Chronic Fatigue and Nutrient Deficiency: What Blood Tests to Ask For

You're getting eight hours. You're sleeping well. And yet you wake up already exhausted. Your GP says you're fine but you're not fine. You're bone-deep tired. The kind of tired that doesn't shift with a weekend off.

If that's your life right now, the problem may not be in your head. It's in your blood.

Why standard GP tests miss nutrient deficiency

Most people who walk into their GP's surgery complaining of chronic fatigue get a basic blood test. Full blood count. Thyroid-stimulating hormone. Maybe a glucose test. The results come back normal and you're left confused. How can everything look fine when you feel this broken?

The problem is that normal, from a standard GP test, often means "within the population reference range." That range is wide. You might be at the lower end of normal for B12, for example, and still be functionally deficient. You might have ferritin levels that technically pass the threshold but are too low for energy production. Standard tests are designed to catch severe deficiency. They're not designed to optimise your health.

Here's the uncomfortable truth: chronic fatigue is almost always multifactorial. It's never just one thing. But nutrient deficiency is almost always part of it. The question is, which nutrients are depleting your energy?

Your energy depends on a complex interplay of nutrients. Iron carries oxygen to your cells. B vitamins convert food into usable energy. Vitamin D regulates immune function and mood. Thyroid hormones set your metabolic rate. If any one of these is out of balance, you're exhausted. If several are out of balance simultaneously, you're exhausted and confused because nobody can figure out why.

Ferritin, iron and energy production

Iron is the mineral that carries oxygen through your bloodstream. Without it, your cells simply cannot generate ATP, the energy currency your body runs on.¹ When iron is low, everything slows down. Mitochondrial function drops. Your brain feels foggy. Your muscles feel heavy.

Most GP tests check serum iron, which is useful but incomplete. What you actually need to know is your ferritin level. Ferritin is the storage form of iron, and it's the real measure of your iron reserves. A normal ferritin range is typically 15 to 200 nanograms per millilitre for women, but research suggests that for energy production and thyroid function, you want to be above 50. Many chronically fatigued people sit at 20 to 35, technically normal but functionally depleted.

This is particularly important if you're a woman. Women lose iron every month through menstruation. If your periods are heavy, if you've been on a plant-based diet without adequate supplementation, or if you have ongoing digestive inflammation that impairs iron absorption, your ferritin will be low. The fatigue that follows is profound and often blamed on depression or laziness when it's simply iron deficiency.

Ask your GP specifically for serum iron, iron saturation, total iron-binding capacity, and ferritin. Don't accept just ferritin alone.

Rebuilding iron stores takes time. If your ferritin is below 30, expect three to six months of consistent dietary iron intake or supplementation before you see real energy improvements. If your ferritin is between 30 and 50, you might see improvements within six to twelve weeks. But the timeline matters. You have to be consistent.

B12 and folate: the energy twins

B12 is essential for nervous system function, energy production, and DNA synthesis.² It's found almost exclusively in animal foods, and the richest sources are organ meats, followed by red meat, fish, eggs, and dairy. When B12 is low, fatigue is often paired with brain fog, poor memory, and sometimes tingling in the fingers or toes.

The problem is that B12 deficiency can be slow. It can creep up on you. You might have a serum B12 level that looks normal but functional deficiency because you're not absorbing it properly or you're not carrying enough of it in the right form.

Ask for both serum B12 and methylmalonic acid (MMA). MMA is a more sensitive marker.² If your B12 is low-normal but MMA is raised, you have functional B12 deficiency even if standard tests say you're fine. This distinction matters because it explains why you're exhausted even though your B12 level technically looks acceptable.

Folate works hand in glove with B12. Both are involved in methylation, the process that keeps energy metabolism running smoothly. Low folate shows up as fatigue, poor concentration, and sometimes depression. Ask for red blood cell folate, not serum folate, as it's a better marker of tissue levels.

If both B12 and folate are low, your energy production is compromised at a fundamental level. You can't make neurotransmitters efficiently. You can't complete your energy-production cycles. Your mitochondria are starving for the nutrients they need to work.

Serum B12, methylmalonic acid, and red blood cell folate are three tests that often reveal the energy gap between normal and how you actually feel.

Vitamin D and chronic fatigue

This one is especially important if you live in the UK, where the sun barely achieves the angle needed for vitamin D synthesis for much of the year. Vitamin D isn't just about bone health. It's deeply involved in immune function, mood regulation, and cellular energy production.

Chronically fatigued people are frequently deficient in vitamin D. The optimal range is debated, but research suggests most people feel better with levels above 40 nanograms per millilitre.³ Many people are walking around at 20 to 30, which technically doesn't qualify as deficiency but is often enough to tank mood and energy simultaneously.

Ask for 25-hydroxyvitamin D, which is the main circulating form and the best indicator of your vitamin D status. If your level is below 30, supplementation is necessary. If it's between 30 and 50, you're in the grey zone. Some people feel okay. Others are exhausted. Testing and correcting this is worth doing.

Vitamin D is fat-soluble, so it requires fat for absorption. If you're supplementing vitamin D without eating adequate fat, you're wasting your money. Take it with food that contains fat. Butter, olive oil, eggs, meat. This matters.