss of appetite, increased thirst, frequent urination, kidney stones, bone and muscle aches, fatigue, and mental changes like depression or confusion. Symptoms of too little calcium look different: muscle cramps, spasms, or stiffness, tingling in your lips, tongue, fingers, or feet, dry skin, brittle nails, and in severe cases, irregular heartbeat or seizures. People with chronic kidney disease are also monitored with regular PTH tests because the kidneys play a central role in how PTH works.
How to Read Your Results
A PTH result only makes sense when you look at it alongside your calcium level. The two numbers together tell a much clearer story than either one alone.
High PTH With High Calcium
This combination points toward primary hyperparathyroidism. One or more of the parathyroid glands is overactive, pumping out too much PTH and driving calcium above normal. The most common cause is a small, benign growth on one of the glands. This is one of the most frequently diagnosed hormonal disorders, especially in women over 50.
High PTH With Low or Normal Calcium
This pattern suggests secondary hyperparathyroidism. The glands are working overtime because something else is keeping calcium low, often chronic kidney disease or severe vitamin D deficiency. The parathyroid glands themselves are healthy but are responding appropriately to a problem elsewhere. In people with advanced kidney disease, PTH levels can climb quite high. Clinical guidelines suggest that PTH above 70 pg/mL in stage 3 kidney disease or above 110 pg/mL in stage 4 warrants closer management, and patients on dialysis often have target ranges of 150 to 300 pg/mL.
Low PTH With Low Calcium
This indicates hypoparathyroidism, meaning the glands aren’t producing enough hormone. The most common cause is damage to the parathyroid glands during thyroid or neck surgery. Other causes include radiation treatment to the head or neck, autoimmune conditions where the body attacks the glands, genetic factors, and low magnesium levels. Magnesium is required for the parathyroid glands to function properly, so a simple magnesium deficiency can suppress PTH production.
Low PTH With High Calcium
This less common pattern means calcium is elevated for a reason unrelated to the parathyroid glands. Possible causes include certain cancers, excessive vitamin D supplementation, or other medical conditions. The parathyroid glands are correctly shutting down in response to the high calcium, so the problem lies elsewhere.
Timing and Preparation
PTH follows a natural daily rhythm. Levels peak around 3 a.m. and again in the late afternoon (around 5:30 p.m.), while dropping to their lowest point mid-morning around 10:30 a.m. For this reason, many labs draw PTH in the morning to get the most consistent baseline reading. Fasting is not typically required, though your doctor may ask you to fast for other tests being drawn at the same time.
One preparation detail worth knowing: if you take biotin supplements, they can interfere with PTH test results. The daily recommended amount of biotin (0.03 mg) doesn’t cause problems, but many supplements contain 20 to 100 mg per pill, and doses used for conditions like multiple sclerosis can reach 300 mg per day. At those levels, biotin can produce inaccurate readings. The FDA has warned that there isn’t enough data to recommend a specific washout period, so mention any biotin supplements to your doctor before the test.
Types of PTH Tests
Most labs use what’s called a second-generation “intact PTH” assay, which is the standard test. A newer third-generation “bio-intact” or “whole PTH” assay exists that measures only the full-length hormone and excludes certain fragments. Results between the two can differ by as much as 47%, so if you’re tracking PTH over time, it’s important that the same type of test is used each time.
For most people being evaluated for a parathyroid problem, the type of assay won’t change the diagnosis. The distinction matters more in specific situations: during parathyroid surgery, the third-generation test reflects successful gland removal about five minutes faster, and in chronic kidney disease, the two assays can give meaningfully different numbers that affect treatment decisions. Major kidney disease guidelines currently recommend the second-generation test because it’s more widely available and better studied.
What Happens After an Abnormal Result
A single abnormal PTH value usually leads to repeat testing. Guidelines for conditions like hyperparathyroidism typically require at least two consecutive abnormal results before a diagnosis is made. Your doctor will likely also check your vitamin D level, phosphorus, magnesium, and kidney function to get the full picture.
If primary hyperparathyroidism is confirmed, imaging may be done to locate the overactive gland. For secondary hyperparathyroidism caused by kidney disease or vitamin D deficiency, treatment focuses on correcting the underlying issue. For hypoparathyroidism, the approach centers on maintaining adequate calcium and active vitamin D levels in the body. The specific next steps depend heavily on which pattern your PTH and calcium results show, which is why interpreting these two numbers together is so important.