Parathyroid hormone, vitamin D, osteoporosis, and Paget disease of bone mechanisms and lab findings tested on USMLE Step 1.
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- Which cells secrete parathyroid hormone (PTH)?
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Chief cells of the parathyroid glands- What stimulus triggers PTH secretion?
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A drop in serum ionized calcium, sensed by the calcium-sensing receptor- How does PTH increase bone resorption?
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It upregulates RANKL on osteoblasts, which activates osteoclasts- Where does PTH increase renal calcium reabsorption?
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The distal tubule- Where does PTH decrease renal phosphate reabsorption?
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The proximal tubule (a phosphaturic effect)- Which renal enzyme does PTH stimulate to activate vitamin D?
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1-alpha-hydroxylase- What is the net effect of PTH on serum calcium and phosphate?
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Increases serum calcium, decreases serum phosphate- Vitamin D from the skin or diet is first hydroxylated in the ____ to form 25-hydroxyvitamin D.
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liver- 25-hydroxyvitamin D is converted to its active form in the ____.
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kidney- What is the active form of vitamin D called?
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Calcitriol (1,25-dihydroxyvitamin D)- What is the effect of calcitriol on intestinal absorption?
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It increases absorption of both calcium and phosphate- What is the most common cause of primary hyperparathyroidism?
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A single parathyroid adenoma- What are the typical serum calcium and phosphate levels in primary hyperparathyroidism?
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High calcium, low phosphate- What is the typical PTH level in primary hyperparathyroidism?
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High (elevated)- What drives secondary hyperparathyroidism in chronic kidney disease?
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Reduced renal activation of vitamin D causes hypocalcemia, which stimulates compensatory PTH release
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