Step by Step
PTH
Parathyroid hormone — raises blood calcium
Released from the chief cells of the four parathyroid glands when blood calcium falls. PTH stimulates bone resorption (via osteoclast activation), increases kidney calcium reabsorption (in the DCT), and stimulates the kidney to activate vitamin D — all working together to raise blood calcium.
CT
Calcitonin — lowers blood calcium
Released from thyroid C cells when blood calcium rises. Calcitonin inhibits osteoclasts, lowering blood calcium — though this effect is relatively minor in adults, playing a more significant role in children.
VitD
Vitamin D (calcitriol) — absorbs calcium from the gut
Vitamin D synthesis begins in the skin (via UV exposure), continues in the liver (forming 25-OH vitamin D), and is completed in the kidney (forming active 1,25-OH calcitriol, a process stimulated by PTH). Calcitriol's most important effect is stimulating calcium absorption from the gut.
Dis
Disorders of calcium regulation
Hypoparathyroidism (decreased PTH) causes decreased blood calcium, leading to tetany, seizures, and positive Chvostek/Trousseau signs. Hyperparathyroidism (increased PTH) causes increased blood calcium, leading to kidney stones, bone pain, and constipation.
Applied Walkthrough
1
When blood calcium drops, the parathyroid glands release PTH, which stimulates bone resorption, increases kidney calcium reabsorption, and activates vitamin D in the kidney — working through all three target organs (bone, kidney, and indirectly the gut) to restore normal calcium levels.
2
The activated vitamin D (calcitriol) then travels to the gut, where it stimulates calcium absorption from food — its single most important effect.
3
If blood calcium instead rises too high, thyroid C cells release calcitonin, inhibiting osteoclasts and modestly lowering blood calcium — though this effect matters much more in children than in adults.
4
A patient with hyperparathyroidism (excess PTH) develops chronically elevated blood calcium, leading to kidney stones, bone pain from ongoing bone resorption, and constipation — a cluster of symptoms directly traceable to PTH's actions on its three target organs taken to an excessive degree.
Exam Application
Exams test whether you can match PTH, calcitonin, and vitamin D to their effects on blood calcium (raise, lower, and absorb from the gut respectively), and whether you can connect hypoparathyroidism and hyperparathyroidism to their opposite symptom patterns.
⚠ Common Trap
The most common trap is underestimating calcitonin's role compared to PTH — calcitonin has only a minor calcium-lowering effect in adults, while PTH is the dominant, more powerful regulator working through three separate target organs.
✓ Quick Self-Check
1. What is the normal blood calcium range?
8.5–10.5 mg/dL.
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2. What does PTH do to blood calcium, and through which three organs does it act?
Raises it, acting on bone (resorption), kidney (reabsorption and vitamin D activation), and indirectly the gut (via activated vitamin D).
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3. What does calcitonin do, and how significant is its effect in adults?
Lowers blood calcium by inhibiting osteoclasts; relatively minor in adults, more significant in children.
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4. What is vitamin D's most important effect regarding calcium?
Stimulating calcium absorption from the gut.
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5. What are the symptoms of hypoparathyroidism, and why do they occur?
Tetany, seizures, and positive Chvostek/Trousseau signs, due to decreased blood calcium.
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