Primary hyperparathyroidism is usually due to an adenoma of one of the parathyroid glands. Secondary hyperparathyroidism is caused by excessive secretion in response to low plasma calcium levels caused by renal insufficiency. The gland secretes excessive amounts of PTH, resulting in hypercalcemia. The adenoma
I. Hyperparathyroidism results in hypocalciuria. II. Hypercalcemia can be caused by vitamin D intoxication.
Extra info: Increased PTH secretion decreases the amounts of calcium being excreted via urine but enhances the amounts of phosphate secreted in urine. Toxicity of vitamin D from excessive administration or, rarely overproduction of active vitamin D (e.g. in sarcoidosis) causes hypercalcemia.