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 is not sensitive to the negative feedback of the high calcium levels: the parathyroid gland keeps secreting PTH.
These high levels cause hypercalcemia, which results in muscle and bone weakness, depression, kidney stones and gastrointestinal complaints. Surgical removal of the tumor brings the calcium levels to normal values. Even one of the four parathyroid glands can maintain normal calcium levels.
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.