What is the likely diagnosis, workup, and management for a patient with elevated hemoglobin, hematocrit, mean corpuscular volume, mean corpuscular hemoglobin, free testosterone, follicle‑stimulating hormone, vitamin B12, and low 25‑hydroxy vitamin D?

Likely Diagnosis: Testosterone-Induced Erythrocytosis vs. Polycythemia Vera

The most likely diagnosis is testosterone-induced erythrocytosis given the constellation of elevated free testosterone, hemoglobin, hematocrit, and macrocytic indices, though polycythemia vera must be excluded given the elevated vitamin B12.

Diagnostic Considerations

Testosterone-Induced Erythrocytosis

• Elevated testosterone directly stimulates erythropoiesis, causing hemoglobin increases of 15-20% through increased erythropoietin production and establishment of a new EPO/hemoglobin set point 1, 2.
• The elevated MCV and MCH suggest macrocytic red blood cells, which can occur with testosterone therapy as larger RBCs accommodate more hemoglobin 3.
• Testosterone therapy commonly causes erythrocytosis, with incidence ranging from 2.8% to 43.8% depending on route of administration (intramuscular injections having highest risk) 1.

Polycythemia Vera Differential

• Elevated vitamin B12 is characteristic of polycythemia vera and is one of the PVSG diagnostic criteria, though it lacks both sensitivity and specificity 1.
• The elevated FSH argues against primary polycythemia vera, as this suggests a testicular or hypothalamic-pituitary issue rather than a primary myeloproliferative disorder.
• PV typically presents with low serum erythropoietin levels (>90% specificity), though sensitivity is only approximately 70% 1.

Essential Workup

Immediate Laboratory Tests

• Serum erythropoietin level is the critical next test: low EPO suggests polycythemia vera (>90% specificity), while normal/elevated EPO with high testosterone suggests secondary erythrocytosis 1.
• JAK2 V617F mutation testing to definitively exclude polycythemia vera, as this mutation is present in >95% of PV cases 1.
• Iron studies including ferritin and soluble transferrin receptor: testosterone increases iron utilization, causing decreased ferritin and hepcidin with increased sTfR 2.
• Repeat vitamin B12 level to confirm elevation, as spurious B12 elevation can occur.

Additional Evaluation

• Medication history is crucial: determine if patient is on exogenous testosterone therapy (injections, gels, patches) or using anabolic steroids 1, 4.
• Assess for secondary causes of elevated testosterone: adrenal or testicular tumors, though elevated FSH makes this less likely.
• Bone marrow examination with cytogenetics if EPO is low and JAK2 is negative, as PV can present with characteristic hypercellularity and panmyelosis 1.

Low Vitamin D Assessment

• The low 25-hydroxy vitamin D requires supplementation but is unrelated to the erythrocytosis 5.
• Initiate empiric vitamin D supplementation without further testing, as routine 25(OH)D screening is not recommended for disease prevention 5.

Management Algorithm

If Testosterone-Induced Erythrocytosis (Most Likely)

Immediate actions based on hematocrit level:

• Hematocrit >54%: This is the traditional threshold requiring intervention, though evidence for this specific cutoff is limited 1, 6.

  • Reduce testosterone dose or switch formulation: transdermal preparations cause less erythrocytosis than intramuscular injections (15.4% vs 43.8% incidence) 1.
  • Consider therapeutic phlebotomy if hematocrit remains >54% despite dose adjustment, though evidence supporting this practice is lacking and may paradoxically increase thrombotic risk through tissue hypoxia and iron depletion 7.
  • Temporary cessation of testosterone may be necessary if hematocrit cannot be controlled 1, 4.

• Monitor hematocrit every 3 months during testosterone therapy, with most changes occurring in the first 3 months 1.

Critical Safety Considerations

Elevated hematocrit above normal range may have grave consequences, particularly in elderly patients, as increased blood viscosity can aggravate coronary, cerebrovascular, or peripheral vascular disease 1.

• Assess cardiovascular risk factors: hypertension, diabetes, smoking, prior thrombotic events 1.
• Screen for conditions that worsen hemoconcentration risk: chronic obstructive pulmonary disease, sleep apnea 1.
• No testosterone-associated thromboembolic events have been reported to date in clinical trials, which is reassuring 1.

If Polycythemia Vera Confirmed

• Therapeutic phlebotomy to maintain hematocrit <45% in men is standard treatment for PV.
• Cytoreductive therapy with hydroxyurea or other agents may be indicated based on thrombotic risk stratification.
• Low-dose aspirin for thrombosis prevention unless contraindicated.

Common Pitfalls to Avoid

• Do not assume normal MCV rules out vitamin B12 issues: sensitivity of elevated MCV for B12 deficiency is only 17-77% depending on population 8.
• Do not use red cell mass measurement routinely: it is costly, rarely changes management, and misses PV patients at the left tail of the distribution 1.
• Do not continue testosterone at same dose if hematocrit >54%: this increases cardiovascular risk through hyperviscosity 1.
• Do not perform therapeutic phlebotomy without considering risks: it may increase thrombotic risk through iron depletion and tissue hypoxia 7.