1️⃣ Big Picture: Who Controls Calcium?

Core idea:

Body must keep extracellular Ca²⁺ and phosphate in a very tight range → critical for nerve, muscle, coagulation, bone, and signaling.

Three main hormones (must-remember trio)

1. PTH (Parathyroid hormone)
• From: Parathyroid glands
• Main actions:
• Mobilizes Ca²⁺ from bone
• Increases renal Ca²⁺ reabsorption
• Increases renal phosphate excretion (phosphaturic)
• So: PTH ↑Ca²⁺, ↓Pi
2. 1,25-dihydroxycholecalciferol (Active Vitamin D / calcitriol)
• From: Vitamin D → liver → kidney hydroxylations
• Main action:
• Increases Ca²⁺ absorption from gut
• Also increases phosphate absorption from gut
• So: Vit D ↑Ca²⁺ AND ↑Pi (mainly via gut)
3. Calcitonin
• From: C cells of thyroid
• Main action:
• Inhibits bone resorption → ↓Ca²⁺ (and ↓Pi)
• Role: Small in adults, more of a “fine-tuning/emergency brake”.

👉 Exam line:

“PTH, calcitriol, and calcitonin act together to maintain stable Ca²⁺ and phosphate levels; PTH and calcitriol are dominant, calcitonin is minor.”

2️⃣ Calcium – What Matters Clinically?

Total body & distribution

• Total body Ca: ≈1100 g
• 99% in bone, 1% in ECF + cells.
• Plasma total Ca ≈ 10 mg/dL (2.5 mmol/L):
• Part protein-bound
• Part diffusible, of which:
• Ionized Ca²⁺ (active)
• Ca²⁺ in small complexes

The star: Ionized Ca²⁺

This is what actually matters for:

• Nerve & muscle excitability
• Blood coagulation
• Signal transduction (second messenger)

⬇ ↓ Extracellular Ca²⁺ = ↑ nerve/muscle excitability

→ Hypocalcemic tetany

• Carpopedal spasms
• Laryngospasm → risk of asphyxia
• Extremely important emergency concept

Protein binding & pH (VERY exam-heavy)

• Ca²⁺ binding to protein ∝ plasma protein concentration.

Alkalosis (↑ pH):

• Plasma proteins become more negatively charged

→ bind more Ca²⁺

→ ionized Ca²⁺ drops, even if total Ca normal

→ tetany symptoms can appear.

• Classic: hyperventilation → respiratory alkalosis → tetany at “normal” Ca.

👉 So you always interpret total Ca with albumin level and pH in mind.

3️⃣ Bone Calcium – Two Pools, Two Systems

Bone Ca²⁺ exists in:

1. Readily exchangeable pool (small, dynamic)
• Rapid exchange with plasma
• About 500 mmol/day moving in and out
• Controlled mainly by PTH and calcitriol to stabilize plasma Ca²⁺
2. Stable structural pool (massive)
• Slowly exchangeable
• Bone remodeling (osteoblasts vs osteoclasts)
• Only ~7.5 mmol/day exchange
• Important for long-term bone health, not minute-to-minute Ca²⁺ control.

👉 High-yield distinction:

Fast system = exchangeable pool (for plasma Ca²⁺ stability).

Slow system = remodeling (for bone strength & growth).

4️⃣ Intestinal Calcium Handling – TRPV6, Calbindin, Vit D

Key steps in enterocyte:

1. Entry at apical (luminal) side
• Through TRPV6 Ca²⁺ channel.
2. Inside the cell
• Ca²⁺ immediately bound by calbindin-D₉k:
• Prevents it from disturbing intracellular signaling.
• Shuttles Ca²⁺ toward basolateral side.
3. Exit at basolateral side
• Either by:
• Na⁺/Ca²⁺ exchanger (NCX1)
• Ca²⁺-ATPase (active pump)
4. Regulation
• 1,25-dihydroxycholecalciferol (Vit D):
• ↑ TRPV6
• ↑ calbindin-D₉k
• ↑ transport machinery → ↑ Ca²⁺ absorption
• As Ca²⁺ rises → Vit D activation falls (negative feedback).

👉 Even if TRPV6 & calbindin-D₉k are absent, other minor pathways still absorb some Ca²⁺, but they are not the main regulated route.

