☀️ Vitamin D & Calcium-Parathyroid Axis Simulator

Baseline Vitamin D & Calcium Balance

Normal vitamin D keeps calcium absorption and PTH steady.

• 30–50: Normal 25-OH-D (ng/mL sufficient)
• ~30%: Normal Ca²⁺ absorption (of dietary intake)
• 10–65: Normal PTH (pg/mL reference range)
• balanced: Bone remodeling (formation = resorption)

Vitamin D activation pathway

Skin, diet, and liver/kidney convert vitamin D into active calcitriol.

Intestinal calcium absorption

Calcitriol upregulates gut transporters that absorb dietary calcium.

Parathyroid set point

Calcium-sensing receptors keep PTH secretion low when calcium is adequate.

Vitamin D Deficiency Develops

Low sun exposure, diet, or malabsorption drop calcitriol output.

• <20: Deficient 25-OH-D (ng/mL threshold)
• ~1 B: Global prevalence (people affected)
• 3+: Main causes (sun, diet, malabsorption)
• weeks: Onset timeline (to become measurable)

Falling calcitriol

Less substrate and enzyme activity reduce active hormone levels.

Absorption transporters idle

Gut calcium channels are no longer upregulated as before.

Still asymptomatic

Blood calcium has not yet fallen — deficiency is silent.

Serum Calcium Begins To Drop

Reduced intestinal absorption outpaces dietary calcium intake.

• 8.5–10.5: Normal serum Ca²⁺ (mg/dL)
• ~10–15%: Absorption falls to (in deficiency)
• blood: Detection method (panel + 25-OH-D)
• days: Compensation lag (before PTH responds)

Absorption shortfall

Less calcium crosses the gut wall into the bloodstream.

Calcium sensing

Parathyroid cells detect the small decline in ionized calcium.

Compensation begins

Falling calcium is the trigger for the PTH response.

Secondary Hyperparathyroidism

PTH rises to pull calcium from bone and kidney.

• >65: Elevated PTH (pg/mL secondary rise)
• ↑: Bone resorption (osteoclast activation)
• ↑: Renal reabsorption (less calcium lost in urine)
• ~normal: Serum calcium (restored by compensation)

PTH surge

Parathyroid glands secrete more hormone to defend blood calcium.

Bone resorption activated

PTH signals osteoclasts to release calcium from bone matrix.

Kidney conserves calcium

Renal tubules reabsorb more calcium, less is excreted.

Chronic Bone Demineralization

Ongoing compensation slowly drains bone mineral reserves.

• adults: Osteomalacia (softened bone matrix)
• children: Rickets (growth plate deformity)
• progressive: BMD decline (over months to years)
• normal Ca²⁺: Masked risk (hides bone loss)

Hidden cost of compensation

Bone is sacrificed to keep blood calcium near normal.

Progressive thinning

Chronic resorption exceeds bone formation over time.

Clinical consequence

Fracture risk and bone pain rise as density declines.