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  Back Blood-tests Explained


PTH


At a Glance

Why Get Tested?

To determine the cause of calcium imbalances; to evaluate parathyroid function; to diagnose and differentiate between primary, secondary, and tertiary hyperparathyroidism; to diagnose hypoparathyroidism; during surgery for hyperparathyroidism to confirm removal of the gland(s) causing the problem

When to Get Tested?

When calcium blood levels are higher or lower than normal; when you are having surgery for hyperparathyroidism; when your health practitioner wants to determine how well your parathyroid glands are functioning

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

Current practices do not require fasting.

PTH levels peak during sleep hours and are lowest during mid-morning to late afternoon hours, the period when most samples are drawn. There may be seasonal fluctuations in PTH due to its inverse relationship with vitamin D. Talk to your health practitioner about sample timing.

The Test Sample

What is being tested?

Parathyroid hormone (PTH) helps the body maintain stable levels of calcium in the blood. It is part of a feedback loop that includes calcium, PTH, vitamin D, and, to some extent, phosphorus (phosphate) and magnesium. Conditions and diseases that disrupt this feedback loop can cause inappropriate elevations or decreases in calcium and PTH levels and lead to symptoms of hypercalcemia or hypocalcemia. This test measures the amount of PTH in the blood.

PTH is produced by four button-sized parathyroid glands that are located in the neck behind the thyroid gland. Normally, these glands secrete PTH into the bloodstream in response to low blood calcium levels. The hormone works in three ways to help raise blood calcium levels back to normal:

  • PTH promotes the release of calcium from bones into the bloodstream.
  • It stimulates the kidneys to convert vitamin D from the inactive to the active form, which in turn increases the absorption of calcium from food in the intestines.
  • It acts on the kidneys to suppress the excretion of calcium in the urine while encouraging excretion of phosphorus.

As calcium levels begin to increase in the blood, PTH normally decreases.

Parathyroid hormone itself is composed of 84 amino acids (sometimes called PTH (1-84)). Intact and fragmented hormone is present in and secreted by the parathyroid gland. The intact hormone represents a smaller fraction, but its portion is increased when calcium levels are low and decreased when calcium levels are high.

Once released into the blood stream, PTH has a very short life span; levels fall by half in less than 5 minutes due to uptake and cleavage in the liver and kidneys. The fragments are referred to as C-terminal fragments and are variably sized, missing anywhere from 6 amino acids to more than half the N-terminal portion of the molecule. C-terminal fragments have a longer half-life, exist in much higher concentrations, and are eventually cleared by the kidneys. Although it was originally thought that the C-terminal fragments were inactive, it now appears that certain fragments may have biologic activities that are able to oppose those of intact PTH.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?

Current practices do not require fasting.

PTH levels peak during sleep hours and are lowest during mid-morning to late afternoon hours, the period when most samples are drawn. There may be seasonal fluctuations in PTH due to its inverse relationship with vitamin D. Talk to your health practitioner about sample timing.

The Test

How is it used?

The so-called intact PTH is the most frequently ordered parathyroid hormone test. It is used to help diagnose the cause of a low or high calcium level and to help distinguish between parathyroid-related and non-parathyroid-related causes. It may also be used to monitor the effectiveness of treatment when an individual has a parathyroid-related condition. PTH is routinely monitored for people with chronic kidney disease or who are on dialysis.

A calcium test is almost always ordered along with a PTH test. It is not just the level in the blood that is important but the balance between calcium and PTH and the response of the parathyroid glands to changing levels of calcium. Usually, health practitioners are concerned about either severe imbalances in calcium regulation that may require medical intervention or persistent imbalances that indicate an underlying problem.

PTH levels can be used to monitor people who have conditions or diseases that cause chronic calcium imbalances or to monitor those who have had surgery or another treatment for a parathyroid tumor.

When is it ordered?

