Table 1.
Normal values in adults.
| Normal range (SI units) | Normal range (Conventional units) | |
|---|---|---|
| Serum calcium | 2.2–2.6 mMol/L | 8.8–10.4 mg/dL |
| Ionized calciuma | 1.1–1.3 mMol/L | 4.4–5.2 mg/dL |
| PTH | 1.6–6.9 pMol/L | 15–65 pg/mL |
| Calcidiol, or 25-hydroxyvitamin D [25(OH)D]b | 50–200 nMol/L | 20–80 ng/mL |
PTH: parathyroid hormone; SI: International System.
a
Albeit all calcium in the body is technically ionized, the term usually only applies to the free ionic fraction that is physiologically active in the blood.
b
In contrast to 25(OH)D, circulating calcitriol, also known as 1,25(OH)2D, is generally not a good indicator of vitamin D status because it has a short half-life of 15 hours and serum concentrations are closely regulated by parathyroid hormone, calcium, and phosphate; levels of 1,25(OH)2D do not typically decrease until vitamin D deficiency is severe.