Vitamin D acts like a hormone controlling calcium, bone, and immune function. Its clearest value is correcting genuine deficiency and preventing the resulting bone disease. Steady daily use may lower cancer deaths, new autoimmune conditions, and infections in deficient people. Benefit concentrates in those who start low; excess intake risks high calcium and kidney stones. (Full Review)
| Marker | Target | Why |
|---|---|---|
| 25-hydroxyvitamin D [25(OH)D] | 40–60 ng/mL | Primary marker of vitamin D status |
| Serum calcium | 9.0–10.0 mg/dL | Detects hypercalcemia from over-supplementation |
| Intact parathyroid hormone (PTH) | 15–40 pg/mL | Signals functional vitamin D insufficiency |
| 24-hour urine calcium | <250–300 mg/day | Detects hypercalciuria before stones form |
| Magnesium (RBC) | 5.0–6.5 mg/dL | Required cofactor for vitamin D activation |
Cadence: Recheck 25(OH)D at 8–12 weeks after starting or changing dose, then every 6–12 months once stable; check serum calcium periodically at higher doses