Primary hyperparathyroidism (PHPT) and Hypoparathyroidism (HP) are two of the most frequent disorder of Calcium-Phosphorus (Ca-P) metabolism. The Ca/P ratio is an accurate tool to differentiate patients with PHPT from healthy subjects, according to a previous single-centre study. The reliability of this index is based on the fact that serum Ca and P are inversely related together either in healthy subjects or in patients with PHPT and HP.

The aim of this study is to investigate the accuracy and diagnostic value of Ca/P ratio in the diagnosis of primary hyperparathyroidism and hypoparathyroidism. The definition of a valid cut-off of serum Ca/P ratio for patients with these disorders will be of help especially in those patients with apparently normal biochemical profile, but suggestive for primary hyperparathyroidism and hypoparathyroidism.

Inclusion Criteria:

• patients with diagnosis of primary hyperparathyroidism
• patients with diagnosis of hypoparathyroidism
• subjects with normal Calcium-Phosphorus metabolism

Exclusion Criteria for both cases and controls will be:

• age younger than 18 or older than 90 years
• severe renal and liver diseases (i.e. glomerular filtration rate (GFR) <30 ml/min)
• hyperparathyroidism secondary to Vitamin D deficiency
• active metabolic bone disease (e.g. Paget's disease of the bone, osteomalacia, rickets, etc)
• any type of cancer
• malnutrition
• severe obesity (BMI > 40 kg/m2)
• a history of gastrointestinal malabsorption
• sarcoidosis
• hypercortisolism
• diabetes insipidus
• hyperthyroidism
• pseudohypoparathyroidism
• familial hypocalciuric hypercalcemia (FHH)
• treatment with steroids, active forms of vitamin D (calcitriol, ergocalciferol, etc), thiazides, phosphate binders, lithium, cinacalcet, bisphosphonates, and denosumab.

Serum Calcium to Phosphorus ratio will be compared among patients with different disorders of calcium-phosphorus metabolism (primary hyperparathyroidism and hypoparathyroidism) and controls.