Abstract
A relationship between serum concentrations of ionized calcium and parathyroid hormone (PTH) in persons without parathyroid overactivity was defined by performing an oral calcium load test. As a result, a serum PTH concentration greater than 2·6 pmol/L in a hypercalcaemic patient was regarded as suggestive of hyperparathyroidism. Fasting serum PTH concentrations in 58 patients with surgically and histologically proven primary hyperparathyroidism (PHPT) were all above 2·7 pmol/L (range 3·2–84·5). Thirteen of 20 patients with familial benign hypercalcaemia (FBH) had fasting serum PTH concentrations greater than 2·6 pmol/L (range 1·6–6·1). There was a significant correlation between serum PTH and age in the FBH patients only. Fasting urine calcium excretion (CaE) ranged from 14 to 222 μmol/L of glomerular filtrate in PHPT and 3–34 μmol/L of glomerular filtrate in FBH. The best biochemical discriminant between patients with PHPT and FBH was a plot of fasting serum PTH against CaE. A plot of post-calcium load PTH against post-load CaE showed no further significant advantage in discriminating between the two conditions.