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The blood markers that quietly drain your energy

6 min read

You sleep enough. You train. And you are still tired. Often the answer is in your blood work — in markers that your doctor calls "normal" because they fall inside a reference range built to catch disease, not to optimise energy. Here are four of the most common culprits.

1. Ferritin (iron stores)

Ferritin can sit at 20 ng/mL — technically "normal" — while leaving you exhausted, short of breath on climbs, and unable to build fitness. Functional-medicine targets for energy and performance sit closer to 70–150. Low ferritin is one of the most common and most missed causes of fatigue, especially in women and endurance athletes.

2. Vitamin D

Roughly 40% of adults are deficient. Low vitamin D impairs muscle recovery, immune function, testosterone, and mood. "Normal" labs often start at 20 ng/mL; the optimal window is 40–60. Most people who live indoors need year-round supplementation with D3 plus K2 to hold it there.

3. Thyroid (TSH)

A TSH above 2.5 mIU/L — still inside most lab ranges — can mean your thyroid is quietly underperforming, bringing fatigue, cold intolerance, weight gain, and stubbornly high cholesterol. It is worth a fuller thyroid panel before you blame yourself for being tired.

4. Vitamin B12

B12 below 400 pg/mL can cause fatigue, brain fog, and poor nerve function even when flagged "normal" (cutoffs are often set near 200). Vegetarians, vegans, and anyone on metformin or acid blockers are especially at risk.

The pattern

None of these show up on your watch. All of them shape how you feel every day. The problem is that a lab report is a wall of numbers with no interpretation — so the signal gets lost.

Urja fixes that: import your lab PDF or sync from Apple Health, and it checks each marker against these functional ranges, tells you in plain language which ones are costing you energy, tracks them over time, and — importantly — reminds you to re-test when results are over a year old, because stale blood work should never drive today's decisions. It even folds your ferritin, vitamin D, thyroid, and B12 into your daily energy picture, so the dots finally connect.