Figure 4. Disordered mineral homeostasis in hyperparathyroidism. In primary sporadic hyperparathyroidism PTH is generally overproduced by a single parathyroid adenoma. Increased PTH secretion leads to a net increase in skeletal resorption with release of Ca+2 and Pi (inorganic phosphate) from bone. PTH also increases renal 1(OH)ase activity leading to increased production of 1,25(OH)2D3 from 25(OH)D3 and increased Ca+2 and Pi absorption from the small intestine. PTH also enhances renal Ca+2 reabsorption and inhibits Pi reabsorption resulting in increased urine Pi excretion. The net result is an increase in ECF calcium and a decrease in ECF phosphate.