PTH is secreted in a discontinuous (pulsatile) fashion. There is a diurnal variation, with highest values at 2 A.M. (midnight to 4 A.M.) and lowest at noon (10 A.M. -2 P.M.. The parathyroids synthesize intact PTH, consisting of 84 amino acids in a single chain. Metabolic breakdown of intact PTH occurs both inside and outside of the parathyroids; outside the parathyroid, breakdown takes place in the liver and to a much lesser extent in the kidneys. This breakdown results in several fragment molecules: a small amino-terminal (N-terminal) fragment containing the PTH amino acid sequence 1-34; a larger midregion fragment containing amino acids 44-68; and a relatively large carboxy-terminal (C-terminal) fragment containing amino acids 53-84. Intact PTH and the N-terminal fragments have metabolic activity but not the midregion or C-terminal fragments. In a normal person, intact PTH constitutes 5%-15% of circulating PTH molecules. All PTH fragments are eliminated by the kidney, primarily through glomerular filtration. The measurable serum half-life of intact PTH is only about 5 minutes; that of the N-terminal fragment is about 2-3 minutes; and that of the C-terminal fragment is about 30 minutes. Renal function impairment will decrease elimination of the C-terminal fragment and also to a lesser extent the N-terminal fragment. In renal failure the C-terminal half-life lengthens to 24-36 hours and the N-terminal half-life lengthens to 30 minutes.

PTH is measured by immunoassay. Original methods were based on antibodies against either the N-terminal fragment or the C-terminal fragment. Current tests use antibody against synthetic portions of the PTH chain, resulting in somewhat better sensitivity and reliability. These tests primarily detect either the C-terminal, the midregion fragment, or the intact PTH molecule. Actually, the C-terminal and midregion assays detect more fragments than the principal one indicated by their name:

Intact PTH
Midregion (sometimes called “total” PTH)

Assay includes
Intact PTH only
N-terminal fragment Intact PTH
C-terminal fragment Intact PTH
Midregion combined with C-terminal fragment
Midregion fragment Intact PTH
Midregion combined with C-terminal fragment

At present, midregion assays have generally been more sensitive in detecting PHPT and separating PHPT from normal persons than C-terminal or intact PTH assays have been. Although there is considerable variation in reported sensitivity due to different kit antibodies and other technical factors, the best midregion kits are claimed to achieve 95% or greater sensitivity in detecting primary hyperparathyroidism. However, they are generally not as good in differentiating hypercalcemia due to PHPT from hypercalcemia of malignancy (the midregion PTH levels of 20%-25% of these cancer patients are normal or sometimes slightly increased rather than suppressed below reference range by the hypercalcemia). Intact PTH, on the other hand, generally is best at separating PHPT and hypercalcemia of malignancy (the PTH values of cancer patients are usually below intact PTH reference range or are in the lower part of the range, whereas the levels of PHPT patients are elevated or in the upper part of the range). The best intact assays are reported to detect PHPT almost as well as the better midmolecule assays. Intact PTH is also more reliable in patients with poor renal function. In azotemia, serum C-terminal and midregion fragments increase much more than intact PTH because of decreased excretion by the diseased kidneys.

In some cases, detection of abnormality can be assisted by correlating PTH values with serum calcium values. PTH values may be within the upper part of the reference range but may still be higher than expected for the degree of serum calcium elevation. A PTH/calcium nomogram should be constructed for each PTH antiserum.

Parathyroid hormone assay interpretation.

Among diseases associated with hypercalcemia, PTH values are elevated in PHPT, in most cases of ectopic PTH syndrome, and in most cases of tertiary hyperparathyroidism. The actual percentage of elevated results in each category varies with the particular antiserum used (e.g., 8%-73% of PHPT patient values have been reported to be within the reference range with different anti-sera). In metastatic carcinoma to bone, the PTH value is normal with the majority of antisera, but there is overlap with PHPT in a significant minority of patients with nearly all antisera (the exact percentage varying with the particular antiserum). Parathyroid hormone values are usually normal or decreased in other conditions producing hypercalcemia.

Parathyroid hormone values are elevated in many (but not all) conditions associated with true hypocalcemia (false hypocalcemia from hypoalbuminemia must be excluded). These include osteomalacia, vitamin D deficiency of dietary origin and in some patients with malabsorption, renal failure, and pseudohypoparathyroidism (congenital nonresponse of kidney to PTH). In PHPT, serum PTH and serum calcium levels are both increased.

There are additional factors in PTH assay interpretation. PTH has a diurnal variation, with the lowest values (trough, nadir) about noon (10 A.M.-2 P.M.) and the peak about 2 A.M. (midnight-4 A.M.). Specimens should be drawn when patients are fasting at about 7-8 A.M. without using specimen anticoagulants. Specimens should be processed at cold temperatures, frozen immediately, and transported in dry ice. Assay of PTH at present is difficult. Reliable antisera are not yet readily available from commercial sources, and, as noted previously, homemade antisera in reference laboratories differ in reactivity.

