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A point that is frequently tested in USMLE is about the two different actions of Vitamin D and Parathyroid hormone on phosphorus homeostasis.

PTH is a phosphaturic hormone which means it encourages the excretion of phosphate in the kidneys.

Conversely, Vitamin D3 stimulates phosphate reabsorption in the proximal tubules and so it's a phosphate conserving hormone.

The biggest prototypical condition that exemplifies these two effects is Nutritional Rickets in which the deficiency of dietary vitamin D leads to impaired phosphate reabsorption and in advanced stages the hypophosphatemia even goes worse as the secondary hyperparathyroidism further exacerbates the loss of phosphate in the urine.

Therefore, if you have low Calcium and high Phosphate levels then think of renal failure or hypoparathyroidism while if you have low Calcium and low Phosphate levels then think Vit. D deficiency.

A good grasp of this concept is important to answer many USMLE questions.
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