Understanding Your Parathyroid Hormone (PTH) and Calcium Balance Tests

Section Editors: Christopher McCudden Ph.D., DABCC, FADLM, FCACB and Kamran Mirza MD PhD
July 27, 2026

Parathyroid hormone (PTH) and calcium balance tests are a group of blood tests used to check the level of calcium in the blood and the hormones and vitamins that keep it in balance. The main tests are calcium, parathyroid hormone (PTH), phosphorus (also called phosphate), and vitamin D. Calcium is essential for strong bones, nerve signaling, muscle function, and blood clotting, so the body keeps its level within a narrow range. These tests are ordered when the calcium level on a blood test is too high or too low, or to investigate bone thinning, kidney stones, or a suspected problem with the parathyroid glands. Because these tests control one another, they are interpreted together as a pattern rather than one value at a time. The combination of calcium and PTH, in particular, is what points to a cause.

This article explains what each test measures, how the results fit together, and what the common patterns of results can mean.

The reference range that applies to your result is the one printed on your laboratory report, not the typical ranges shown here. Reference ranges vary between laboratories based on the equipment used, the population tested, and individual factors such as age, sex, and pregnancy status. Always compare your result to the reference range printed on your own report, and discuss any abnormal result with your doctor.

How calcium balance works

Understanding how the body controls calcium makes these test results much easier to interpret. The level of calcium in the blood is kept within a narrow range by several organs working together, and parathyroid hormone (PTH) is the main control signal. PTH is made by the parathyroid glands, four small glands in the neck that sit just behind the much larger thyroid gland.

When the calcium level in the blood falls, the parathyroid glands release more PTH. PTH raises calcium in three ways: it releases calcium from the bones, it tells the kidneys to hold on to calcium (while getting rid of phosphate), and it activates vitamin D, which helps the intestines absorb more calcium from food. When the calcium level rises high enough, the parathyroid glands release less PTH, and calcium falls back toward normal. This feedback loop is key to interpreting the results: a calcium level is only meaningful when you also know what PTH is doing in response to it. Phosphorus and vitamin D are measured because they are part of the same system, and abnormal levels can point to where the problem lies.

What do these tests measure?

A calcium balance panel usually includes calcium, PTH, phosphorus, and vitamin D. Your report may include some or all of them, depending on what is being investigated.

Calcium

Calcium testing measures the amount of calcium in the blood. Most calcium in the blood is attached to a protein called albumin, and only the unattached (free) portion, called ionized calcium, is biologically active. A standard test measures total calcium, which includes both the attached and free portions. Because a low albumin level can make total calcium look falsely low, the result is often adjusted for albumin (this is called corrected calcium), or the ionized calcium is measured directly for a more accurate value. A high calcium level is called hypercalcemia, and a low level is called hypocalcemia. A typical adult total calcium is approximately 8.5 to 10.2 mg/dL (2.1 to 2.6 mmol/L), and a typical ionized calcium is approximately 4.6 to 5.3 mg/dL (1.15 to 1.33 mmol/L), although the exact range depends on the laboratory. Because so many conditions can change the calcium level, an abnormal calcium level is almost always investigated together with PTH.

Parathyroid hormone (PTH)

PTH is the main hormone that controls the calcium level in the blood, and it is measured together with calcium because the two must be read as a pair. The important question is not simply whether PTH is high or low, but whether it is appropriate for the calcium level. Normally, a high calcium should switch PTH off, producing a low PTH, and a low calcium should switch PTH on, producing a high PTH. A typical range for intact PTH is approximately 15 to 65 pg/mL (about 1.6 to 6.9 pmol/L). What matters most, though, is the level relative to the calcium: when calcium is high, PTH would normally be suppressed to a low level, often below about 20 pg/mL (2.1 pmol/L), so a PTH that is not suppressed in that setting is inappropriately high even if it falls within the normal range. When the PTH does not match the calcium in this expected way, it points to a specific problem, as described in the patterns below.

Phosphorus (phosphate)

Phosphorus, also called phosphate, is a mineral closely linked to calcium and part of the same balance system. PTH lowers blood phosphate by increasing the amount lost in the urine. As a result, phosphate tends to be low when PTH is high (as in primary hyperparathyroidism) and high when PTH is low (as in hypoparathyroidism) or when the kidneys cannot clear it (as in chronic kidney disease). A typical adult phosphate range is approximately 2.5 to 4.5 mg/dL (0.8 to 1.45 mmol/L). Phosphate therefore adds useful supporting information to the calcium and PTH results.

Vitamin D

Vitamin D helps the body absorb calcium from food, so it is an important part of calcium balance. The usual test measures 25-hydroxyvitamin D, the storage form, which reflects the body’s overall vitamin D level. A low level (vitamin D deficiency) is common and can lower calcium and, in response, raise PTH. Many laboratories consider a 25-hydroxyvitamin D level below about 20 ng/mL (50 nmol/L) to be deficient, 20 to 29 ng/mL (50 to 74 nmol/L) to be insufficient, and 30 ng/mL (75 nmol/L) or above to be sufficient, although the exact thresholds vary between guidelines. A separate active form, called 1,25-dihydroxyvitamin D, is measured only in specific situations. Vitamin D is checked alongside calcium and PTH because a deficiency needs to be identified and corrected before some of the other results can be interpreted correctly.

