Summary

• Parathyroid hormone (PTH) regulates calcium levels in your blood and keeps them stable. 
• Testing PTH levels can help diagnose the reason for low or high calcium levels and can show if there is a problem with your parathyroid gland or if it is due to other causes.
• A PTH test is performed together with a calcium test because it is not just the levels of PTH and calcium in the blood that are important, but the balance between them and the response of the parathyroid glands to changing calcium levels. 
• Often blood tests for phosphate and/or vitamin D are ordered along with PTH.

What is parathyroid hormone?

Parathyroid hormone (PTH) regulates calcium levels in your blood and keeps them stable. It is made by the parathyroid glands that sit just behind your thyroid gland at the base of your neck.

Calcium is one of the most important minerals in your body and is needed for building bones, teeth, blood vessels, muscles and nerves. It is important that levels are kept in balance – not too high or too low but just right.

Hormones are chemical messengers that are made by your glands and travel through your bloodstream to specific tissues and organs. When they reach their target, they attach to a cell’s receptors, stimulating a response. They affect many different processes, including growth and development, metabolism (how your body uses energy), sexual function, reproduction and mood.

When calcium levels are low, the parathyroid glands release PTH to bring calcium levels back to normal.  As calcium levels rise, the parathyroid glands slow the production of PTH.

How blood calcium levels are controlled

Your body tries to keep your calcium levels at just the right level. It does this through a feedback system.

• When levels of calcium in your blood are low, your parathyroid glands sense this and make PTH.
• The PTH is released into the blood, and it triggers your kidneys to slow the amount of calcium being removed from your blood and passed into your urine. At the same time the PTH signals to cells in your bones to release calcium into your blood. As a result, blood calcium levels rise.  
• PTH also stimulates your kidneys to make calcitriol which is the activated form of vitamin D. This increases the amount of calcium being absorbed from food in your intestine.
• As calcitriol levels rise, and calcium levels go back to normal, PTH levels fall. In healthy people, PTH and vitamin D keep blood calcium at stable levels.
• When levels of calcium in your blood are high, your parathyroid glands sense this and less PTH is released into your bloodstream. As a result, calcium is no longer released from your bones, the kidneys allow it to pass into the urine.

For more on how calcium levels are controlled see the ionised calcium test.

Why get tested?

PTH testing is used to help diagnose the reason for a low or high calcium level and to help show whether you have a problem that is related to the parathyroid or due to other causes.  PTH may be ordered to monitor the effectiveness of treatment if you have a parathyroid-related condition. It can also be used to monitor calcium metabolism (how well your kidneys process calcium) in kidney failure.

Usually, the PTH test is ordered when there is concern about either a severe imbalance in calcium regulation that may need medical intervention, or when there are ongoing imbalances that could be due to another underlying problem.

PTH is ordered along with calcium. It is not just the levels in the blood that are important, but the balance between PTH and calcium and the response of the parathyroid glands to changing levels of calcium.

PTH also plays an important role in controlling the blood levels of phosphate and vitamin D. Your medical team may order blood tests for phosphate and/or vitamin D to help with the diagnosis.

High levels of PTH in the blood

This may be due to hyperparathyroidism, when your parathyroid glands release too much PTH into the bloodstream.

Primary hyperparathyroidism – caused by a problem with the parathyroid gland. This is usually due to a parathyroid tumour that releases PTH without feedback control. This puts PTH release constantly in the ‘ON’ position. These tumours are always benign, but this causes constantly high calcium levels which can lead to kidney stones, calcium deposits in organs and decalcification of the bones. With primary hyperparathyroidism, people generally have high calcium and high PTH levels, while phosphate levels are often low.

Secondary hyperparathyroidism – caused by a problem elsewhere in the body. This is usually due to kidney disease in which the kidneys are unable to get rid of phosphate efficiently, which they do through the urine. This disrupts the phosphate – calcium balance. Kidney disease may also stop production of the active form of vitamin D and this means that you are unable to absorb calcium properly from your diet. As phosphate levels build up and calcium levels fall, the parathyroid glands release more PTH.

Secondary hyperparathyroidism can be caused by other conditions that reduce calcium levels, such as malabsorption of calcium due to intestinal disease and vitamin D deficiency. With secondary hyperparathyroidism, you can expect to have high PTH levels and low or normal calcium levels. Sometimes, you can develop high blood calcium levels and still have a high PTH level. This condition is called tertiary hyperparathyroidism: where the production of PTH has escaped all normal body feedback controls and so persists even in cases where the initial cause (low blood calcium) has been corrected.

Low levels of PTH in the blood

This may be due to hypoparathyroidism (where your parathyroid gland does not release enough PTH) caused by a variety of conditions that may be persistent, progressive or temporary. Causes include an autoimmune disorder, parathyroid damage or removal during thyroid surgery, a genetic condition and severe illnesses. People with hypoparathyroidism will generally have low PTH levels and low calcium levels with high phosphate levels.

Low levels of PTH may also be seen when blood calcium levels are high due to causes not related to the parathyroid glands.

Having the test

Sample

Blood.

Any preparation?
No special preparation is needed for this test. However, since PTH levels vary during the day, they are usually collected first thing in the morning along with a fasting calcium sample. This means you will need to fast before the blood test.

Your results

Reading your test report

Your results will be presented along with those of your other tests on the same form.  You will see separate columns or lines for each of these tests. 

The close relationship between your calcium and PTH levels mean that the two must always be interpreted together.  

Reference intervals

Your results will be compared to reference intervals (sometimes called a normal range).

• Reference intervals are the range of results expected in healthy people 
• When compared against them your results may be flagged high or low if they sit outside this range
• Many reference intervals vary between labs so only those that are standardised or harmonised across most laboratories are given on this website.
• Parathyroid testing is not standardised (harmonised) and reference intervals may vary between labs.

If your results are flagged as high or low this does not necessarily mean that anything is wrong. It depends on your personal situation. Your results need to be interpreted by your doctor.

Questions to ask your doctor

The choice of tests your doctor makes will be based on your medical history and symptoms. It is important that you tell them everything you think might help. 

You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely. 

Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods or supplements. These may affect your results. Ask:

• Why does this test need to be done?
• Do I need to prepare (such as fast or avoid medications) for the sample collection?
• Will an abnormal result mean I need further tests?
• How could it change the course of my care?
• What will happen next, after the test?

More information

Pathology and diagnostic imaging reports can be added to your My Health Record. You and your healthcare provider can now access your results whenever and wherever needed.

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