Kidney stones can cause a great deal of pain and may require surgical removal. A single stone the size of a grain of sand can cause such excruciating pain already. There are several factors which could facilitate the formation of kidney stones and hyperparathyroidism is one of them. But why is a hormone concerned mainly with bones involved in the formation of kidney stones? First, we must explore the functions of the parathyroid hormone itself.
The Parathyroid Hormone
The parathyroid hormone increases calcium and phosphate absorption from the bone. It does this by utilizing two mechanisms. One is a rapid phase in which the osteocytes are activated to pump calcium and phosphate ions from the bone fluid into the extracellular fluid. Another requires the activation of cells called osteoclasts. Unlike the osteocytes, parathyroid hormone does not directly exert its influence on the osteoclasts. Instead a signal from the activated osteocytes will further activate the osteoclasts to increase bone resorption activity.
Furthermore, parathyroid hormone also increases renal tubular reabsorption of calcium while it diminishes phosphate reabsorption. It also increases the rate of reabsorption of magnesium and hydrogen ions while it decreases the reabsorption of sodium, potassium, and amino acid ions in almost the same way it affects phosphate. Calcium reabsorption is increased mainly in the late distal tubules, the collecting tubules, the early collecting ducts, and even the ascending loop of Henle to a certain extent.
This mechanism maintains the amount of calcium in the body. If it were not for this effect of the parathyroid hormone on the kidneys, there would be constant loss of calcium through the urine. In time, this would deplete both the extracellular fluids and the bone of such a mineral.
Formation of Kidney Stones in Hyperparathyroidism
Some patients with mild hyperparathyroidism may show signs of bone disease but most of hyperparathyroid patients have the tendency to form kidney stones. This may be due to the fact that the excess calcium and phosphate absorbed by the intestines or mobilized from the bones under the influence of parathyroid hormone must be excreted by way of the kidneys, causing an increase in the concentration of these substances in the urine. Because of this, calcium phosphate crystals tend to form in the kidney, forming calcium phosphate stones. Calcium oxalate stones may also develop because even normal levels of oxalate will cause calcium precipitation at high calcium levels.
Most renal stones are also only slightly soluble in alkaline media. This is the reason why there is a greater tendency for renal stones to form in alkaline urine than in acidic urine. This is the logic behind the acidotic diets and acidic drugs frequently used to treat renal calculi or kidney stones.