To determine whether PTH concentrations are responding normally to changes in blood calcium concentration; to distinguish the cause of calcium imbalances, and to evaluate parathyroid function. In some laboratories, PTH may be measured during surgery for hyperparathyroidectomy. This is to help confirm removal of the parathyroid gland(s).
A blood sample taken from a vein in your arm.
Parathyroid hormone (PTH) helps the body maintain stable amounts of calcium in the bloodstream. It is part of a ‘feedback loop’ that includes calcium, PTH, vitamin D, and to some extent phosphate and magnesium. Conditions and diseases that disrupt this feedback loop can cause inappropriate elevations or decreases in calcium and PTH and lead to symptoms of hypercalcaemia (raised blood concentrations of calcium) or hypocalcaemia (low blood concentrations of calcium).
PTH is produced by four parathyroid glands that are located in the neck beside the thyroid gland. Normally, these glands secrete PTH into the bloodstream in response to low blood calcium concentration. Parathyroid hormone then works in three ways to help raise blood calcium back to normal. It takes calcium from the body’s bone, stimulates the activation of vitamin D in the kidney (which in turn increases the absorption of calcium from the intestines), and suppresses the excretion of calcium in the urine (while encouraging excretion of phosphate). As blood calcium concentrations begin to rise, the concentration of PTH will normally decrease.
Parathyroid hormone itself is composed of 84 amino acids (sometimes called PTH (1-84)). Once it is released from the parathyroid gland into the blood stream, it has a very short life-span; levels fall by half in less than 5 minutes. The fall is caused primarily by the breakdown of PTH to smaller fragments.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
PTH is requested to help determine the cause of a low or high calcium concentration, to help distinguish between parathyroid-related and non-parathyroid-related causes. It may also be requested to monitor the effectiveness of treatment when a patient has a parathyroid-related condition. PTH should always be requested with calcium - it is not just the amounts of each in the blood that are important, but the balance between them, and the response of the parathyroid glands to changing concentration 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 concentrations (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 concentrations and a low or normal calcium result. Sometimes, people with secondary hyperparathyroidism develop high serum calcium concentrations, and still have high PTH - a condition known as tertiary hyperparathyroidism.
Low blood calcium concentrations (called ‘hypocalcaemia’) may be due to hypoparathyroidism, where there is a failure of the parathyroid gland to produce PTH.
PTH 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.
When is it requested?
PTH may be requested when a blood calcium result 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 way of 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). This is not available in all UK laboratories.
What does the test result mean?
A PTH result 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 results are normal, and appropriate relative to each other, then it is likely that the body’s calcium regulation system is functioning properly.
Low concentrations 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 concentrations are low and PTH concentrations 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 concentrations are low and PTH concentrations are normal or low, then PTH is not responding to the change in calcium appropriately, and you may have hypoparathyroidism.
If calcium concentrations are high and PTH concentrations 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 concentrations are high and PTH concentrations are low, then your calcium regulation system is working normally but your doctor will do some further investigations to check for non-parathyroid related reasons for your elevated calcium.
If calcium concentrations are normal and PTH concentrations are high, then your parathyroid gland is having to produce more PTH than normal to maintain your blood calcium level. This is most commonly due to either vitamin D deficiency or chronic kidney disease.
Is there anything else I should know?
‘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.
Drugs that may increase PTH concentrations include: phosphates, anticonvulsants, steroids, isoniazid, lithium, and rifampin.
Drugs that may decrease PTH include cimetidine and propranolol.
Can I have an abnormal PTH level without having symptoms?
What does vitamin D have to do with PTH?
If you do not have enough vitamin D, your body will not be able to absorb calcium properly. Vitamin D regulates the intestinal absorption of calcium, while PTH regulates the activation of vitamin D. Too much and too little vitamin D can imbalance calcium metabolism.