In a patient with low parathyroid hormone and normal or high‑normal calcium, should the initial evaluation be ordered by the primary‑care physician, an endocrinologist, or a hematologist?

Specialist Referral for Low PTH with Normal/High-Normal Calcium

The initial evaluation should be ordered by the primary care physician (PCP), who can then refer to an endocrinologist if the diagnosis remains unclear or if specialized management is needed; a hematologist is not indicated for this presentation. 1

Initial PCP Responsibilities

The primary care physician should initiate the diagnostic workup because the evaluation requires straightforward laboratory tests that do not necessitate subspecialty expertise at the outset. 2

Essential First-Line Laboratory Panel

The PCP should order:

• Serum calcium (corrected for albumin) and ionized calcium to confirm the calcium status, as total calcium can be misleading if albumin is abnormal 1
• Intact parathyroid hormone (PTH) measured in EDTA plasma rather than serum, as PTH is most stable in EDTA plasma at 4°C 1, 3
• 25-hydroxyvitamin D to exclude vitamin D deficiency or intoxication as secondary causes 1, 4
• Serum creatinine and estimated GFR to assess kidney function, since impaired renal function (GFR <60 mL/min/1.73 m²) alters PTH interpretation 2, 1
• Serum phosphorus to help differentiate causes of calcium-PTH abnormalities 2
• Serum magnesium because severe hypomagnesemia can suppress PTH secretion 1

When to Refer to Endocrinology

Refer to an endocrinologist if:

• The diagnosis remains uncertain after initial laboratory evaluation, particularly if PTH is inappropriately low for the calcium level and secondary causes have been excluded 1
• Hypoparathyroidism is confirmed (low PTH with hypocalcemia or normal calcium), as this requires specialized management with calcium and active vitamin D supplementation 1
• Post-surgical hypoparathyroidism is suspected (history of thyroid or parathyroid surgery), even if PTH appears "normal," because normal PTH in the setting of hypocalcemia represents parathyroid insufficiency 5
• Genetic or familial syndromes are suspected, requiring genetic counseling and coordinated subspecialty care 1

The endocrinologist's role includes confirming the diagnosis, excluding secondary causes, determining the underlying etiology (e.g., post-surgical, autoimmune, genetic), and establishing long-term management strategies. 1, 3

Why Not a Hematologist?

A hematologist is not indicated for isolated low PTH with normal or high-normal calcium. This presentation does not suggest a primary hematologic disorder. Hematology referral would only be appropriate if there were concurrent findings suggesting bone marrow pathology, plasma cell dyscrasia, or hematologic malignancy—none of which are suggested by the PTH-calcium pattern described. 2

Critical Measurement Considerations

• PTH assays vary by up to 47% between different generations, so always use assay-specific reference ranges when interpreting results 1, 3
• Biotin supplementation can interfere with PTH immunoassays, leading to falsely low or high values; patients should discontinue biotin at least 72 hours before testing 1, 3
• Biological variation of PTH is substantial (~20% in healthy individuals), so a single borderline result may require repeat measurement 1, 3

Common Diagnostic Pitfalls

• Failing to correct calcium for albumin can lead to misclassification of calcium status; always calculate corrected calcium or measure ionized calcium directly 1
• Interpreting "normal" PTH as reassuring when calcium is low represents a missed diagnosis of hypoparathyroidism, because PTH should be elevated in response to hypocalcemia 5
• Not assessing vitamin D status can obscure the diagnosis, as vitamin D deficiency causes secondary hyperparathyroidism while vitamin D intoxication suppresses PTH 1, 4
• Overlooking medication history, particularly thiazide diuretics, lithium, or calcium/vitamin D supplements, which can alter calcium-PTH relationships 4