PTH and osteoporosis: Several investigations have underlined the role of serum calcium variations in etiopathogenesis of osteoporosis and fracture occurrence; low blood calcium level activates PTH which, in turn, stimulates the production in the kidney of vitamin D that enhances the calcium uptake in the active sites of duodenum [15], while PTH decreases the urinary excretion of calcium and stimulates calcium resorption from bone [16].