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