Bone tests: calcium, phosphorus, calcitonin, parathyroid hormone, vitamin D, osteocalcin, pyridinolines and telopeptide
Bones are made of tissue that is constantly renewed to keep them healthy and strong, a process involving several factors. So, to find out whether a person has a bone disease causing loss of bone density, the recommended laboratory tests are:
- Calcium (Ca), or blood calcium
- Phosphorus (P)
- Calcitonin (CT)
- Parathyroid hormone
- Vitamin D
- Osteocalcin (OC)
- Pyridinolines or telopeptide.
Another test, called hydroxyprolinuria, was also used in the past; this urine test is now obsolete because it is easily affected by external factors such as food, and it has been replaced by the more precise pyridinoline test.
What they mean
Calcium (Ca), or blood calcium: calcium gives bone its rigidity. • If the result is higher than normal, there is hypercalcaemia, which may be caused by hyperparathyroidism (i.e. the parathyroid glands produce too much hormone), excess vitamin D in the blood, kidney failure, the use of diuretics or contraceptives, or a bone tumour.
If the result is lower than normal, it could be hypoparathyroidism (i.e. the parathyroid glands produce too little hormone), vitamin D deficiency, insufficient dietary calcium intake, or excessive calcium loss in the stool or urine for reasons that need investigating.
Phosphorus (P): this element ensures that bone is not excessively rigid.
If the result is higher than normal, it could be due to hypoparathyroidism, kidney failure or excess vitamin D in the blood.
If the result is lower than these values, it could be due to hyperparathyroidism, intestinal malabsorption, prolonged fasting or liver disease.
Calcitonin (CT): this hormone tries to slow down the breakdown of bone to keep its renewal in balance.
If the result is higher than normal or close to the upper limit, it could indicate a thyroid tumour (medullary thyroid carcinoma).
Parathyroid hormone: this hormone tries to keep the amount of calcium in the blood stable.
If the result is higher than normal, it indicates hyperparathyroidism, which may be caused by a low blood calcium level (the hormone then acts on bone cells to release calcium from bone) or by a parathyroid tumour that stops the gland working properly.
• If the result is lower than these values, it indicates hypoparathyroidism, which may be caused by too much calcium in the blood (so parathyroid hormone is not stimulated) or by a parathyroid tumour.
Vitamin D: a vitamin, activated by the kidneys, that promotes the absorption of calcium from food.
If the result is higher than normal, it could indicate excessive intake and absorption in the intestine, in which case it becomes toxic to the body.
If the result is lower than normal, it indicates insufficient intake from food, too little exposure to sunlight (which stimulates its production) or liver disease. This can cause rickets in children and bone disease (osteomalacia) in adults.
Osteocalcin (OC): this test shows the activity of osteoblasts, the cells that form bone.
If the result is higher than normal, it could indicate hyperthyroidism or osteoporosis (i.e. thinning of the bone).
If the result is lower than normal, it indicates that bone formation by osteoblasts is slow and reduced.
Pyridinolines: this test shows the activity of osteoclasts, which “break down” bone so that it can be renewed.
If the result is higher than normal, osteoporosis is present.
If the result is lower than normal, the bone is not renewing itself enough and the cause needs to be investigated (for example, it could be due to the hormones produced by the parathyroid glands).
Telopeptide: this test has the same meaning as the pyridinoline test and shows osteoclast activity.
If the result is higher than normal, osteoporosis is present.
If the result is lower than normal, the bone is not renewing itself enough and the cause needs to be investigated.
Why they are done
These tests are carried out to diagnose whether or not a bone disease or bone loss is present. In particular, calcium is a routine test that the doctor prescribes to keep a constant check on the state of the bones. Blood calcium normally varies very little, so if it is abnormal, further investigation with the other tests is advisable to find out where the bone renewal system has broken down. During the menopause, when a woman’s hormonal status changes naturally because she no longer has periods, and this leads to bone loss, the doctor will generally prescribe a bone density scan (DEXA) as well as a calcium test. This is a radiological test that can accurately picture the health of bone tissue.
How they are done
Almost all of these are laboratory tests involving a simple blood test; only the pyridinoline and telopeptide tests are urine tests. For these, the urine sample should preferably be collected first thing in the morning, when it is more concentrated and easier to analyse. The disposable container is provided by the laboratory when you have the test, or it can be bought at a pharmacy. Fasting is recommended for these tests to prevent food from affecting the result, except for parathyroid hormone, for which fasting is required, and for pyridinolines and telopeptide, which can also be done on a random sample. Medicines do not affect the result, although your doctor should always be told about any treatment you are taking.
Results
NORMAL VALUES
Calcium (Ca or blood calcium) 8.6-10.5 mg/dl
Phosphorus (P) up to 1 year 5.0-7.5 mg/dl
2-10 years 4.5-5.6 mg/dl
over 10 years 2.5-4.8 mg/dl
Calcitonin 0-10 pg/ml
Parathyroid hormone 12-72 pg/ml
Vitamin D 8-42 ng/ml
Osteocalcin premenopausal women 1-11 ng/ml
postmenopausal women 1-13 ng/ml
men 1-11 ng/ml
Pyridinolines women 3-7.4 nM
men 2.3-5.4 nM
Telopeptide 1.8-5 ng/l