A PTH test may be ordered when a test for calcium is abnormal. It may be ordered when someone has symptoms associated with hypercalcemia, such as:

  • Fatigue
  • Nausea
  • Abdominal pain
  • Thirst

PTH may also be ordered when a person has symptoms associated with hypocalcemia, such as:

  • Abdominal pain
  • Muscle cramps
  • Tingling fingers or toes

A health practitioner may order a PTH test along with a calcium test at intervals when someone has been treated for a disease or condition that affects calcium regulation, such as the removal of a parathyroid tumor, or when a person has a chronic condition such as kidney disease.

Sometimes, an intraoperative PTH test will be ordered when someone who has hyperparathyroidism is undergoing surgery to have abnormal parathyroid tissue removed to ensure removal of all of the abnormal glands, which can occasionally vary in number and location.

What does the test result mean?

A health practitioner will evaluate both calcium  and PTH results together to determine whether the levels are appropriate and are in balance as they should be. If both PTH and calcium levels are normal, then it is likely that the body's calcium regulation system is functioning properly.

Low levels of PTH may be due to conditions causing hypercalcemia or to an abnormality in PTH production causing hypoparathyroidism. Excess PTH secretion may be due to hyperparathyroidism, which is most frequently caused by a benign parathyroid tumor. In rare cases, it may be caused by cancer.

The table below summarizes results that may be seen:

Calcium PTH Interpretation
Normal Normal Calcium regulation system functioning OK
Low High PTH is responding correctly; may run other tests to check for other causes of hypocalcemia
Low Normal or Low PTH not responding correctly; probably have hypoparathyroidism
High High Parathyroid gland producing too much PTH; may do imaging studies to check for hyperparathyroidism
High Low PTH is responding correctly; may run other tests to check for non-parathyroid-related causes of elevated calcium
Normal High Mild hyperparathyroidism

Calcium - PTH Relationship

  • If calcium levels are low and PTH levels high, then the parathyroid glands are responding appropriately, producing appropriate amounts of PTH. Depending on the degree of hypocalcemia, a health practitioner may investigate a low calcium level further by measuring vitamin D, phosphorus, and magnesium levels.
  • If calcium levels are low and PTH levels are normal or low, then PTH is not responding properly and the person tested probably has hypoparathyroidism. Hypoparathyroidism is a failure of the parathyroid glands to produce sufficient PTH. It may be due to a variety of conditions and may be persistent, progressive, or transient. Causes include an autoimmune disorder, parathyroid damage or removal during surgery, a genetic condition, or a severe illness. Those affected will generally have low PTH levels, low calcium levels, and high phosphorus levels.
  • If calcium levels are high and PTH levels are high, then the parathyroid glands are producing inappropriately high amounts of PTH. A health practitioner may order X-rays or other imaging studies to help determine the cause and evaluate the severity of hyperparathyroidism. Hyperparathyroidism is a group of conditions characterized by an overproduction of PTH by the parathyroid glands that is separated into primary, secondary, and tertiary hyperparathyroidism. [See Parathyroid Diseases for more on this.]
  • If calcium levels are high and PTH levels are low, then the parathyroid glands are responding properly, but a health practitioner is likely to perform further investigations to check for non-parathyroid-related reasons for the elevated calcium, such as rare mutations in calcium receptors or tumors that secrete apeptide that has PTH-like activity and increases calcium concentration, which in turn decreases PTH.

Is there anything else I should know?

Currently, the most prevalent assay for PTH is the intact PTH assay, which typically measures the whole molecule (PTH 1-84) and the larger fragments (predominantly PTH 1-74). While this assay is known as the "intact" assay, it measures large fragments that are slightly smaller than the truly intact hormone. There are less common but newer assays that detect only the whole molecule, which has the greatest bioactivity. The "intact" and "whole or bioactive" assays have different reference ranges, but typically their results yield similar interpretations. The effects of fragmentation and disease are still being studied, and there are instances when the two assays may yield a different interpretation.

Drugs that may increase PTH levels include phosphates, anticonvulsants, steroids, isoniazid, lithium, and rifampin.



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