Summary

• Calcium, phosphate and magnesium are electrolytes. These are minerals that have an electric charge.
• Although best known for the roles they play in bone and teeth building, they are also essential for keeping many of the body’s systems working including heart rhythms, nerves and muscles. They are involved in helping maintain the right amount of fluid and the acid-base balance in your body.
• Measuring them together or separately is often done to check a wide range of health conditions, especially those associated with kidney function, bone disorders and parathyroid gland disorders.
  

What are calcium, phosphate and magnesium?

Calcium, phosphate and magnesium are some of the most important minerals in your body.

They are electrolytes, which are minerals that have an electric charge when dissolved in water – or blood. In your body, electrolytes help regulate many systems including heart rhythms, nerves and muscles. They are also involved in helping maintain the right amount of fluid and the acid-base balance in your body.

Most of the calcium and phosphate in your body is stored in your bones and teeth, which need both calcium and phosphate for strength. Only small amounts are found in your blood. You get all of your calcium, phosphate and magnesium through your diet.

Different forms of calcium

There are several different forms of calcium in your blood and two are measured. These are free calcium which circulates in your blood and is also called ionised calcium, and bound calcium which is mainly bound to the protein albumin in your blood.

How you get calcium, phosphate and magnesium from your diet

Intestines absorb calcium, phosphate and magnesium from your diet. If you have low levels of any of these it may be because you are not getting enough in your diet or you have vomiting or diarrhoea. Conditions in your gut which make your intestines ability to absorb these electrolytes such as coeliac disease or inflammatory bowel disease (Crohn’s disease and ulcerative colitis) may also affect the levels of these electrolytes.

How excess calcium, phosphate and magnesium are removed from your body

Kidneys filter excess calcium, phosphate and magnesium from your blood and remove them through your urine. If your levels are low, your kidneys respond by reducing the amount being removed from the blood. Abnormal levels are seen in disorders that cause too little or too much to be removed by the kidneys.

How calcium and phosphate levels in your blood are controlled

Your body works to keep calcium and phosphate at just the right balance. Levels are kept in balance through a feedback system under the control of your parathyroid glands, which are tiny glands located at the back of your thyroid.

• When circulating levels of free calcium are low in your blood and your body needs more, your parathyroid glands sense this and make more parathyroid hormone (PTH). Hormones are the body’s chemicals messengers. They are made by various glands and travel through the bloodstream to regulate body functions.
• When PTH is released into your blood, it triggers your kidneys to reduce the amount of calcium being filtered from your blood and removed through your urine. At the same time PTH signals to cells in your bones to release calcium into your blood. As a result of both actions, blood calcium levels rise.
• PTH also stimulates your kidneys to make calcitriol, the activated form of vitamin D. This increases the amount of calcium being absorbed from food in your intestine.
• PTH has the opposite effect on phosphate. It triggers your kidneys to remove more phosphate from your blood and is passed into your urine. As phosphate levels fall and calcium levels rise, less PTH is made.
• Calcium and phosphate work together in an inverse relationship. In other words, when calcium levels rise, phosphate levels fall, and when calcium levels fall, phosphate levels rise. This is because phosphate binds to free calcium, lowering the amount circulating in your blood. When you have blood tests for calcium and phosphate, the higher the phosphate level, the lower the level of free calcium.

Magnesium

Magnesium is needed by your body to properly absorb calcium from your diet and for your body to use calcium effectively. About half of the body’s magnesium is combined with calcium and phosphate to form your bones. About 40 percent is found in muscles and soft tissues. Only a very small amount (about one percent of the total magnesium in the body) is found in the blood.

Magnesium helps convert vitamin D into its active form, calcitriol, which is needed for calcium absorption. It is also needed for making and releasing PTH. When magnesium levels are low, not enough PTH is produced, leading to lower calcium levels. A low magnesium level can, over time, cause persistently low calcium and potassium levels. This means that measuring magnesium can be helpful in diagnosing disorders that involve calcium, potassium, phosphate and/or PTH.

Why get tested?

Calcium and phosphate are often ordered together to test for conditions associated with kidney function, bone building, parathyroid gland disorders and disorders that affect your intestines.

Heart function

Calcium is essential for normal heart function, but high blood calcium levels can contribute to irregular heart rhythms. High blood calcium can also lead to calcium being deposited in your blood vessels, making the blood vessels stiff and increasing your cardiovascular risk (your risk of heart attack and stroke). High phosphate levels, which can be due to chronic kidney disease or high PTH levels, can also cause blood vessels to become stiff and increased cardiovascular risk.

Cancer
High calcium levels is a common complication in cancer (called hypercalcemia of malignancy).

Bone cancers that have spread from cancers of the breast, prostate and multiple myeloma (a type of blood cancer), can significantly affect calcium and phosphate levels.

On the other hand, cancer can cause low calcium levels. When cancer cells break down rapidly and release large amounts of their contents into the bloodstream, this can cause phosphate levels to rise which in turn leads to low calcium levels.

Medications
Certain medications directly impact calcium and phosphate levels. Bisphosphonates, treatments for osteoporosis and some cancers may temporarily lower calcium levels. Diuretics (such as loop diuretics, which are medications that make you urinate) increase calcium loss into the urine, while thiazides medications the amount of calcium removed from the body through urine. Drugs like tacrolimus or cyclosporine, immunosuppressants used in organ transplantation and autoimmune conditions, can also affect phosphate levels.

Because magnesium is used in so many of the body’s functions, measuring magnesium levels is used to diagnose and manage a wide range of health conditions. The test is most often ordered where electrolyte imbalances, nerve and muscle function disorders, or certain kidney disorders are suspected.

For more detailed information on these tests and what your results can mean please see

Calcium

Ionised calcium

Phosphate

Magnesium

More to know?

Newborns, especially premature and low birthweight infants and those born to a diabetic mother, are often monitored during the first few days of life for neonatal hypocalcaemia (high calcium levels). This can occur because of an immature parathyroid gland and does not always cause symptoms. The condition may resolve itself or may require treatment with calcium given orally or by injection.

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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