In an adult with a markedly elevated 25‑hydroxy‑vitamin D level, which primary‑care laboratory tests should be ordered to exclude endogenous causes such as granulomatous disease or lymphoma, and what does each test indicate?

Laboratory Workup for Markedly Elevated 25‑Hydroxyvitamin D

Order serum calcium, parathyroid hormone (PTH), and 1,25‑dihydroxyvitamin D to distinguish exogenous vitamin D excess from endogenous overproduction by granulomatous disease or lymphoma. 1, 2

Primary‑Care Laboratory Panel

Essential First‑Line Tests

• Serum corrected total calcium – Detects hypercalcemia, which occurs in approximately 50% of patients with granulomatous or lymphoproliferative disorders producing excess 1,25‑dihydroxyvitamin D 1, 2
• Intact parathyroid hormone (PTH) – Distinguishes PTH‑mediated hypercalcemia from vitamin D‑mediated hypercalcemia; PTH is suppressed (<20 pg/mL) when 1,25‑dihydroxyvitamin D drives calcium absorption 2, 3
• Serum 1,25‑dihydroxyvitamin D – The active vitamin D metabolite; inappropriately elevated levels (>80 pmol/L or >33 pg/mL) in the setting of high 25‑hydroxyvitamin D indicate endogenous extrarenal production by granulomas or lymphoma cells 1, 2, 3

Interpretation Algorithm

Finding	Interpretation	Next Step
Normal calcium, normal PTH, normal 1,25‑D	Exogenous vitamin D excess (supplementation or intoxication)	Discontinue all vitamin D; recheck in 3 months [4]
Elevated calcium, suppressed PTH, elevated 1,25‑D	Endogenous 1,25‑D production by granulomas or lymphoma	Proceed to imaging and tissue diagnosis [1,2,3]
Elevated calcium, elevated PTH, normal 1,25‑D	Primary hyperparathyroidism (unrelated to vitamin D elevation)	Refer to endocrinology for parathyroid evaluation

What Each Test Indicates

Serum Calcium

• Elevated (>10.2 mg/dL or >2.54 mmol/L) with high 25‑hydroxyvitamin D signals either vitamin D intoxication or endogenous 1,25‑dihydroxyvitamin D overproduction 1, 2
• Normal calcium with markedly elevated 25‑hydroxyvitamin D strongly favors exogenous supplementation rather than granulomatous disease 1

Parathyroid Hormone (PTH)

• Suppressed PTH (<20 pg/mL) in the presence of hypercalcemia confirms vitamin D‑mediated calcium absorption rather than parathyroid‑driven bone resorption 2, 3
• Normal or elevated PTH excludes vitamin D‑mediated hypercalcemia and points to primary hyperparathyroidism or other causes 2

1,25‑Dihydroxyvitamin D (Active Vitamin D)

• Inappropriately elevated (>80 pmol/L or >33 pg/mL) despite high 25‑hydroxyvitamin D indicates unregulated extrarenal conversion by granulomatous tissue (sarcoidosis, tuberculosis) or lymphoma‑associated macrophages 1, 2, 3
• Normal or low 1,25‑dihydroxyvitamin D with elevated 25‑hydroxyvitamin D confirms exogenous vitamin D intake without pathologic activation 1, 2
• Granulomatous macrophages and tumor‑adjacent macrophages in lymphoma express 1α‑hydroxylase, the enzyme that converts 25‑hydroxyvitamin D to 1,25‑dihydroxyvitamin D, bypassing normal renal regulation 1, 3

Clinical Context and Pitfalls

Granulomatous Diseases

• Sarcoidosis is the most common granulomatous cause of elevated 1,25‑dihydroxyvitamin D, occurring in 20–30% of patients with extrapulmonary disease 1, 5
• Tuberculosis rarely causes this abnormality despite being a granulomatous process 1
• The unregulated production of 1,25‑dihydroxyvitamin D by granulomas is controlled by glucocorticoids, which suppress macrophage 1α‑hydroxylase activity 1

Lymphoproliferative Disorders

• Hodgkin lymphoma and T‑cell lymphomas are the most frequent lymphomas associated with elevated 1,25‑dihydroxyvitamin D and hypercalcemia 2, 6
• Immunohistochemistry demonstrates that tumor‑adjacent macrophages, not the lymphoma cells themselves, produce 1α‑hydroxylase and synthesize 1,25‑dihydroxyvitamin D 3
• Nearly 50% of hypercalcemic lymphoma patients have inappropriately elevated 1,25‑dihydroxyvitamin D concentrations 3

Common Diagnostic Errors

• Failing to measure 1,25‑dihydroxyvitamin D when 25‑hydroxyvitamin D is markedly elevated; this is the only test that distinguishes exogenous excess from endogenous overproduction 1, 2
• Assuming all elevated vitamin D is from supplementation; granulomatous disease and lymphoma can produce extreme elevations of 25‑hydroxyvitamin D through increased substrate availability for extrarenal 1α‑hydroxylase 2, 3
• Ordering a chest X‑ray before biochemical confirmation; obtain calcium, PTH, and 1,25‑dihydroxyvitamin D first to determine whether imaging is warranted 1, 2

When to Refer or Image

• If 1,25‑dihydroxyvitamin D is elevated with suppressed PTH, order chest CT to evaluate for hilar lymphadenopathy (sarcoidosis) or mediastinal masses (lymphoma) 1, 2, 3
• If hypercalcemia is present, refer urgently to hematology‑oncology or pulmonology depending on imaging findings 2, 3
• If all tests are normal, reassure the patient that the elevation is from supplementation, discontinue vitamin D, and recheck 25‑hydroxyvitamin D in 3 months 4