This test measures the amount of parathyroid hormone (PTH) in the blood. PTH is released by the parathyroid glands to regulate calcium levels in the body. If PTH secretion is excessive - hyperparathyroidism - it can lead to high levels of calcium or hypercalcaemia. If PTH secretion is too low then low levels of calcium or hypocalcamia can occur. As well as the PTH test being used to diagnose these conditions it can also be used to monitor calcium metabolism in renal failure.
Why get tested?
Parathyroid glands are four small glands that help the body maintain stable amounts of calcium. They sit just behind the thyroid gland. Although the names are similar, parathyroid glands have a different function to the thyroid.
Calcium is essential in building healthy bones, teeth, blood vessels, muscles, and nerves. It is important that levels are kept in balance – not too high or too low.
When calcium levels are low, the parathyroid glands release parathyroid hormone (PTH) to bring calcium levels back to normal. As calcium levels rise, the parathyroid glands will stop making PTH.
PTH is part of a feedback loop that includes PTH, calcium, phosphate and vitamin D. It works in three ways to help lift blood calcium levels back to normal.
A PTH blood test is ordered when calcium levels are abnormal. Calcium levels should be tested at the same time as PTH. It is not just the levels in the blood that are important, but the balance between the two, and the response of the parathyroid to changing levels of calcium. Usually, doctors are concerned about either severe imbalance in calcium metabolism that may require medical intervention, or in persistent imbalances that indicate an underlying problem.
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 constantly high calcium levels 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 caused by a variety of conditions that may be persistent, progressive, or transient. 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
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 and collected with a fasting calcium sample.
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.
Calcium regulation system functioning normally
PTH is responding correctly. Other tests may be needed to investigate low blood calcium levels.
PTH is not responding correctly. Suggests hypoparathyroidism.
Parathyroid glands producing too much PTH. May require imaging investigation.
PTH is functioning correctly. May need more tests to check for non-parathyroid related causes of high blood calcium levels.
Table 1. Possible results of calcium and PTH levels.
Your results will be compared to reference intervals (sometimes called a normal range).
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:
Any more to know?
PTH plays an important role in controlling the blood levels of phosphate and vitamin D. Your doctor may order blood tests for phosphate and/or vitamin D to help with the diagnosis.
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