5️⃣ Renal Calcium Handling – Where PTH Acts

• Filtered Ca²⁺: almost all is reabsorbed (98–99%).
• Sites:
• ≈60% in proximal tubule
• Rest in loop of Henle (ascending limb) + distal tubule
• Distal tubule:
• Uses TRPV5 (similar to TRPV6).
• TRPV5 expression is upregulated by PTH.

👉 So PTH:

• Directly increases distal Ca²⁺ reabsorption.
• Helps prevent hypocalcemia.

6️⃣ Phosphate – Parallel but Linked System

Distribution

• Total body phosphorus: 500–800 g
• 85–90% in bone (as hydroxyapatite).
• Plasma phosphorus ≈ 12 mg/dL total:
• ~⅔ in organic form (ATP, proteins, etc.).
• Remaining = inorganic phosphate (Pi): PO₄³⁻ / HPO₄²⁻ / H₂PO₄⁻.

Roles of Pi (high yield list)

• Component of:
• ATP, cAMP, 2,3-DPG, phosphoproteins
• Critical for:
• Energy transfer
• Signal transduction
• Buffering (acid–base)
• Structural in bone

7️⃣ Kidney & Intestinal Phosphate Transport – Key Transporters

Renal Pi handling

• Plasma Pi is filtered at glomerulus.
• 85–90% is reabsorbed, mainly in proximal tubule.
• Main transporters: NaPi-IIa and NaPi-IIc (Na⁺-dependent cotransporters).
• PTH powerfully inhibits NaPi-IIa:
• Causes internalization + degradation of transporter.
• Result: ↓ Pi reabsorption → ↑ Pi excretion (phosphaturia).

👉 Again:

PTH = “phosphate-wasting” hormone → prevents Ca–Pi product from getting too high and causing soft tissue calcification.

Intestinal Pi absorption

• Via NaPi-IIb transporter in duodenum and small intestine:
• Secondary active transport, driven by Na⁺ gradient (Na⁺/K⁺ ATPase).
• Exact basolateral exit pathway is unknown, but:
• 1,25-dihydroxycholecalciferol ↑ NaPi-IIb expression and/or insertion.
• So Vit D increases BOTH Ca²⁺ and Pi absorption.

🧠 Calcium–Phosphate Regulation: Complete High-Yield Table (Zero Omission)

VITAMIN D & THE HYDROXYCHOLECALCIFEROLS

✔️ 1. The Vitamin D Pathway (Absolute Core)

This is the single most high-yield flowchart in the whole topic:

SKIN → LIVER → KIDNEY → ACTIVE HORMONE

1. Skin (UVB sunlight)
• 7-dehydrocholesterol → Pre-vitamin D₃ → Vitamin D₃ (cholecalciferol)
• Also obtained from diet.
2. Liver
• Vitamin D₃ → 25-hydroxycholecalciferol (25-OH D₃ / calcidiol)
• Major circulating storage form
• Plasma level ≈ 30 ng/mL
• NOT tightly regulated (just reflects supply).
3. Kidney (proximal tubule)
• 25-OH D₃ → 1,25-dihydroxycholecalciferol (calcitriol / 1,25-(OH)₂D₃)
• Active form
• Level ≈ 0.03 ng/mL (100 pmol/L)
4. Also produced in:
• Placenta
• Keratinocytes
• Macrophages

👉 Exam line:

25-OH D₃ = storage; 1,25-OH₂ D₃ = active; kidney 1-α hydroxylase is rate-limiting.

✔️ 2. Regulation of Active Vitamin D (CRUCIAL)

What increases 1-α-hydroxylase → ↑ active Vit D (1,25-D)?

• Low plasma Ca²⁺
• High PTH (most important stimulator)
• Low phosphate (Pi)

What decreases 1-α-hydroxylase → ↓ active Vit D?

• High Ca²⁺
• High phosphate
• Direct negative feedback from 1,25-OH₂ D₃
• FGF23
• α-Klotho deficiency → ↑ 1,25-D → hypercalcemia / hyperphosphatemia / calcifications

FGF23 + α-Klotho = phosphate-reducing, vitamin D–reducing duo

👉 Exam line:

PTH ↑ active Vit D; phosphate ↓ active Vit D.

✔️ 3. Actions of 1,25-dihydroxycholecalciferol (The Triple Effect)

⭐ 1. INTESTINE — MAIN ACTION (most examined)

↑ Ca²⁺ absorption via:

• ↑ TRPV6 channels
• ↑ calbindin-D₉k, D₂₈k
• ↑ Ca-ATPase + NCX1

↑ Phosphate absorption via:

• ↑ NaPi-IIb

👉 High-yield:

Both Ca²⁺ AND phosphate absorption ↑ → provides minerals for bone.

