Diagnostic Studies

All patients with recurrent renal calculi and or severe osteopenia or osteoporosis should be screened for PHPT because of the benefits provided by surgical correction. Patients with the combination of an elevated serum ionized calcium and an elevated iPTH have hyperparathyroidism, even in the absence of elevated serum total calcium. An elevated serum iPTH in the absence of elevated ionized or total calcium does not confirm the diagnosis of normocalcemic hyperparathyroidism. The iPTH can be elevated in the absence of an elevated ionized or total calcium in the renal leak form of idiopathic hypercalciuria and vitamin D deficiency.3644 The iPTH may return to normal with treatment with a thiazide diuretic in the renal leak form of idiopathic hypercalciuria. Patients with vitamin D deficiency have a low 25-hydroxyvitamin D level, and their serum PTH cannot be corrected by vitamin D replacement. In patients in whom the combination of 3 consecutive days of concomitant serum ionized and total calcium and intact parathyroid hormone screening cannot provide a definitive diagnosis, the oral calcium loading study may be helpful.26 44

An elevated serum iPTH with no elevation in the ionized or total calcium is not uncommon. In a study of 178 patients with PHPT 27 patients (15%) had no elevation of the total or ionized calcium. The diagnosis in these patients was established by an oral calcium loading study showing the serum ionized calcium increasing to a supranormal value with only a minimal decrease in iPTH.26

An oral calcium loading study can be accomplished in an office setting. The patient is given an oral dose of 1000 mg of elemental calcium. A baseline serum iPTH is obtained prior to giving the oral calcium load, and subsequent serum iPTH values are obtained at 30, 60, and 120 minutes after giving the oral calcium load. Figure 45-4 illustrates the suppression of iPTH in 18 normal controls, which shows that all but 2 of these patients exhibited suppression to 70% or more of the baseline level of iPTH at 60 minutes after the oral calcium load. Figure 45-5 shows the results of the oral calcium loading study in 6 patients with recurrent renal calculi with normocalcemic or subtle hyperparathyroidism. Five of these 6 patients did not suppress to less than 70% of the baseline iPTH. These results and our continuing

IRMA PTH (Percent of Baseline)

140 T

20--

BASELINE 30 60 120

TIME (Minutes)

FIGURE 45-4. Percentage of change in intact parathyroid hormone values expressed as a percentage of the baseline values during the oral calcium loading test in 18 normal control subjects. IRMA PTH = immunoradiometric assay of parathyroid hormone. (From Monchik JM, Lamberton RP, Roth U. Role of the oral calcium loading test with measurement of intact parathyroid hormone in the diagnosis of symptomatic subtle primary hyperparathyroidism. Surgery 1992;112:1103.)

experience with the oral calcium-loading study emphasize that no single test can be expected to reliably identify all patients with normocalcemic hyperparathyroidism. Subsequent unpublished data from our institution have provided further confirmation that a completely normal suppression of iPTH can occur with oral calcium loading in patients with PHPT.

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