In a patient with multiple myeloma, will parathyroid hormone‑related protein (PTHrP) be elevated?

PTHrP is Usually NOT Elevated in Multiple Myeloma

In patients with multiple myeloma (MM), PTHrP is typically NOT elevated, as MM primarily causes hypercalcemia through local osteolytic mechanisms rather than humoral mechanisms. However, rare cases of PTHrP-producing MM do exist and should be considered when hypercalcemia occurs with suppressed PTH.

Pathophysiology of Hypercalcemia in MM

The underlying mechanism of hypercalcemia in MM differs fundamentally from solid tumors:

• Primary mechanism: Local osteolytic hypercalcemia (LOH) mediated by cytokines like IL-1, IL-6, TNF-beta, and MIP-1α that directly stimulate osteoclastic bone resorption 1
• NOT humoral hypercalcemia: Unlike squamous cell carcinomas, breast, and renal carcinomas where PTHrP-mediated humoral hypercalcemia of malignancy (HHM) is the dominant mechanism 1

When PTHrP May Be Elevated in MM

While uncommon, PTHrP can be elevated in MM in specific circumstances:

Rare PTHrP-Producing MM Cases

• Case reports document MM cells expressing and secreting PTHrP 2, 3
• PTHrP can be detected in myeloma cells via immunohistochemistry and in situ hybridization 2, 3
• When present, PTHrP contributes through both:
  • Intracrine effects: Mid-region PTHrP fragments accumulate in nuclei, promoting MM cell proliferation and survival 4
  • Autocrine/paracrine effects: Full-length PTHrP stimulates PTH-R1 on MM cells, increasing RANKL and MCP-1 production, further driving osteoclastogenesis 4

Dual Mechanism Cases

• Rare patients may have both elevated PTHrP AND elevated MIP-1α, creating a "double hit" of humoral and local osteolytic hypercalcemia 5
• This combination can produce particularly severe hypercalcemia

Clinical Implications for Diagnosis

When evaluating hypercalcemia in MM patients:

1. Expect suppressed PTH with normal or low 1,25(OH)₂ vitamin D₃ in typical MM-related hypercalcemia

2. Check PTHrP if:

   • Hypercalcemia is present with suppressed PTH
   • Hypercalcemia is particularly severe or refractory
   • Clinical picture suggests humoral mechanism

3. Interpret PTHrP levels carefully:

   • Mild PTHrP elevations (<7.0 pmol/L) with PTH ≥30 pg/mL suggest non-malignant causes (positive predictive value for cancer <5%) 6
   • Markedly elevated PTHrP (>7.2 pmol/L) with suppressed PTH is more consistent with malignancy-associated hypercalcemia 5

Important Caveats

• Do not assume PTHrP elevation equals MM-related hypercalcemia: Consider concomitant primary hyperparathyroidism, which can coexist with MM 7
• PTHrP measurement is not routine in MM workup unless hypercalcemia is present and mechanism is unclear
• The majority of MM patients with hypercalcemia will have normal or low PTHrP levels, distinguishing them from solid tumors where PTHrP-mediated HHM predominates