⭐ 2. KIDNEY

• ↑ Ca²⁺ reabsorption (via ↑ TRPV5)
• Helps maintain Ca²⁺ during hypocalcemia

⭐ 3. BONE

• Stimulates osteoblasts → which then ↑ osteoclast activity (indirect)
• Necessary for normal matrix mineralization

👉 Clinically:

Vitamin D deficiency → osteoid forms normally but does not mineralize → rickets/osteomalacia.

✔️ 4. Clinical Core: Rickets & Osteomalacia

Cause

• Vitamin D deficiency
• Lack of sun
• Liver disease → ↓ 25-OH D
• Kidney disease → ↓ 1-OH D
• 1-α hydroxylase mutations → Type I Vit D–resistant rickets
• Receptor mutations → Type II Vit D–resistant rickets

Problem

• Failure of Ca²⁺ + phosphate delivery to mineralization sites
• Bone matrix formed but not calcified

Children: Rickets

• Bow legs
• Weak bones
• Hypocalcemia
• Dental defects

Adults: Osteomalacia

• Bone pain
• Fragile bones
• Often subtle (insidious)

Response to treatment

• Liver/kidney intact → respond to Vitamin D
• Kidney 1-α hydroxylase defects → respond ONLY to 1,25-OH₂ D₃
• Vitamin D receptor defects → respond to neither

👉 Exam trap:

“Type II vitamin D–resistant rickets = VDR mutation = NO RESPONSE to 1,25-D.”

✔️ 5. Binding & Transport

• Vitamin D and metabolites are carried by DBP (vitamin D–binding protein).

Small but easy marks..

🧾 Vitamin D & Hydroxycholecalciferols — Complete Integrated Table

🧾 Regulation of Renal 1-α-Hydroxylase (ACTIVE Vit D Control)

Exam lock:

PTH ↑ active Vit D; phosphate ↓ active Vit D.

FGF23 + α-Klotho = phosphate-reducing + vitamin-D-reducing duo.

🧾 Actions of 1,25-Dihydroxycholecalciferol (Calcitriol)

🧾 Clinical Correlation Table: Rickets & Osteomalacia

🧾 Age-Specific Manifestations

🧠 Final Exam One-Liners (Lock These)

• 25-OH D₃ = storage form
• 1,25-(OH)₂D₃ = active hormone
• Kidney 1-α-hydroxylase = rate-limiting
• Type II Vit D–resistant rickets = VDR mutation = NO response to 1,25-D

THE PARATHYROID GLANDS

1️⃣ Anatomy & Cell Types (Short but high yield)

• Number & location
• Usually 4 parathyroid glands:
• 2 at superior poles of thyroid
• 2 at inferior poles
• Cell types
• Chief cells
• Small, abundant
• Have Golgi + rER + secretory granules
• Main function: synthesize and secrete PTH
• Oxyphil cells
• Larger, many mitochondria
• Few before puberty, ↑ with age
• Function unclear (exam: “unknown” is ok)

👉 Key memory: Chief cells = PTH factory.

2️⃣ PTH Synthesis, Structure & Clearance (Mechanism logic)

• PTH = 84–amino acid peptide, MW ≈ 9500.
• Made as:
1. PreproPTH (115 aa) in rER
2. Leader removed → proPTH (90 aa)
3. Further processing in Golgi → PTH (84 aa) → stored in granules → secreted
• Plasma:
• Normal intact PTH = 10–55 pg/mL
• Half-life ≈ 10 min (short)
• Metabolism:
• Rapidly cleaved to fragments in liver (Kupffer cells)
• Cleared by kidney
• Modern assays detect intact 84-aa PTH (biologically active form).

👉 Take-home: Short half-life + sensitive regulation = “fast responder” hormone for Ca²⁺.

3️⃣ Core Actions of PTH (This is exam GOLD)

Think: PTH = “Raise Ca, Waste Phosphate, Activate Vit D.”

1. Bone

• Directly increases bone resorption → Ca²⁺ released.
• Long term: stimulates both osteoblasts and osteoclasts (via osteoblast-mediated signaling).

