How does excess PTH cause kidney damage?

Excess parathyroid hormone (PTH) can cause kidney damage through several mechanisms:

1. Hypercalcemia: Elevated PTH levels lead to increased calcium reabsorption from the bones and decreased calcium excretion by the kidneys, resulting in hypercalcemia. High calcium levels can directly damage the kidney tissue and impair renal function.

2. Renal vasoconstriction: PTH causes vasoconstriction of the renal blood vessels, reducing blood flow to the kidneys. This can lead to ischemia and damage to the kidney cells, affecting their ability to filter waste products and maintain electrolyte balance.

3. Increased phosphate excretion: Excess PTH promotes phosphate excretion by the kidneys, which can lead to hypophosphatemia. Low phosphate levels can interfere with various cellular processes and impair renal function.

4. Alterations in vitamin D metabolism: PTH is involved in the regulation of vitamin D metabolism. Excess PTH can disrupt this process, leading to increased production of 1,25-dihydroxyvitamin D (1,25(OH)2D3). High levels of 1,25(OH)2D3 can further increase calcium absorption from the gut and worsen hypercalcemia, contributing to kidney damage.

5. Fibrosis and calcification: Chronic PTH excess can lead to the development of renal fibrosis and calcification. Fibrosis involves the excessive deposition of scar tissue in the kidneys, while calcification refers to the accumulation of calcium deposits. These changes can disrupt the normal architecture of the kidneys and impair their function.

It's important to note that kidney damage caused by excess PTH is typically associated with long-standing hyperparathyroidism, such as in cases of primary hyperparathyroidism (where the parathyroid glands are overactive) or secondary hyperparathyroidism (which can occur in response to chronic kidney disease). Early detection and management of PTH imbalances are crucial to prevent or minimize kidney damage.