It can feel ridiculous to go to the doctor complaining of fatigue. You’re busy. Work, kids, life — of course, you’re tired. With a teething baby keeping me up this week, I feel you.
Differentiating “I need more sleep” from “something is actually wrong” is hard enough. And telling your doctor there’s a problem when you’re also running on four hours a night feels laughable.
Nevertheless, fatigue is often the first sign of something that becomes a much bigger problem if ignored.
I talk a lot about iron here because it’s so common in women, and doctors usually suspect it first. But there’s another micronutrient that starts with almost identical symptoms: vitamin B12.
And while iron gets all the attention, B12 is the sneaky one that can cause irreversible neurological damage if left untreated long enough.
The (endlessly) frustrating part? Many doctors test iron, see that hemoglobin levels are “normal” and stop there. Unless you’re willing to push (hi, me 🙋🏻♀️), they won’t dig further.
If you’re dealing with inexplicable fatigue and on the fence about testing, here’s a simple framework to figure out what might be worth asking for.
A simple 5-step decision tree to start
Check symptoms in this order to narrow down what might be going on:
STEP 1: Do you have neurological symptoms?
This is the emergency step. Check for:
Tingling or “pins and needles” in hands and/or feet that’s there when you wake up and has lasted weeks.
Balance test: Stand on one foot with eyes closed for 10 seconds. Wobbly?
Forgetting words mid-sentence regularly? I’m talking like multiple times a day.
Dropping things, trouble with buttons, stumbling more than usual?
If YES, B12 deficiency is likely and go get tested (see Step 5).
STEP 2: Check your mouth and tongue
Look in the mirror. Do you have:
A “beefy red” or unusually smooth tongue
A sore tongue
Cracks at the corner of your mouth
If YES, B12 deficiency looks likely. Move to Step 4 to check your risk factors, then get tested.
STEP 3: Do you have classic iron deficiency signs?
If you have three or more of these, iron is the likely culprit:
Very pale skin, pale inner eyelids, pale nail beds
Cold hands and feet constantly
Breathless walking upstairs or carrying groceries
Overwhelming fatigue — barely functional, not just tired
Ice cravings or craving dirt, clay, paper
Hair loss, hair shedding in the shower
Brittle nails or nails that curve upward
If YES (3+ symptoms), iron deficiency is likely. Move to Step 4 to check your risk factors, then get tested.
STEP 4: Check your diet and risk factors
Your lifestyle and medications can lean you towards one deficiency or another. Here’s what to consider:
Lean toward B12 if:
You’re vegan or vegetarian and don’t already take B12 supplements.
You take PPIs (omeprazole, pantoprazole) or metformin.
You have gut issues like celiac, Crohn’s, or IBS.
You’re over 50 (stomach acid decreases with age, which tanks B12 absorption).
Lean toward iron if:
You have heavy periods.
You eat little to no red meat.
You eat a primarily plant-based diet.
You take NSAIDs regularly (ibuprofen, naproxen) because they can cause GI bleeding.
You are pregnant, breastfeeding, or recently postpartum.
Consider BOTH if:
You have a history of gut disorders, which block absorption of everything.
You take multiple medications.
You have autoimmune conditions.
STEP 5: What to actually ask your doctor for
This is where you get real answers. Here’s what to ask your doctor for:
If you suspect B12:
Serum B12
MMA (methylmalonic acid)
Homocysteine
CBC to check for macrocytic anemia (large red blood cells)
What you’re looking for: B12 below 400 pg/mL, elevated MMA or homocysteine. Even if your serum B12 looks “normal,” elevated MMA/homocysteine means you have a functional deficiency.
Iron:
Full iron panel: ferritin, serum iron, TIBC, transferrin saturation
CBC to check hemoglobin, hematocrit, and MCV (small red blood cells)
What you’re looking for: Ferritin below 50 ng/mL. Labs call 15 ng/mL “normal,” but multiple reports suggest that you need 50-100 ng/mL to actually feel good.
What to do
Get tested, and don’t settle for “normal.”
The only way to get definitive answers for overwhelming fatigue is to get tested. And here’s the important part: don’t accept “normal” if you still feel terrible.
Why “normal” isn’t good enough.
Reference ranges are built for populations, not individuals. Your body might need ferritin at 70 ng/mL (or more!) to feel normal, even though the lab notes anything above 15 as “fine.”
Same with B12. Some people feel awful at 300 pg/mL even though it’s technically in range.
What to do if your labs say “normal”:
Ask for your actual numbers.
Compare them to optimal ranges.
Trust yourself. You know your body best!
