Iron Deficiency: The Diagnosis You’re Probably Missing (and Why It Matters)

You know the patient.

She’s tired — really tired. Not just “I stayed up too late scrolling” tired, but bone-deep fatigue. Maybe she’s struggling to keep up with work, skipping workouts she used to enjoy, or brushing off brain fog as stress. Her labs? “Normal.” Hemoglobin? Fine.

So we reassure, maybe suggest better sleep, maybe check a TSH and move on.

But here’s the uncomfortable truth: we might be missing one of the most common, most treatable conditions in medicine.

Iron deficiency.

Not anemia. Not yet, anyway. Just iron deficiency, and it matters more than we were trained to think.

Iron Deficiency Isn’t Just About Anemia

Many of us were taught to think of iron deficiency as synonymous with anemia. No anemia? No problem.

That thinking is outdated.

Iron deficiency is a clinical syndrome that precedes anemia — sometimes by a long time. The body prioritizes hemoglobin production over everything else, so by the time anemia shows up, iron stores have already been depleted for a while.

And during that time? Patients feel it.

They may experience:

• Fatigue and decreased exercise tolerance
• Poor sleep or insomnia
• Cognitive changes or “brain fog”
• Mood symptoms, including depression
• Restless legs

This isn’t subtle. This is quality-of-life altering.

And it’s incredibly common. Iron deficiency is the most common nutritional deficiency worldwide, and it disproportionately affects menstruating individuals and adolescents.

So if you’ve ever thought, “I have a lot of patients with these symptoms”… you’re probably right.

The Lab That Matters Most (Hint: It’s Not Iron)

Let’s talk diagnostics, because this is where things often go sideways.

Iron panels can feel… messy. Serum iron, TIBC, transferrin saturation — it’s a lot. And to be honest, most of it isn’t that helpful in isolation.

If you remember one thing, make it this:

Ferritin is your best test.

Ferritin reflects iron stores. And while the “normal” range in your EMR might dip as low as 10–15 ng/mL, that doesn’t mean it’s clinically adequate.

Here’s a more practical framework:

• Ferritin <15 ng/mL: highly specific, but misses many cases
• Ferritin <30 ng/mL: consistent with iron deficiency
• Ferritin <45–50 ng/mL: likely deficiency, especially if symptomatic

In practice, many experts use <50 ng/mL as a threshold when patients have symptoms.

And yes — ferritin is an acute phase reactant. It can be falsely elevated in inflammation.

So in patients with chronic disease, consider:

• Ferritin <100 ng/mL (or <300 with low transferrin saturation <20) as suggestive of deficiency

Or, when in doubt: a therapeutic trial of iron can be diagnostic.

Who Should You Be Thinking About?

Some patients should immediately raise your suspicion.

Iron deficiency is especially common in:

• Menstruating individuals (particularly with heavy menstrual bleeding)
• Adolescents
• Endurance athletes
• Patients with GI conditions (celiac, IBD, H. pylori)
• Those on chronic PPIs or anticoagulants
• Patients post-bariatric surgery

And importantly: you don’t need anemia to justify testing.

If the story fits, check ferritin.

The Workup: Don’t Stop at the Diagnosis

Diagnosing iron deficiency is just step one. The next question is: why?

In premenopausal patients, heavy menstrual bleeding is often the culprit. But don’t assume; that’s how we miss other causes.

Consider:

• Celiac disease screening (even without GI symptoms)
• H. pylori testing
• GI evaluation if red flags are present (age >40, family history, persistent deficiency)

In men and postmenopausal patients, the threshold for GI workup should be lower, because the likelihood of pathology is higher.

Sometimes, “time” is part of the diagnostic process. Treat the suspected cause, replete iron, and reassess.

Treatment: Simple in Theory, Messy in Practice

Once you’ve made the diagnosis, it’s time to treat.

Oral Iron: Still First-Line (Usually)

Oral iron works. It’s cheap, accessible, and effective.

But let’s be honest, it’s also frustrating.

Patients struggle with:

• GI side effects (constipation, nausea)
• Confusing instructions
• Poor adherence

Here’s a simplified approach:

• Use ferrous sulfate or ferrous gluconate
• Dose once daily or every other day (better tolerated, similar efficacy)
• Take separately from calcium, coffee, and tea (these reduce absorption significantly)
• Taking with meat can actually improve absorption
• Vitamin C? Optional — it may help, but isn’t essential

One memorable takeaway from hematology: “Once a day, at dinner, with a meaty meal.” A bacon-wrapped iron pill is the way to go!

Simple enough to stick.

Expect:

• Hemoglobin rise of ~1 g/dL in 2–4 weeks
• Several months of therapy to fully replete stores

IV Iron: Not as Scary as You Think

If oral iron is the slow road, IV iron is the express lane.

And yet, many clinicians hesitate. That hesitation is understandable, but often outdated.

IV iron is:

• Safe (serious reactions are extremely rare if not nonexistent)
• Effective (rapid repletion)
• Appropriate in many common scenarios

The preferred formulation is low molecular weight iron dextran (INFeD). Give 1g in a single infusion — this is the fastest and cheapest method. INFeD is preferred over Injectafer (may cause hypophosphatemia) or Ferumoxytol (can cause artifacts on MRI). Following the infusion, you want to recheck a CBC and iron panel in about 6 weeks.

Consider IV iron for:

• Intolerance to oral iron
• Ongoing blood loss (e.g., heavy menses)
• Malabsorption (e.g., bariatric surgery, IBD)
• Significant symptoms or need for rapid correction

And the infamous “infusion reactions”?

With newer formulations, these reactions are not allergic. The “Fishbane reaction” (transient flushing, myalgias related to the complement pathway, not anaphylaxis) occurs in ~1% of patients, resolves quickly, and does not require epinephrine or antihistamines. Stop the infusion, wait 15 minutes, and restart at a slower rate. In fact, premedicating with antihistamines may make things worse, so do not premedicate despite what your EHR order sets may want you to believe!

Why This Matters (More Than You Think)

Iron deficiency is easy to miss and easy to dismiss.

But when we catch it early, we can:

• Improve energy, cognition, and mood
• Enhance physical performance
• Prevent progression to anemia
• Meaningfully improve quality of life

And maybe just as importantly, we validate our patients’ experiences.

That patient who felt brushed off?
Who was told everything was “normal”?

Sometimes, the most impactful thing we can say is:

“I think I know what’s going on — and we can treat it.”

Takeaways You Can Use Tomorrow

If you’re walking into clinic and want a quick mental checklist, here it is:

• Think of iron deficiency as a syndrome, not just anemia
• Check ferritin early, don’t wait for hemoglobin to drop
• Use <50 ng/mL as a practical threshold in symptomatic patients
• Treat with oral iron first, but don’t hesitate to escalate
• Consider the underlying cause every time
  

Iron deficiency is bread-and-butter primary care, but it’s also one of those topics that’s easy to underestimate.

Once you start looking for it, you’ll find it. And when you treat it well, the impact can be profound.

And that’s the kind of medicine that keeps us coming back.

To hear more on this topic, subscribe and listen to our episode, "Don't Get Rusty: Iron Deficiency Diagnosis"  on Primary Care Reviews and Perspectives podcast or listen on The Monthly Rounds on Spotify or Apple Podcasts.