PTH

PTH is ordered to help diagnose the reason for 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 ordered along with calcium. 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 imbalance in calcium regulation, that may require medical intervention, or in persistent imbalances that indicate an underlying problem.

High calcium levels, called hypercalcemia, may be due to hyperparathyroidism, a group of conditions characterized by an overproduction of PTH by the parathyroid gland. Hyperparathyroidism 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, patients will generally have high calcium and high PTH levels, while phosphate levels are often low.

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; this is sometimes called tertiary hyperparathyroidism.

Low calcium levels, called hypocalcemia, may be due to hypoparathyroidism – a failure of the parathyroid gland to produce sufficient PTH. Hypoparathyroidism 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. Affected patients will generally have low PTH levels and low calcium levels and will also have high phosphate levels.

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

PTH may be ordered when a test for calcium is abnormal. PTH may be ordered when you have symptoms associated with hypercalcemia such as fatigue, nausea, abdominal pain, and thirst. PTH may also be ordered when you have symptoms associated with hypocalcemia, such as abdominal pain, muscle cramps, and tingling fingers. Your doctor may order a PTH, along with calcium, at intervals when you have been treated for diseases or conditions that affect calcium regulation, such as the removal of a parathyroid tumor, or when you have a chronic condition 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 he has removed all of the abnormal glands. 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).

Your doctor will determine whether calcium and PTH concentrations 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.

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, your doctor may investigate the low calcium further by measuring your vitamin D, phosphorus, 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 glands are producing inappropriate amounts of PTH. Your doctor may order 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 the parathyroid glands are responding properly, but your doctor is likely to perform further investigations to check for non-parathyroid-related reasons for your elevated calcium.

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 fragment. Some PTH assays (often called "biointact" or "PTH (1-84)" tests) do not measure this fragment. There is no clear evidence at present that tests that do not measure PTH (7-84) fragments actually are more helpful in kidney failure patients than tests that measure both PTH (1-84) and PTH (7-84); however, results are always lower with the tests that measure only PTH (1-84).

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.