Reliability of Serum Calcium to Phosphorus (Ca/P) Ratio as an Accurate and Inexpensive Tool to Define Disorders of Ca-P Metabolism

Inclusion Criteria: * patients with diagnosis of primary hyperparathyroidism * HIV-infected patients with reduced phosphorus but normal calcium * 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.