Differentiation of hypercalcaemia due to malignancy from primary hyperparathyroidism: the value of parathyroid hormone and plasma bicarbonate measurements

The value to the surgeon of parathyroid hormone assays in primary hyperparathyroidism

The role of various parathyroid hormone (PTH) radio-immunoassays in the diagnosis of primary hyperparathyroidism (PHP) is controversial. A series of 204 patients with surgically proven PHP was studied. Serum total calcium, serum ionized calcium, amino (N)-terminal PTH and carboxyl(C)-terminal PTH were assessed in relation to the volume and weight of adenomatous or hyperplastic parathyroid tissue excised at operation. N-terminal PTH was elevated above the normal laboratory range in only 24% of patients and correlated relatively poorly with the volume of abnormal parathyroid tissue (r = 0.20, P = 0.05). C-terminal PTH was elevated above the normal range in 91% of patients and had a strong correlation with the volume of abnormal parathyroid tissue (r = 0.63, P less than 0.001). The correlation coefficients between C-terminal PTH and serum total calcium and serum ionized calcium were both 0.63 (P less than 0.001). In contrast, there was no correlation between N-terminal PTH and serum total calcium (r = -0.02), serum ionized calcium (r = -0.04) or C-terminal PTH (r = 0.09). A combination of hypercalcaemia and elevated C-terminal PTH can be regarded as strong diagnostic evidence of PHP. Furthermore, the level of C-terminal PTH can assist the surgeon by approximately predicting the amount of adenomatous or hyperplastic parathyroid tissue that may be expected at surgical exploration.

Differential laboratory diagnosis of hypercalcemia

It is proposed that this review will adopt the following format: establishment of hypercalcemia. This demands a discussion of the problem of normal ranges, the usage of either total calcium or ionized calcium in making this decision and where total calcium is used whether adjustment of this value for serum protein concentration should be used and if so, the formulae which have been cited to perform this. Having established hypercalcemia why is it necessary to differentiate this? This will involve reviewing those clinical situations in which differentiation of hypercalcemia has been attempted and will include an attempt to produce an up to date indication of conditions in which hypercalcemia has been described. When hypercalcemia has been established the laboratory tests which have been further used to discriminate will be divided into single tests such as N- or C- terminal parathormone, 1,25- dihydroxycholecalciferol, cyclic AMP; the combination tests which have been used including phosphate clearance, chloride vs. bicarbonate etc. proceeding to those groups which have used discriminant function to help in the decision making; dynamic testing will also be discussed particularly with reference to steroid suppression but will also include other known suppressants such as Mithramycin and Calcitonin. A final section will be included attempting to assess overall the present state of art in differentiating laboratory diagnosis of hypercalcemia and will also attempt to highlight those areas which appear to be most fruitful areas of progress in the future.

Importance of dietary sodium in the hypercalciuria syndrome

Daily urinary calcium excretion in renal stone-forming subjects is shown to vary directly with moderate changes in dietary sodium intake. The changes produced are sufficient to alter the basic diagnostic classification from 'hypercalciuric' to 'normocalciuric' because dietary sodium is reduced from 200 to 80 mM/day. Similar changes were observed in fasting morning 'spot' urine samples, resulting in alteration of diagnostic subclassification between so-called 'absorptive' and 'renal' categories, in the absence of demonstrable change in parathyroid function. Diagnostic and therapeutic studies in stone-forming subjects require control of both dietary calcium and dietary sodium if misinterpretations are to be avoided. Habitual high sodium intake may be an etiological factor in the generation of excessive excretion of calcium, sodium, and phosphate--the hypercalciuria syndrome.