The aim of this article is to discuss the diagnostic approach of an increased serum PTH concentration in a normocalcemic, normophosphatemic patient. Detection of this biological presentation is frequent in routine practice all the more that PTH reference values established in vitamin D replete subjects with a normal renal function are used by the clinical laboratories. The first step in this diagnostic approach will be to rule out a cause of secondary hyperparathyroidism (SHPT). Among these, the most frequent are vitamin D deficiency, very low calcium intake, impaired renal function, malabsorptions, drugs interfering with calcium/bone metabolism, such as lithium salts and antiresorptive osteoporosis therapies, hypercalciuria due to a renal calcium leak. If no cause of SHPT are evidenced, the diagnosis of normocalcemic primary hyperparathyroidism (PHPT) should be considered. A calcium load test is a very useful tool for this diagnosis if it shows that serum PTH is not sufficiently decreased when calcemia rises frankly above the upper normal limit. In a normocalcemic patient with hypercalciuria and a high serum PTH concentration, a thiazide challenge test may help to differentiate SHPT due to a renal calcium leak from normocalcemic PHPT. Beyond the discussion of this diagnostic flowchart, we also discuss some points about the merits and the difficulties of measuring and interpreting ionized calcemia and 24-h calciuria.
Keywords: Hormone parathyroïdienne; Hypercalciuria; Hypercalciurie; Hyperparathyroïdie primitive; Hyperparathyroïdie secondaire; Parathyroid hormone; Primary hyperparathyroidism; Secondary hyperparathyroidism; Vitamin D; Vitamine D.
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