Calcium with Creatinine Urine Test, 24-Hour
Calcium is the fifth most common element in the body. It is a fundamental element necessary to form electrical gradients across membranes, is an essential cofactor for many enzymes, and is the main constituent in bone. Under normal physiologic conditions, the concentration of calcium in serum and in cells is tightly controlled. Calcium exists in 3 states in the body; bound to protein, bound to small anions, and in the free (ionized) state. The concentration of serum calcium in the ionized state is regulated by parathyroid hormone (PTH) and 1,23 dihydroxy vitamin D.
Circulating calcium is excreted by glomerular filtration and reabsorbed in the proximal tubules. Calcium reabsorption in the proximal tubule is affected by tubular sodium concentration, whereas PTH induces calcium uptake in the distal tubule and the collecting duct. Excess is excreted in the urine and the feces.
Because PTH increases renal tubular reabsorption of calcium, one would expect patients with hyperparathyroidism to be hypocalciuric. However, high urinary calcium/creatinine ratios were found in most hyperparathyroid patients, and less frequently in patients with hypercalcemia due to other causes.
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