The purpose of this study is to assess if subjects with hyperparathyroidism with normal serum calcium levels have different responses in the calcium regulating hormonal handling compared to a) patients with primary hyperparathyroidism and high serum calcium levels; b) healthy subjects.

The differences will be evaluated with oral peptones load and subsequent blood samples collected every 15 minutes for two hours. Ionized calcium, phosphate, gastrin and PTH levels will be evaluated.

Inclusion Criteria:

1. normocalcemic hyperparathyroidism

   • presence of high serum levels of intact PTH
   • serum ionized calcium levels in the high normality range

2. hypercalcemic hyperparathyroidism

   • presence of high serum levels of intact PTH
   • high serum ionized calcium levels

Exclusion Criteria:

1. potential causes of secondary hyperparathyroidism like renal insufficiency, liver disease, malabsorption, hypercalciuria, Paget's disease, thiazide diuretic or lithium use.
2. bone disease of any origin