Problems and some solutions in PTH assay. Theoretically, any PTH assay should differentiate parathyroid tumor from various other etiologies of hypercalcemia, since PHPT should have increased PTH serum levels and hypercalcemia of all other etiologies should show decreased PTH secretion. Unfortunately, when tested with presently available antisera, some patients with PHPT may have PTH values within laboratory reference range (5%-73% in reports from different laboratories). In addition, some patients with hypercalcemia not due to PHPT may have values that are within the reference range rather than decreased. In most publications, results in hypercalcemia of malignancy fall within the reference range or below, but a few antisera permit some elevated values. In most reports there is substantial overlap between patients with PHPT and patients with hypercalcemia of malignancy when their values fall within the reference range, averaging about 10%-20% (literature range, 0%-73%). Diagnosis of parathyroid adenoma can be assisted by correlating PTH assay with serum calcium levels, based on the fact that the PTH level normally decreases as the serum calcium level increases. A parathyroid adenoma may produce a PTH level that is within population normal range but is higher than expected in relation to the degree of calcium elevation. A nomogram should be constructed for each PTH antiserum, correlating PTH values with serum calcium values obtained from patients with surgically proved PHPT. This nomogram also should provide data on PTH and calcium findings in other calcium-phosphorus disorders, such as metastatic carcinoma to bone, ectopic PTH syndrome, myeloma, and renal disease. The nomogram may permit separation of these conditions when it would be impossible with the numerical values alone. For example, in one report 45% of PHPT patients had PTH values within the PTH reference range; but with the use of the nomogram, 87% of the PHPT patients could be separated from normal persons. Therefore, with the majority of antisera, such a nomogram is almost essential when PTH values are interpreted, especially since results from different reference laboratories on the same patient specimens have shown considerable difference in behavior among different antisera when tested on patients with calcium disorders. These differences in results exist not only between C-terminal, midregion, and N-terminal categories of antisera but also between individual antisera within the same category. Use of a nomogram can reduce overlap between PHPT and malignancy to 5%-10% (literature range, 0%-15%).

If the serum albumin level is low, the calcium (total calcium) level will be falsely decreased and could alter patient position in the nomogram. Correction of the calcium-albumin relationship by formula may help but may not be accurate. Also, the diagram block areas appear to clearly separate different categories of calcium disease, but, in fact there may be overlap between patient values in certain disorders, and the amount of overlap is different with each individual antiserum. To choose the best laboratory for PTH assay, I strongly suggest that each laboratory under consideration be required to supply a nomogram for each type of PTH assay they perform showing actual values from patients with proven calcium diseases, including hypercalcemia of malignancy, plotted on the diagram in the form of individual symbols, the symbols (dots, circles, triangles) representing the different diseases. It is necessary to have a substantial number of patient results in each disease category, especially in both PHPT and malignancy, to obtain an accurate picture. This way, it is possible to obtain a more meaningful comparison of actual PTH test results in different laboratories. The best PTH assay is one that not only clearly separates different diseases from the reference range but also has the least overlap between disease categories, especially in the area between PHPT and hypercalcemia of malignancy.

If a patient has significant hypoalbuminemia, it may be better to ask for measurement of ionized calcium (which is not affected by the albumin level as total calcium is) and a PTH-calcium nomogram using ionized calcium and PTH values. The nomogram should have a scattergram of known PHPT and hypercalcemia of malignancy cases, not an empty block diagram only.

Lesser used or historically important tests

Tubular reabsorption of phosphate (phosphate reabsorption index). This procedure indirectly measures PTH by estimating PTH action on renal phosphate reabsorption. The patient should be on a normal phosphate (PO4) diet; a low-PO4 diet (<500 mg/day) raises tubular reabsorption of PO4 (TRP) normal values, whereas a high-PO4 diet (3,000 mg/day) lowers TRP normal values limits.

The patient drinks several glasses of water and then voids completely. One hour after voiding, a blood sample is obtained for phosphorus and creatinine measurement. Exactly 2 hours after beginning the test, the patient again voids completely, and the urine volume and urine concentration of creatinine and phosphate are determined. It is then possible to calculate the creatinine clearance rate and find the amount of phosphorus filtered per minute by the glomeruli. Comparing this with the actual amount of phosphate excreted per minute gives the amount reabsorbed by the tubules per minute, or the TRP value. A rough approximation is afforded by the formula:

%TRP = [1 – (UrinePO4 x serum creatinine /  Urine creatinine x serum PO4)]

An index value of less than 80% means diminished TRP value and suggests PHPT. This test becomes increasingly unreliable in the presence of renal insufficiency. About 5% of patients with renal stones but without parathyroid tumor have TRP values of 70%-80%, whereas about 20% of patients with parathyroid tumors have normal TRP values. Therefore, a TRP reduction is more significant than a normal result. Hypercalcemia due to malignancy is usually associated with a decreased TRP value. In addition, some patients with other conditions such as sarcoidosis and myeloma have been reported to have reduced TRP values.

X-ray findings. Bone changes highly suggestive of hyperparathyroidism may be found radiologically in about 15% of PHPT patients (literature range, 9%-36%), although the older literature reports some type of change in up to 46% of cases with skeletal surveys. The incidence of bone change has considerably decreased because of earlier diagnosis. The most typical findings are subperiosteal cortical bone resorption in the phalanges. Patients with chronic renal disease (secondary or tertiary hyperparathyroidism) may also demonstrate these abnormalities but do not have elevated serum calcium levels (except in tertiary hyperparathyroidism, in which case there should be obvious long-term renal failure). Serum alkaline phosphatase elevation in PHPT is highly correlated with the presence of bone changes. It would be unlikely to find skeletal changes in hand x-ray films if the serum alkaline phosphatase level is not elevated. Of course, the serum alkaline phosphatase level could be elevated for a variety of reasons in any individual patient with hypercalcemia.

Serum chloride. Primary hyperparathyroidism tends to develop a hyperchloremic acidosis. Serum chloride is often elevated in PHPT (40%-50% of cases, if one excludes patients with conditions that lower serum chloride levels such as vomiting, diarrhea, or diuretic use). Less than 10% of patients with non-PHPT etiologies of hypercalcemia have elevated serum chloride levels. In one series, these were all patients with thyrotoxicosis or the ectopic PTH syndrome. A chloride/phosphorus ratio has also been proposed. This was found to be greater than 33 in about 94% of PHPT patients (without renal failure). However, results in other hypercalcemias have been variable, with the percentage of patients reported with a ratio greater than 33 having ranged from 4%-39%.