How the tests work together: common patterns

Calcium balance tests are most meaningful when calcium, PTH, phosphate, and vitamin D are read together, because the combination usually points to one of a few patterns. The patterns below are the ones seen most often.

Primary hyperparathyroidism

In primary hyperparathyroidism, one or more parathyroid glands make too much PTH on their own. The typical pattern is a high calcium with a PTH that is high, or in the middle of the normal range, which is still inappropriately high for the raised calcium, often together with a low phosphate. The most common cause is a single non-cancerous parathyroid adenoma, and this is the most common reason for a high calcium found on a routine blood test. Over time, it can lead to kidney stones and thinning of the bones.

Secondary hyperparathyroidism

In secondary hyperparathyroidism, the parathyroid glands are working hard for an understandable reason. The pattern is high PTH with low or normal calcium. This is the normal response to something that lowers calcium, most often vitamin D deficiency or chronic kidney disease. Here the high PTH is the body compensating rather than a primary parathyroid problem. When several glands enlarge in this setting, it is called parathyroid hyperplasia.

Tertiary hyperparathyroidism

After long-standing secondary hyperparathyroidism, usually in advanced kidney disease, the parathyroid glands can begin to act on their own and keep making too much PTH even after the original cause is treated. This is called tertiary hyperparathyroidism, and it shows high PTH together with high calcium.

High calcium with a low PTH

When calcium is high, but PTH is low, the parathyroid glands are correctly switching off in response to the high calcium, which means the extra calcium is coming from somewhere else. The most common cause of this pattern is cancer, either a tumor that makes a PTH-like substance or cancer that has spread to the bones. Other causes include too much vitamin D and certain medications. This pattern usually prompts a search for a cause outside the parathyroid glands.

Hypoparathyroidism

In hypoparathyroidism, the parathyroid glands make too little PTH. The pattern is low calcium with low PTH and often high phosphate. The most common cause is injury to the parathyroid glands during neck or thyroid surgery. A low calcium can cause tingling, muscle cramps, and, when severe, more serious symptoms, so it is treated promptly.

Vitamin D deficiency

Vitamin D deficiency is common and is shown by a low 25-hydroxyvitamin D level. Because vitamin D helps the body absorb calcium, a deficiency can lower calcium and, in response, raise PTH, which is a form of secondary hyperparathyroidism. Since low vitamin D affects both calcium and PTH, it is usually corrected first, and the other results are then rechecked to see the true picture.

What can affect the results?

Several factors can change calcium balance results, which is why they are interpreted carefully and often repeated.

Albumin level — Total calcium depends partly on albumin, so a low albumin (from illness or poor nutrition) can make total calcium look falsely low. This is why corrected calcium or ionized calcium is often used.
Medications and supplements — Thiazide water pills and lithium can raise calcium, while some bone medications can lower it. Calcium and vitamin D supplements raise the measured levels, so it helps to tell your doctor what you take.
Kidney function — Reduced kidney function affects phosphate, the activation of vitamin D, and PTH, so kidney results are considered alongside these tests.
Biotin supplements — High-dose biotin (vitamin B7) can interfere with some laboratory hormone tests, including PTH, and produce misleading results.
Sample handling and timing — Leaving a tourniquet on too long can slightly raise the measured calcium, and phosphate can vary with recent meals.
Season and sun exposure — Vitamin D levels are naturally lower in winter and with less time outdoors.

What happens after an abnormal result?

An abnormal calcium balance result guides the next steps rather than giving a diagnosis on its own, and the tests are usually rechecked and interpreted as a set. Because vitamin D deficiency can affect both calcium and PTH, it is often corrected first, and the results are then repeated. When calcium is high, a 24-hour urine calcium test is sometimes done, mainly to tell primary hyperparathyroidism apart from a benign inherited condition called familial hypocalciuric hypercalcemia, which causes mildly high calcium with low urine calcium and does not require surgery. If primary hyperparathyroidism is confirmed, imaging such as an ultrasound of the neck and a nuclear medicine scan (a sestamibi scan) may be used to locate the overactive gland before surgery. Surgery to remove the gland is often considered when the calcium is more than about 1 mg/dL (0.25 mmol/L) above the upper limit of normal, or when there is bone loss, a kidney stone, or reduced kidney function, although this decision is individualized. Depending on the situation, a bone density scan may be done to check for bone loss, and kidney imaging may be done if stones are a concern. Many people are referred to a hormone specialist (an endocrinologist), and to a surgeon if an operation is being considered. Your doctor interprets the full set of results together with your symptoms and history to decide what, if anything, needs to happen next.

Questions to ask your doctor

Which of my calcium balance tests were abnormal, and by how much?
Was my calcium corrected for my albumin level, or was ionized calcium measured?
What does the combination of my calcium and PTH results suggest?
Was my vitamin D level checked, and does it need to be corrected first?
Could any of my medications or supplements be affecting these results?
Do I need a 24-hour urine calcium test to rule out an inherited cause?
If I have primary hyperparathyroidism, do I need imaging to locate the gland?
Do I need a bone density scan or a check for kidney stones?
Should I be referred to an endocrinologist or a surgeon?
How often should these tests be repeated?