A 55-year-old man is found incidentally to have hypercalcaemia during a routine health screen


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A 55-year-old man is found incidentally to have hypercalcaemia during a routine health screen.
Which one of the following biochemical findings would be most suggestive of this being caused by primary hyperparathyroidism rather than any other cause of hypercalcaemia?

A Elevated 24-hour urinary calcium excretion
B Elevated serum alkaline phosphatase activity
C Low serum concentration of calcitriol (1 ,25dihydroxycholecalciferol)
0 Normal or elevated serum phosphate concentration
Serum parathyroid hormone (PTH) concentration within the normal range

Explanation
Hypercalcaemia
Although parathyroid hormone (PTH) concentrations are often
increased in patients with hyperparathyroidism. they are not
always so PTH secretion should be suppressed by
hypercalcaemia from any other cause. so that a PTH value in
the normal range is consistent with the diagnosis.
• U1·inary calcium excretion is increased in many cases of
hypercalcaemia (except in familial hypocalciuric
hypercalcaemia). including hypercalcaemia due to
hyperparathyroidism. • Serum alkaline phosphatase activity can also be elevated with
hypercalcaemia, regardless of the cause (with the exception of
myeloma). Serum phosphate concentrations tend to be reduced in
hyperparathyroidism (PTH is phosphaturic) Calcitriol concentrations tend to be increased in
hyperparathyroidism (PTH stimulates the formation of this
hormone).