The Test
How is it used?
An intact PTH is the most frequently ordered parathyroid hormone test. It is ordered to help diagnose the reason for a low or high calcium level and 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. A calcium test is almost always ordered along with a PTH test. It is not just the levels 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 imbalances in calcium regulation that may require medical intervention or in persistent imbalances that indicate an underlying problem.
PTH levels can be used to monitor people who have conditions or diseases that cause chronic calcium imbalances or to monitor those who have had surgery or other treatment for parathyroid tumors.
When is it ordered?
A PTH test may be ordered when a test for calcium is abnormal. It may be ordered when someone has symptoms associated with hypercalcemia, such as:
- Fatigue
- Nausea
- Abdominal pain
- Thirst
PTH may also be ordered when a person has symptoms associated with hypocalcemia, such as:
- Abdominal pain
- Muscle cramps
- Tingling fingers
A doctor may order a PTH, along with calcium, at intervals when someone has been treated for diseases or conditions that affect calcium regulation, such as the removal of a parathyroid tumor, or when a person has a chronic condition such as kidney disease.
Sometimes, an intraoperative PTH test will be ordered when someone who has hyperparathyroidism is undergoing surgery to have abnormal parathyroid tissue removed.
What does the test result mean?
A doctor will evaluate both calcium and PTH results together to determine whether the levels are appropriate and 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 hypercalcemia 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 tumor.
The table below summarizes results that may be seen:
Calcium - PTH Relationship
- If calcium levels are low and PTH levels high, then the parathyroid glands are responding as they should and producing appropriate amounts of PTH. Depending on the degree of hypocalcemia, a doctor may investigate a low calcium level further by measuring vitamin D, phosphorus, and magnesium levels.
- If calcium levels are low and PTH levels are normal or low, then PTH is not responding and the person tested probably has hypoparathyroidism. Hypoparathyroidism is a failure of the parathyroid glands to produce sufficient PTH. It may be due to a variety of conditions and may be persistent, progressive, or transient. Causes include an autoimmune disorder, parathyroid damage or removal during surgery, a genetic condition, and severe illnesses. Those affected will generally have low PTH levels, low calcium levels, and high phosphorus levels.
- If calcium levels are high and PTH levels are high, then the parathyroid glands are producing inappropriate amounts of PTH. The doctor may order x-rays or other imaging studies to help determine the cause and evaluate the severity of hyperparathyroidism. Hyperparathyroidism is a group of conditions characterized by an overproduction of PTH by the parathyroid glands. It is separated into primary, secondary, and tertiary hyperparathyroidism. Primary hyperparathyroidism is most frequently due to a parathyroid tumor (usually benign) that secretes PTH without feedback control. This puts PTH constantly in the "ON" position, where it can cause hypercalcemia and can lead to kidney stones, calcium deposits in organs, and decalcification of bone. With primary hyperparathyroidism, people will generally have high calcium and high PTH levels, while phosphorus levels are often low.
Secondary hyperparathyroidism is usually due to kidney failure. In people with kidney disease and/or failure, phosphorus may not be excreted efficiently, disrupting its balance with calcium. Kidney disease may also make those affected 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 phosphorus 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, people will generally have high PTH levels and low or normal calcium levels.
Sometimes, people with chronic secondary hyperparathyroidism develop high serum calcium and still have high PTH; this is sometimes called tertiary hyperparathyroidism.
- If calcium levels are high and PTH levels are low, then the parathyroid glands are responding properly, but the doctor is likely to perform further investigations to check for non-parathyroid-related reasons for the elevated calcium.
Is there anything else I should know?
Because there are many fragments of PTH, tests for PTH may measure one or more of the fragments. None of the assays for intact PTH measure PTH (35-84), which is actually the fragment of PTH present in highest amounts in blood. Many intact PTH assays measure PTH (7-84) as well. In most people, this fragment is present in much lower amounts than PTH (1-84), so this is not a concern. In kidney failure, a common setting for measuring PTH levels, PTH (7-84) levels increase compared to PTH (1-84), and sometimes over half of what is measured as PTH represents this N-terminal truncated fragment. Some intact PTH assays do not measure this fragment and will give lower PTH results when increased PTH (7-84) is present.
PTH levels will vary during the day, peaking at about 2 a.m. Specimens are usually drawn about 8 a.m.
Drugs that may increase PTH levels include phosphates, anticonvulsants, steroids, isoniazid, lithium, and rifampin.
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