Other characteristic features of primary hyperparathyroidism which help distinguish it from other types of hypercalcaemia include: measurements of serum chloride (most patients have mild hyperchloraemic acidosis); serum phosphate (patients are hypophosphataemic due to renal phosphate wasting); urinary cyclic AMP (patients may have an increased urinary cyclic AMP due to the effects of PTH on the renal tubules); serum 1,25-dihydroxyvitamin D (often found to be normal or slightly elevated in patients); and urinary calcium excretion (calcium excretion is usually increased due to an increased filtered load of calcium, despite the effects of PTH to promote renal tubular calcium reabsorption) .
