What is the differential diagnosis for a 78-year-old female with a history of long-term corticosteroid (cortical steroid) use for an autoimmune disease, presenting with anemia (low red blood cell count), macrocytosis (high Mean Corpuscular Volume (MCV)), normal hemoglobin, normal hematocrit, and symptoms of burning pain and tingling in her feet at night, with laboratory results showing a low normal Thyroid-Stimulating Hormone (TSH) level, low normal free Thyroxine (T4) level, and low normal free Triiodothyronine (T3) level?

Differential Diagnosis

The patient's presentation is complex, with multiple factors to consider, including long-term cortical steroid use, abnormal blood cell counts, and symptoms of burning pain and tingling in the feet. Here is a categorized differential diagnosis:

• Single Most Likely Diagnosis

  • Vitamin B12 Deficiency: The patient's high MCV (mean corpuscular volume) at 106.9 suggests a macrocytic anemia, which is often associated with vitamin B12 or folate deficiency. The symptoms of burning pain and tingling in the feet at night are also consistent with vitamin B12 deficiency, which can cause neuropathy.

• Other Likely Diagnoses

  • Folate Deficiency: Similar to vitamin B12 deficiency, folate deficiency can cause macrocytic anemia, which is suggested by the patient's high MCV.
  • Steroid-Induced Changes: Long-term cortical steroid use can affect blood cell counts and may contribute to the patient's anemia and other laboratory abnormalities.
  • Anemia of Chronic Disease: Although the patient's CRP and ESR are normal, anemia of chronic disease is still possible, especially given her autoimmune disease and long-term steroid use.

• Do Not Miss Diagnoses

  • Myeloproliferative Neoplasm (MPN): Although less likely, MPN can cause an elevated platelet count and abnormal blood cell counts. It is essential to rule out this diagnosis due to its potential for serious complications.
  • Hypothyroidism: Although the patient's TSH is low normal, her free T4 and free T3 are also low normal. Hypothyroidism can cause macrocytic anemia and neuropathic symptoms, making it a crucial diagnosis not to miss.

• Rare Diagnoses

  • Copper Deficiency: Copper deficiency can cause macrocytic anemia and neuropathy, although it is a rare condition.
  • Orotic Aciduria: This is a rare genetic disorder that can cause macrocytic anemia, although it is unlikely given the patient's age and presentation.
  • Acquired Sideroblastic Anemia: This is a rare condition that can cause anemia and abnormal blood cell counts, although it is less likely given the patient's laboratory results.