Disorders of hypercalcemia and increased parathyroid hormone can be difficult to diagnose. The calcium and parathyroid levels can be used to direct the initial evaluation of the patient.
The normal serum calcium varies with age:
Age
Normal Serum Calcium
2 to 12 years
8.8-10.8 mg/dL
12 to 18 years
8.4 to 10.2 mg/dL
18 to 60 years
8.6-10.0 mg/dL
60 to 90 years
8.8-10.2 mg/dL
> 90 years
8.2-9.6 mg/dL
Parathyroid hormone exists in different forms in the blood. Intact hormone has a half-life of 2-5 minutes. It is rapidly degraded in the blood to an N-terminal peptide (amino acids 1-34) and a C-terminal peptide fragment (amino acids 35-84). The N-terminal peptide has a short half life (< 5 minutes) and has a biological activity similar to intact PTH. The C-terminal peptide has a longer half life (25-30 minutes) but is nonfunctional; it is cleared by the kidney, and in renal failure levels can accumulate to high levels.
The normal PTH serum level depends on the assay employed. For the intact molecule by immunoradiometric assay (IRMA) or immunochemoluminescent assay (ICMA) it is 1-5 pmol/L
Serum Calcium
Serum PTH
Consider
increased (10-18 mg/dL)
normal to decreased
hypercalcemia of malignancy
normal to slightly increased (8-11 mg/dL)
increased (10-90 pmol/L)
renal failure
normal to increased (9.5-18 mg/dL)
normal to increased (2-90 pmol/L)
primary hyperparathyroidism
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