2. Kidney

• Distal tubule:
• ↑ Ca²⁺ reabsorption (via TRPV5 etc.)
• Proximal tubule:
• ↓ phosphate reabsorption by inhibiting NaPi-IIa → phosphaturia → ↓ plasma phosphate.

3. Vitamin D

• ↑ 1α-hydroxylase in kidney → ↑ 1,25-dihydroxycholecalciferol (active Vit D)
• → ↑ intestinal Ca²⁺ absorption (+ phosphate).

👉 Summary line:

PTH ↑ Ca²⁺ (bone + kidney + Vit D), ↓ phosphate (kidney).

4️⃣ PTH Receptors & Pseudohypoparathyroidism (Pathophys favourite)

Receptors

• hPTH/PTHrP receptor (PTH1 receptor)
• Binds PTH + PTHrP
• Coupled to:
• Gs → ↑ cAMP
• Gq → ↑ PLC → ↑ IP₃/DAG → ↑ Ca²⁺, PKC
• PTH2 receptor
• Does NOT bind PTHrP
• Found in brain, placenta, pancreas
• CPTH receptor (proposed)
• Binds C-terminal of PTH

Pseudohypoparathyroidism

• Clinical picture: hypocalcemia, hyperphosphatemia, NORMAL or HIGH PTH.
• Problem = tissue resistance, not hormone deficiency.
• Types:
1. Common form:
• Congenital 50% reduction of Gs activity
• PTH fails to ↑ cAMP → poor response.
2. Variant form:
• cAMP response normal
• But phosphaturic effect is defective.

👉 Key exam idea:

Low Ca + high PTH = resistance (pseudohypoparathyroidism), not absence.

5️⃣ Regulation of PTH Secretion (Core physiology)

Main regulator: Ionized Ca²⁺

• CaSR (Calcium-sensing receptor) on chief cells:
• GPCR that responds to extracellular Ca²⁺.
• High Ca²⁺ → CaSR activated → inhibits PTH release.
• Low Ca²⁺ → less CaSR signalling → ↑ PTH release.

So:

• High Ca²⁺ → ↓ PTH → deposit Ca²⁺ in bone.
• Low Ca²⁺ → ↑ PTH → mobilize Ca²⁺ from bone, preserve Ca²⁺ in kidney, ↑ Vit D.

Other regulators

• 1,25-dihydroxycholecalciferol (active Vit D):
• ↓ preproPTH mRNA in parathyroid → ↓ PTH synthesis.
• Phosphate
• High plasma phosphate:
• ↓ free Ca²⁺ (complexing)
• ↓ formation of 1,25-D

→ ↑ PTH

• Low phosphate:
• Opposite → ↓ PTH.
• Magnesium
• Normal Mg²⁺ needed for PTH release.
• Severe Mg²⁺ deficiency:
• Impaired PTH release
• Impaired PTH action

→ Hypocalcemia that doesn’t correct until Mg²⁺ is fixed.

👉 Exam pearl: Hypocalcemia + low/normal PTH + low Mg²⁺ = Mg deficiency causing “functional hypoparathyroidism.”

6️⃣ PTHrP & Hypercalcemia of Malignancy (Clinical must-know)

PTHrP basics

• Parathyroid hormone–related protein (PTHrP)
• 140 amino acids, gene on chromosome 12.
• Shares N-terminal homology with PTH → can bind PTH1 receptor.
• Mainly acts as paracrine factor in local tissues:
• Cartilage (endochondral bone development)
• Brain (protects neurons)
• Placenta (Ca²⁺ transport)
• Skin, smooth muscle, teeth (eruption dependent on PTHrP).

Hypercalcemia of malignancy

• Rough breakdown:
• 20%: local bone destruction by metastases → osteolysis (e.g., via PGE₂).
• 80%: humoral hypercalcemia of malignancy:
• Tumors secrete PTHrP, which:
• Acts like PTH on bone and kidney
• Causes hypercalcemia + hypophosphatemia
• Suppresses endogenous PTH.
• Tumors commonly involved:
• Breast
• Kidney
• Ovary
• Squamous cell cancers (skin, lung etc.)

👉 Big exam phrase: “Hypercalcemia with LOW PTH and HIGH PTHrP → humoral hypercalcemia of malignancy.”

