The accurate explanation
Her serum phosphorus is low before surgery. Parathyroid hormone (PTH) raises serum calcium through effects on bone, kidney, and vitamin D, and at the kidney it increases phosphate excretion. With excess hormone, calcium rises while phosphorus falls, so the two move in opposite directions. The accurate explanation is that parathyroid hormone increases phosphorus loss in the urine.
How parathyroid hormone works
PTH has three main targets. In bone, it stimulates resorption, releasing both calcium and phosphate into the blood. In the kidney, it increases calcium reabsorption and decreases phosphate reabsorption in the proximal tubule, so phosphate is lost in the urine. It also stimulates the kidney to activate vitamin D, which increases intestinal absorption of calcium and phosphate. The net effect is a high serum calcium and a low serum phosphorus, because the renal phosphate loss outweighs the phosphate released from bone.
| Target | Effect on calcium | Effect on phosphate |
|---|
| Bone | Released | Released |
| Kidney | Reabsorbed | Excreted in urine |
| Intestine (via vitamin D) | Absorbed | Absorbed |
| Net in hyperparathyroidism | High | Low |
Why the other explanations are wrong
Binding of phosphorus by excess calcium in the blood is not the mechanism; the low phosphorus results from a hormone effect on the kidney. Saying PTH blocks phosphorus release from bone is the reverse of the truth: PTH releases both calcium and phosphate from bone. Calcitonin from the thyroid lowers calcium, but it is not the cause of the low phosphorus in this client; the excess parathyroid hormone explains it.
Clinical relevance
The pattern of high calcium with low phosphorus is a laboratory signature of primary hyperparathyroidism. After successful surgery, PTH falls, and the opposite pattern can appear temporarily. Understanding this relationship helps the nurse anticipate laboratory changes and recognize postoperative hypocalcemia.
Exam takeaway
Key point PTH raises calcium and lowers phosphate by increasing renal phosphate excretion. Calcium and phosphorus move in opposite directions in parathyroid disorders.