Serum Calcium to Phosphorous (Ca/P) Ratio in the Diagnosis of Ca-P Metabolism Disorders: a Multicentre Study

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.