7️⃣ Clinical: Too Little vs Too Much PTH

🔻 Hypoparathyroidism (e.g. post-surgical)

• Cause: inadvertent parathyroidectomy during thyroid surgery.
• Effects:
• ↓ Ca²⁺, ↑ phosphate
• Neuromuscular hyperexcitability → hypocalcemic tetany
• Clinical signs:
• Chvostek sign: tap facial nerve → facial twitch.
• Trousseau sign: inflate BP cuff → carpopedal spasm (flexed wrist, thumb; extended fingers).
• Treatment:
• PTH replacement (injection)
• Calcium salts IV for acute relief

🔺 Hyperparathyroidism

Primary hyperparathyroidism (adenoma)

• ↑ PTH → hypercalcemia, hypophosphatemia
• Often detected incidentally (asymptomatic mild hypercalcemia).
• May cause:
• Stones (kidney stones)
• Bones (bone pain)
• Groans (GI discomfort)
• Moans (psychic changes)
• Treatment: sometimes subtotal parathyroidectomy, especially if severe.

Secondary hyperparathyroidism

• Due to chronic hypocalcemia (e.g. CKD, rickets).
• Kidneys can't make adequate 1,25-D → ↓ Ca²⁺ → chronic PTH stimulation.
• Parathyroids hypertrophy.

CASR mutations (very exammy)

• Inactivating CASR mutation (heterozygous):
• Familial benign hypocalciuric hypercalcemia:
• CaSR “thinks” Ca²⁺ is low → ↑ PTH set point.
• Mildly ↑ plasma Ca²⁺
• PTH normal or slightly ↑
• Low urinary Ca²⁺ (hypocalciuria).
• Inactivating CASR mutation (homozygous):
• Neonatal severe primary hyperparathyroidism:
• Very high PTH
• Severe hypercalcemia
• Gain-of-function CASR mutation:
• Familial hypercalciuric hypocalcemia:
• CaSR oversensitive → suppresses PTH at lower Ca²⁺.
• Plasma Ca²⁺ low, PTH low/normal
• Urinary Ca²⁺ high (hypercalciuria).

👉 CASR concept: “Set-point disease” of Ca²⁺–PTH feedback.

🟣 PARATHYROID GLANDS — COMPLETE HIGH-YIELD TABLE SET

Table 1 — Anatomy & Cell Types

Table 2 — PTH Synthesis, Structure & Clearance

Table 3 — Core Physiologic Actions of PTH (EXAM GOLD)

Table 4 — PTH Receptors

Table 5 — Pseudohypoparathyroidism (Pathophysiology Favorite)

Table 6 — Regulation of PTH Secretion

A. Calcium (Primary Regulator)

B. Other Regulators

Table 7 — PTHrP & Hypercalcemia of Malignancy

Hypercalcemia of Malignancy

Common Tumors

Breast

Kidney

Ovary

Squamous cell carcinomas (lung, skin)

Table 8 — Hypoparathyroidism

Table 9 — Hyperparathyroidism

Primary

Secondary

Table 10 — CaSR Mutations (VERY EXAMMY)

Core Concept

CaSR mutations = altered Ca²⁺–PTH set point

CALCITONIN & EFFECTS OF OTHER HORMONES

1️⃣ Calcitonin – What You Actually Need to Know

📍 Origin

• Produced by:

Parafollicular C cells of the thyroid (also called clear cells).

• Discovered as a Ca²⁺-lowering hormone when perfusing thyroparathyroid region lowered plasma Ca²⁺.

👉 One-liner:

Thyroid C cells → calcitonin → Ca²⁺-lowering hormone.

📈 Secretion & Metabolism (High-yield control points)

• Human calcitonin: 32–amino acid peptide, MW ≈ 3500.
• Main stimulus = high plasma Ca²⁺
• Starts to rise around 9.5 mg/dL
• Above this, higher Ca²⁺ → more calcitonin.
• Other stimulants:
• β-agonists, dopamine, estrogens
• Gut hormones: gastrin (most potent), CCK, glucagon, secretin
• Gastrin can greatly raise calcitonin in Zollinger–Ellison syndrome and pernicious anemia
• But physiological meals don’t give enough gastrin to have a major Ca²⁺-lowering role.
• Half-life:
• Very short, < 10 minutes → effect is brief.

👉 Key idea: calcitonin is acutely responsive to high Ca²⁺, but its overall long-term importance is small.

🎯 Actions (Simple but exam-tested)

• Receptors: in bone and kidney.

Main effects:

1. Bone
• Direct inhibition of osteoclasts → ↓ bone resorption.
• Result: ↓ Ca²⁺ and ↓ phosphate release from bone.
2. Kidney
• ↑ urinary Ca²⁺ excretion
• Contributes further to ↓ plasma Ca²⁺.

