Around 90% of kidney stones contain calcium either as calcium oxalate or calcium phosphate. Nearly 80% of all kidney stones are composed of calcium and oxalate.
Some stone formers will have high levels of calcium in their urine (hypercalciuria). This can occur due to increased absorption of dietary calcium from the gut, loss of calcium from bones, or problems calcium regulation by the kidneys. Hypercalciuria can also be caused by dietary, metabolic and genetic factors. For example, high levels of parathyroid hormone (PTH) can lead to high calcium levels in the blood and/or urine.
Calcium oxalate stones can also form due to excess amounts of oxalate in the urine (hyperoxaluria). This is most commonly discovered in people with a history of inflammatory disease or surgery of the intestinal tract.
Rarely, calcium phosphate stones can result from a condition called renal tubular acidosis, where the kidney is unable to appropriately acidify the urine.
Uric acid is produced when the body breaks down protein. Humans have a level of uric acid that is 10 times higher than other mammals because they lack the enzyme which breaks down uric acid. Urine becomes saturated with uric acid when the it becomes more acidic (pH <5.5). People who consume high animal protein (meat) diets are more likely to form uric acid stones. Gout, chemotherapy, and disorders where there are high turnover of cells (e.g. myeloproliferative disorders) are also risk factors for these stones. Occasionally there is a genetic cause.
Struvite stones are composed of mixtures of magnesium, ammonium, phosphate and calcium carbonate phosphate crystals. These stones develop as a consequence of recurrent or chronic urinary tract infections. Some urinary bacteria can split the urea in urine to form ammonium and also to make urine less acidic. Some struvite stones can grow very large, filling the contours of the kidney - these are known as staghorn stones due to their shape.
Cystine stones account for only 1% of all kidney stones. They are due to a rare inherited genetic condition that results in high levels of cystine in the urine. These stones can occur in childhood.
Kidney stones can rarely occur as a result of medication.