PTH is requested to help determine the cause of a low or high calcium level, to help distinguish between parathyroid-related and non-parathyroid-related causes.  It may also be ordered to monitor the effectiveness of treatment when a patient has a parathyroid-related condition.  PTH is requested with calcium - it is not just the levels of each in the blood that are important, but the balance between them, and the response of the parathyroid glands to changing levels of calcium. Usually doctors are concerned about either severe imbalance in calcium regulation, that may require medical intervention, or in persistent imbalances that indicate an underlying problem.

High blood calcium levels (called ‘hypercalcaemia’) may be due to a condition called hyperparathyroidism, where there is overproduction of PTH by the parathyroid gland. Hyperparathyroidism is separated into primary and secondary hyperparathyroidism. Primary hyperparathyroidism is most frequently due to a parathyroid tumour (usually benign, but very rarely cancerous) that secretes PTH without feedback control. This puts PTH constantly in the “ON” position, where it can cause hypercalcaemia, and can lead to kidney stones, calcium deposits in organs, and decalcification of bone. With primary hyperparathyroidism, patients will generally have high calcium and high PTH levels.

Secondary hyperparathyroidism is usually due to kidney failure. In patients with kidney disease and/or failure, phosphate may not be excreted efficiently, disrupting its balance with calcium. Kidney disease may also make the patient unable to produce the active form of vitamin D, and this in turn means that they are unable to absorb calcium properly from the diet. As phosphate levels build up and calcium levels fall, PTH is secreted. Secondary hyperparathyroidism can also be caused by any other condition that causes low calcium, such as malabsorption of calcium due to intestinal disease and vitamin D deficiency.  In secondary hyperparathyroidism, patients will generally have high PTH levels and low or normal calcium levels.  Sometimes, persons with secondary hyperparathyroidism develop high serum calcium, and still have high PTH - a condition known as tertiary hyperparathyroidism.

Low blood calcium levels (called ‘hypocalcaemia’) may be due to hypoparathyroidism, where there is a failure of the parathyroid gland to produce PTH.

PTH levels can also be used to monitor patients who have conditions or diseases that cause chronic calcium imbalance, and to monitor those who have had surgery or other treatment for parathyroid tumours.

PTH may be requested when a test for calcium is abnormal. PTH may be requested when you have hypercalcaemia, which may cause symptoms such as tiredness, sickness, stomach pain, and thirst. PTH may also be requested when you have hypocalcaemia, which may cause symptoms such as muscle cramps and tingling fingers. Your doctor may request a PTH, along with calcium (and other tests) as a monitoring changes when you have had treatment for diseases or conditions that affect calcium regulation, such as the removal of a parathyroid tumour, or when you have chronic conditions such as kidney disease.

 

When a person has hyperparathyroidism, the usual treatment is surgery to remove the enlarged gland or glands. About 85-90% of the time in primary hyperparathyroidism, only one abnormal parathyroid gland is present, but in the remaining cases two or more of the glands are abnormal. In secondary hyperparathyroidism, usually all four of the parathyroid glands are affected.  During surgery, it is important for the surgeon to make sure that all of the abnormal glands have been removed.  If all are abnormal, this usually means removing three glands completely and part of the fourth, leaving behind just enough parathyroid tissue to prevent hypoparathyroidism.  One way to be sure that all of the abnormal tissue has been removed is to measure PTH before and after an apparently abnormal gland has been removed.  If all the abnormal tissue is gone, PTH levels will fall by over 50% within 10 minutes.  To be useful, this requires that the laboratory be able to provide the results quickly (this is often called rapid or intraoperative PTH measurement).

NOTE: A standard reference range is not available for this test. Because reference values are dependent on many factors, including patient age, gender, sample population, and test method, numeric test results have different meanings in different labs. Your lab report should include the specific reference range for your test. Lab Tests Online strongly recommends that you discuss your test results with your doctor. For more information on reference ranges, please read Reference Ranges and What They Mean.

A PTH level needs to be evaluated relative to a calcium level measured at the same time; your doctor will look to see if they 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 hypercalcaemia, 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 tumour.

 

Calcium - PTH Relationship

•   If calcium levels are low and PTH levels high, then PTH is responding as it should. Depending on the degree of hypocalcaemia, your doctor may investigate the low calcium further by looking at your vitamin D, phosphate, and magnesium levels.

•   If calcium levels are low and PTH levels are normal or low, then PTH is not responding and you probably have hypoparathyroidism.

•   If calcium levels are high and PTH levels are high, then your parathyroid gland is producing inappropriate amounts of PTH and your doctor may request X-rays or other imaging studies to check for the cause and severity of hyperparathyroidism.

•   If calcium levels are high and PTH levels are low, then your calcium regulation system is working normally but your doctor will do further investigation to check for non-parathyroid related reasons for your elevated calcium.

‘Intact’ PTH is broken down into several molecular fragments including: an N-terminal, a C-terminal, and a mid-region fragment. While each of these fragments can give the doctor information about calcium regulation, intact PTH is measured most frequently as it is the major biologically active form.

 

PTH levels will vary during the day, peaking at about 2 am. Specimens are usually taken at about 8 am.

 

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

Drugs that may decrease PTH include cimetidine and propranolol.