👉 Short version:

Calcitonin = “anti-PTH” on bone: inhibits osteoclasts → ↓ Ca²⁺, ↑ Ca²⁺ loss in urine.

🧠 But what is its real physiologic role?

This is where exam questions love to trick you.

• Human thyroid calcitonin content is low.
• After total thyroidectomy:
• If parathyroids preserved → Ca²⁺ and bone density stay normal.
• Only transient Ca²⁺ handling changes with a Ca²⁺ load.
• Patients with medullary carcinoma of thyroid:
• Very high calcitonin levels
• But normal bones and no clear hypocalcemia due to calcitonin.
• No deficiency syndrome described for calcitonin.

So:

• In adults, calcitonin is NOT essential for day-to-day Ca²⁺ homeostasis.
• It may have roles in:
• Children / skeletal development
• Pregnancy & lactation: possibly helps protect maternal bone from excessive resorption while:
• Fetus and infant demand ↑ Ca²⁺
• 1,25-(OH)₂D₃ is high (which would normally promote bone resorption)

👉 Exam-takeaway:

Calcitonin exists, lowers Ca²⁺ acutely, but PTH + Vit D dominate long-term regulation. No major disease from its absence.

2️⃣ Summary: The Big 3 Hormones in Ca²⁺ Homeostasis

This paragraph is pure exam gold.

🔺 PTH (Parathyroid Hormone)

• Increases plasma Ca²⁺ by:
• ↑ bone resorption (osteoclast activation via osteoblasts)
• ↑ renal Ca²⁺ reabsorption (distal tubule)
• ↑ 1,25-(OH)₂D₃ formation → ↑ intestinal Ca²⁺ absorption
• Decreases plasma phosphate:
• ↓ proximal tubular phosphate reabsorption → phosphaturia

👉 Mnemonic: “PTH = Pulls Ca²⁺ High, Pushes Phosphate Out.”

🌞 1,25-dihydroxycholecalciferol (Active Vitamin D)

• Increases Ca²⁺ absorption from intestine
• Increases Ca²⁺ reabsorption from kidney
• Also ↑ phosphate absorption from gut

👉 Think: Vit D = bring Ca²⁺ (and Pi) into the body from gut.

🔻 Calcitonin

• Inhibits bone resorption (blocks osteoclasts)
• Increases Ca²⁺ in urine

👉 Net: weak Ca²⁺-lowering, short-lived, minor player in adults.

3️⃣ Other Hormones that Influence Calcium/Bone (Easy marks if you remember directions)

These are often used in integrated questions or clinical vignettes.

🧬 Glucocorticoids

• Short term:
• ↓ osteoclast formation & activity → ↓ Ca²⁺ (initially).
• Long term (chronic steroids):
• ↓ protein synthesis in osteoblasts → ↓ bone formation
• ↑ bone resorption → osteoporosis
• ↓ intestinal absorption of Ca²⁺ and Pi
• ↑ renal excretion of Ca²⁺ and Pi
• ↓ plasma Ca²⁺ → ↑ PTH → more bone resorption

👉 Net chronic effect: osteoporosis + high fracture risk.

📈 Growth Hormone & IGF-1

• GH:
• ↑ Ca²⁺ excretion in urine
• BUT ↑ intestinal Ca²⁺ absorption → often net positive Ca²⁺ balance.
• IGF-1 (produced under GH influence):
• Stimulates protein synthesis in bone → bone growth.

👉 So GH/IGF-1 generally support bone formation.

🔥 Thyroid Hormones

• Excess thyroid hormones:
• Can cause hypercalcemia (↑ bone turnover).
• ↑ urinary Ca²⁺ loss (hypercalciuria).
• Over time → osteoporosis in some patients.

👉 Chronic hyperthyroidism = bone loss risk.

♀ Estrogens

• Estrogens protect bone:
• Inhibit cytokines that stimulate osteoclasts.
• Reduce bone resorption.

👉 Menopause → ↓ estrogen → ↑ osteoclast activity → postmenopausal osteoporosis.

🩸 Insulin

• Increases bone formation.
• Untreated diabetes → insulin deficiency → bone loss.

👉 Insulin is anabolic for bone.

