A 50-year-old man is seen in the office for new-onset constipation. Family history is unremarkable. Vital signs are within normal limits. Physical examination reveals normal findings. Serum laboratory results are as follows: Which of the following is the most likely mechanism causing this patient's low serum phosphate?

Question

A 50-year-old man is seen in the office for new-onset constipation.  Family history is unremarkable.  Vital signs are within normal limits.  Physical examination reveals normal findings.  Serum laboratory results are as follows:  Which of the following is the most likely mechanism causing this patient's low serum phosphate?

Quizplus · Accepted Answer

11ec160b_3cf7_3d4c_9249_fd4047186129_MD0001_00 This patient has mild hypercalcemia associated with hypophosphatemia, suggesting an excess of parathyroid hormone (PTH). PTH is released in response to hypocalcemia and acts to raise plasma calcium and lower plasma phosphorus by the following mechanisms: Primary hyperparathyroidism is characterized by over-secretion of PTH, despite normal (or elevated) calcium levels. Causes include parathyroid adenoma, multiglandular parathyroid hyperplasia, or (rarely) parathyroid carcinoma. Manifestations are largely related to hypercalcemia and include constipation, abdominal pain, kidney stones, fatigue, and bone pain. (Choice B) Hungry bone syndrome can occur after resection of a parathyroid adenoma in patients with primary hyperparathyroidism. The abrupt loss of resorptive stimulus can lead to rapid bone formation, resulting in hypophosphatemia and hypocalcemia. However, this patient has not yet undergone parathyroidectomy. (Choice C) Phosphate binders (eg, sevelamer) are synthetic polymers that bind intestinal phosphate and are subsequently eliminated in the feces. They are used to lower plasma phosphorus in patients with hyperphosphatemia due to chronic kidney disease. In patients not taking these medications, most phosphorus is eliminated in the urine and very little in the feces. (Choice D) Phosphate is filtered at the glomerulus and reabsorbed in the absence of PTH. Hyperparathyroidism results in decreased reabsorption in the proximal tubule; however, phosphate is not secreted. (Choice E) Reintroduction of carbohydrates in chronically malnourished patients (eg, anorexia nervosa) leads to a surge in insulin, which drives phosphorus intracellularly for use in cellular metabolism (ie, ATP production). This causes refeeding syndrome, characterized by severe hypophosphatemia, weakness, rhabdomyolysis, and arrhythmias. This patient's mild hypercalcemia and constipation are more suggestive of hyperparathyroidism. Educational objective: Primary hyperparathyroidism is characterized by oversecretion of parathyroid hormone despite normal (or elevated) serum calcium levels. Parathyroid hormone raises serum calcium and lowers serum phosphorus by increasing bone resorption (freeing calcium and phosphate), increasing renal reabsorption of calcium, and decreasing proximal tubular reabsorption of phosphate.

A 50-Year-Old Man Is Seen in the Office for New-Onset