Secondary hyperparathyroidism is caused by any condition causing a chronic depression in the serum calcium level. Low serum calcium levels lead to conpensatory overactivity of the parathyroid glands. Renal failure is the most common cause of secondary hyperparathyroidism. Nutritional causes such as inadequate calcium intake/absorption or excess phosphorus intake are the other main cause of secondary hyperparathyroidism. Chronic renal insufficiency leads to decreased phosphate excretion and hyperphosphatemia which directly lowers serum calcium levels by the mass law Ca x P = K. Chronic renal insufficiency also leads to decreased levels of the active form of Vitamin D3 which leads to decreased Ca absorption from the gut and hypocalcemia which stimulates the parathyroid gland. ( Robbins) Parathyroid hormone (PTH) activates osteoclasts, thereby mobilizing calcium from bone, increases renal tubular reabsorption of calcium, thereby conserving free calcium, increases urinary phosphate excretion, thereby lowering serum phosphate level, and increases conversion of vitamin D to its active dihydroxy form in the kidneys, thereby augmenting GI calcium absorption. Bone resorptive disease is a significant sequelae to longterm hyperparathyroidism.