🦴 CALCIUM & BONE REGULATION — MASTER TABLE (EXAM-LOCK)

🧠 ULTIMATE EXAM LOCK (1-Line Truth)

PTH + Vitamin D control long-term Ca²⁺ balance. Calcitonin only fine-tunes acute Ca²⁺ rises and is dispensable in adults.

BONE PHYSIOLOHY

1️⃣ What Bone Really Is (Composition + Function)

• Bone = connective tissue with:
• Collagen type I framework (≈ 90% of matrix protein) → tensile strength (like steel cables).
• Hydroxyapatite crystals (Ca₁₀(PO₄)₆(OH)₂) → compressive strength (hardness).
• Functions:
• Mechanical: support, protection, locomotion.
• Metabolic: major Ca²⁺ and phosphate reservoir, constantly exchanging with ECF.
• Vascular: very well perfused (200–400 mL/min in adults).

👉 Key idea: bone is living, dynamic tissue, not a “dead” scaffold.

2️⃣ Two Types of Bone – Why It Matters

🦴 Cortical (compact) bone

• 80% of bone mass, forms outer shell.
• Low surface-to-volume ratio.
• Organized into osteons (Haversian systems):
• Central Haversian canal with blood vessels.
• Concentric lamellae around it.
• Osteocytes in lacunae, connected via canaliculi.

🕸️ Trabecular (spongy) bone

• 20% of bone mass, but much higher surface area.
• Plates/spicules with many cells on the surface.
• Metabolically more active:
• Faster turnover.
• Preferentially lost in osteoporosis (→ vertebral + hip fractures).

👉 Exam point: Trabecular bone = more metabolically active → first to suffer in osteoporosis.

3️⃣ How Bones Grow in Length & Shape

A. Types of ossification

• Endochondral ossification:
• Most bones.
• Bone replaces preformed cartilage model.
• Intramembranous ossification:
• Clavicles, mandible, some skull bones.
• Mesenchymal cells → bone directly.

B. Epiphyseal plate (growth plate)

• Epiphysis separated from shaft by epiphyseal cartilage plate.
• Plate proliferates → new bone laid down → bone length increases.
• Width of plate ∝ growth rate.
• Controlled by hormones, especially GH + IGF-1.
• At epiphyseal closure:
• Chondrocytes stop proliferating.
• Become hypertrophic, secrete VEGF, attract blood vessels.
• Plate ossifies → no more linear growth.
• Bone age = which epiphyses are open vs closed on X-ray.

C. Periosteum

• Outer fibrous layer + inner cellular layer.
• Highly vascular and innervated.
• Source of bone-forming cells (especially in youth).
• Thins with age → bones more vulnerable.

👉 So: length = growth plate; thickness/shape = periosteal activity + remodeling.

4️⃣ Osteoblasts vs Osteoclasts – The Core Mechanism

🧱 Osteoblasts – bone builders

• Origin: mesenchyme → fibroblast-like → osteoblast.
• Key transcription factor: Runx2 (Cbfa1)
• Runx2 knockout mice → skeleton only cartilage, no ossification.
• Functions:
• Lay down type I collagen.
• Mineralize matrix → form new bone.

👉 Think: Runx2 ON = bone formation can happen.

🧨 Osteoclasts – bone resorbers

• Origin: monocyte/macrophage lineage.
• Need two signals to differentiate:

1. RANKL–RANK interaction
• RANKL expressed on:
• Bone marrow stromal cells
• Osteoblasts
• T-lymphocytes
• RANK on monocyte precursors.
2. M-CSF–CSF1R pathway
• Non-monocytic cells secrete M-CSF.
• Binds CSF1R on monocytes.

• OPG (osteoprotegerin):
• Secreted by precursor cells.
• Acts as a decoy receptor for RANKL.
• Binds RANKL → prevents it from binding RANK → limits osteoclast formation.

👉 Memory hook:

RANKL + M-CSF = “license” precursors to become osteoclasts. OPG = “safety cap” that blocks RANKL.

🔬 How osteoclasts resorb bone

1. Attach to bone via integrins → form sealing zone.
2. Create sealed compartment between cell and bone.
3. Pump H⁺ into this space via H⁺-ATPase (proton pump) → pH ≈ 4.0.
4. Acid dissolves hydroxyapatite; acid proteases digest collagen.
5. Digested products are endocytosed and transcytosed → released into ECF.
6. Collagen breakdown products have pyridinoline structures → can be measured in urine as marker of bone resorption.

