The kidneys play a significant role in maintaining the body’s overall health by filtering waste products and balancing various substances. Calcium, a mineral abundant in the body, is also crucial for many bodily functions beyond just strong bones. Its proper balance in the bloodstream is maintained to support nerve function, muscle contraction, and blood clotting.
Kidneys’ Role in Calcium Balance
Healthy kidneys maintain a stable level of calcium in the blood through filtration, reabsorption, and hormone signaling. As blood passes through the kidneys, calcium is filtered out, but most is reabsorbed to prevent loss in urine. This reabsorption process is tightly regulated to adjust calcium levels as needed.
The kidneys also perform the function of converting an inactive form of vitamin D into its active form, calcitriol. Calcitriol is necessary for the intestines to absorb calcium from food and maintains bone health. Without this activation step, the body struggles to get enough calcium from dietary sources.
The parathyroid glands, located in the neck, produce parathyroid hormone (PTH), which acts as a primary regulator of blood calcium levels. When blood calcium levels decrease, PTH is released, signaling the kidneys to reabsorb more calcium and to increase their production of active vitamin D. This active vitamin D further aids intestinal calcium absorption.
Conversely, when blood calcium levels are elevated, another hormone called calcitonin, produced by the thyroid gland, can help lower them. Calcitonin primarily acts to reduce calcium reabsorption by the kidneys and can also decrease its release from bones. This hormonal system ensures that calcium levels remain within a narrow, healthy range.
How Kidney Dysfunction Affects Calcium
When kidneys are not functioning properly, such as in chronic kidney disease (CKD), the balance of calcium and other minerals is disrupted, leading to a condition known as Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD). One of the initial problems is the kidneys’ reduced ability to convert inactive vitamin D into its active form. This deficiency in active vitamin D leads to less calcium being absorbed from the diet, resulting in lower blood calcium.
Another issue in kidney dysfunction is the impaired excretion of phosphorus. Healthy kidneys remove excess phosphorus from the blood, but in CKD, phosphorus builds up, leading to hyperphosphatemia, or high phosphorus. This elevated phosphorus further contributes to calcium imbalances by directly affecting calcium levels and stimulating the parathyroid glands.
The combination of low blood calcium and high phosphorus triggers the parathyroid glands to produce excessive amounts of parathyroid hormone, a condition called secondary hyperparathyroidism. The body releases more PTH to raise blood calcium by drawing it from bones, weakening them over time. This continuous overproduction of PTH can cause damage to the skeletal system, leading to bone disorders known as renal osteodystrophy.
Renal osteodystrophy can manifest as weak and brittle bones, increasing the risk of fractures and bone pain. Beyond bone health, these mineral imbalances can also contribute to vascular calcification, where calcium deposits accumulate in blood vessels. This calcification can stiffen arteries, increasing the risk of cardiovascular problems, a common complication in individuals with advanced kidney disease.
Managing Calcium Imbalances in Kidney Disease
Managing calcium imbalances in kidney disease involves a multifaceted approach to restore the balance of minerals and hormones. Dietary modifications are often a first step, focusing on phosphorus restriction. Limiting foods high in phosphorus, such as dairy products, nuts, and processed foods, helps reduce the mineral load that the impaired kidneys must handle.
Medications play a role in managing these imbalances. Phosphate binders are taken with meals to bind to phosphorus in the digestive tract, preventing its absorption into the bloodstream. This helps to lower blood phosphorus levels, which in turn can reduce the stimulus for PTH release.
Active vitamin D analogs are prescribed to replace the active vitamin D that the kidneys can no longer produce adequately. These medications improve calcium absorption and directly suppress the overproduction of PTH by the parathyroid glands. Administering these analogs helps to normalize calcium levels and reduce the strain on the bones.
Calcimimetics are another class of medications that work by making the parathyroid glands more sensitive to calcium. This increased sensitivity means the glands produce less PTH, even with slightly low calcium levels, thus helping to control secondary hyperparathyroidism. The goal of these medical interventions is to maintain healthy levels of calcium, phosphorus, and PTH to prevent bone disease and vascular calcification.
For individuals with severe, uncontrolled secondary hyperparathyroidism, surgical intervention may be considered. A parathyroidectomy, the surgical removal of overactive parathyroid glands, can significantly reduce PTH levels. Additionally, dialysis, a treatment that filters waste products and excess fluids from the blood when kidneys fail, also helps remove excess phosphorus and manage mineral imbalances.