Laboratory Workup for Elevated Serum Calcium in Post-Menopausal Women

Initial Essential Laboratory Panel

The American College of Physicians recommends measuring intact parathyroid hormone (iPTH) first to distinguish between primary hyperparathyroidism and malignancy, which are responsible for 90% of hypercalcemia cases 1
Serum albumin is required to calculate corrected calcium using the formula: Corrected calcium (mg/dL) = Total calcium + 0.8 × [4.0 - Serum albumin (g/dL)] [1]
Ionized calcium is preferred over corrected calcium to avoid pseudo-hypercalcemia from hemolysis or improper sampling 1
Serum phosphorus helps differentiate causes of hypercalcemia, with low levels often seen in primary hyperparathyroidism and variable levels in malignancy 1
Serum creatinine and BUN are essential to assess renal function, as chronic kidney disease significantly alters calcium metabolism and PTH interpretation 1
Magnesium levels should be checked, as hypomagnesemia can suppress PTH secretion and must be corrected 1

The Endocrine Society recommends checking 25-hydroxyvitamin D to exclude vitamin D deficiency as a secondary cause of elevated PTH 2, 1
A 24-hour urinary calcium test can distinguish primary hyperparathyroidism from familial hypocalciuric hypercalcemia (FHH), which presents with low urinary calcium excretion 3
Pre-operative localization studies, such as 99mTc-sestamibi parathyroid scan with SPECT/CT, are highly sensitive for localizing parathyroid adenomas 2, 4
Neck ultrasound can be combined with sestamibi for improved sensitivity in localizing parathyroid adenomas 2

PTH-related peptide (PTHrP) should be checked, as it is elevated in humoral hypercalcemia of malignancy, the most common mechanism in solid tumors 1
The National Comprehensive Cancer Network recommends checking 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D to evaluate for vitamin D intoxication or granulomatous diseases 1
Serum protein electrophoresis (SPEP) and immunofixation should be performed to screen for multiple myeloma 1
Measuring both 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D together is crucial for diagnostic accuracy, as checking only one can lead to missed diagnoses 1

The American Geriatrics Society recommends inquiring about thiazide diuretics, lithium, and calcium supplements >500 mg/day, as these can cause mild hypercalcemia or hyperparathyroidism 1
Vitamin D supplements >400 IU/day can increase the risk of toxicity, and patients should be advised accordingly 1
Calcitriol or vitamin D analogs can cause hypercalcemia in 22.6-43.3% of patients, and their use should be carefully monitored 1

The American College of Physicians advises against relying on corrected calcium alone and recommends measuring ionized calcium when possible to avoid misdiagnosis from pseudo-hypercalcemia 1
Measuring both 25-OH and 1,25-dihydroxy forms of vitamin D together is essential for complete assessment, and checking only one can lead to missed diagnoses 1