👉 Key exam sentence:

Osteoclast = sealed acidic micro-lysosome on the bone surface, created by H⁺-ATPase and proteases.

5️⃣ Bone Remodeling – Dynamic Balance

• Old bone is always being removed and replaced:
• Ca²⁺ turnover:
• 100%/year in infants
• ~18%/year in adults
• Remodeling occurs via bone-remodeling units:
1. Osteoclasts resorb bone.
2. Osteoblasts follow and lay down new bone.
3. One cycle ≈ 100 days.
4. At any time, about 5% of skeleton is being remodeled by ~2 million units.
• Renewal rates:
• Compact bone ≈ 4% per year
• Trabecular bone ≈ 20% per year (more active).
• Remodeling is influenced by:
• Mechanical load (gravity, muscle pull).
• Hormones:
• PTH → speeds bone resorption.
• Estrogens → inhibit resorption.
• Leptin:
• Central (brain, intracerebroventricular) leptin → ↓ bone formation (via hypothalamic pathways).
• Peripheral leptin → may ↑ bone mass via direct osteoblast/pre-osteoblast signaling.

👉 Concept: bone is constantly renewing and adapting to mechanical and hormonal signals.

6️⃣ Bone Disease: Two Opposites to Remember

1. Osteopetrosis – “Stone bones” (too much bone, but bad quality)

• Osteoclasts defective → cannot resorb bone.
• Osteoblasts keep working → bone density increases, but:
• Marrow cavities narrowed → hematologic problems (anemia, infections).
• Foramina narrowed → neurological defects (nerve compression).
• Seen in:
• Mice lacking c-fos.
• Mice lacking PU.1.

→ Shows these factors are vital for osteoclast development.

👉 Core idea: no osteoclast function → bone accumulates but becomes pathologic.

2. Osteoporosis – “Porous bones” (too much resorption)

• Relative excess of osteoclastic activity vs osteoblast activity.
• Marked loss of bone matrix → increased fracture risk.
• Sites prone to fracture:
• Distal radius (Colles’ fracture)
• Vertebral bodies (→ vertebral collapse, kyphosis / “widow’s hump”)
• Hip
• These areas are rich in trabecular bone, which is lost faster.

Involutional (age-related) osteoporosis

• Normal pattern: gain bone in youth → plateau → slow loss with age.
• If bone loss is exaggerated → osteoporosis.
• Postmenopausal women at highest risk:
• Less peak bone mass than men.
• Lose bone faster after menopause.

Role of Estrogen

• Estrogen normally:
• Inhibits cytokines (IL-1, IL-6, TNF-α), which otherwise stimulate osteoclast development.
• Stimulates TGF-β, which promotes osteoclast apoptosis.
• After menopause:
• ↓ estrogen → ↑ osteoclast survival and activity → rapid bone loss.

Disuse Osteoporosis

• Seen with immobilization or space flight.
• Bone resorption > formation.
• Plasma Ca²⁺ not dramatically high, but:
• ↓ PTH
• ↓ 1,25-(OH)₂D₃
• ↑ urinary Ca²⁺ loss.

7️⃣ Treatment Principles (Just Enough for Exams)

• Lifestyle:
• ↑ Calcium intake (preferably from diet like milk).
• Moderate weight-bearing exercise (effect modest but beneficial).
• Drugs:
• Bisphosphonates (e.g. etidronate):
• Inhibit osteoclasts.
• ↑ mineral content of bone.
• ↓ new vertebral fractures.
• Fluoride:
• Stimulates osteoblasts, makes bone denser.
• But not very effective clinically.
• Estrogen replacement:
• Can preserve bone, but ↑ risk of breast/uterine cancer and no real CV protection → no longer first-line.
• Raloxifene (SERM):
• Mimics estrogen’s protective effect on bone with less cancer risk (but still some side effects like risk of clots).
• Calcitonin, teriparatide (PTH analogue):
• Used in selected cases to modulate bone resorption/formation.
• Physio / rehab:
• Improve strength, balance, and mechanical loading → ↓ falls and fractures, ↑ quality of life.

🦴 BONE PHYSIOLOGY — MASTER CONSOLIDATED TABLE (ZERO-OMISSION)

🔒 One-Line Exam Reflex

Bone is a living, mechanically responsive Ca²⁺ reservoir maintained by Runx2-driven osteoblasts and RANKL-regulated osteoclasts; imbalance causes osteopetrosis or osteoporosis.