What 'energy' actually is
Energy in the everyday sense — your felt vitality and focus — is the net result of three systems working together: oxygen transport (red cells and iron), metabolic burn (thyroid, blood sugar, mitochondria) and recovery (sleep and autonomic balance). A shortfall in any of the three can show up as 'just tired', while the cause is usually possible to pin down.
The body is surprisingly good at compensating — until it isn't. People with low ferritin or a sluggish thyroid often feel 'a little off' for years before their numbers are abnormal enough for a GP to flag.[1,4]
Iron status and ferritin
Iron is required to carry oxygen in the blood and to produce energy inside cells. A shortfall usually starts as iron depletion (low ferritin without anaemia) and ends, untreated, in iron-deficiency anaemia with a low Hb. Fatigue, reduced exercise capacity and brain fog tend to appear in that first stage — before a routine haemoglobin test raises an alarm.[1,2]
Ferritin is therefore the most sensitive early marker. For premenopausal women, athletes, vegetarians and people with heavy menstrual bleeding, ferritin is the first thing to measure when fatigue is structural. Many guidelines treat a value below ~30 µg/L as suboptimal even in the absence of anaemia.1
Thyroid and metabolic pace
The thyroid sets the tempo at which your body runs. An underactive thyroid (hypothyroidism) typically causes fatigue, cold intolerance, weight gain and sluggishness; an overactive thyroid (hyperthyroidism) causes palpitations, weight loss and inner restlessness. A large fraction of symptomatic people have a subclinical form, with mildly abnormal TSH but normal fT4 — easy to miss without a targeted test.4
TSH (first-line) and, if TSH is off, free T4 are the standard measurements. At Optimize both are part of the Baseline and our triage system automatically flags values that fall outside optimal ranges.
Vitamin B12 and vitamin D
Vitamin B12 deficiency causes fatigue, brain fog, tingling in the hands and feet and — if untreated — neurological damage. At-risk groups: vegetarians and vegans, people over 60, and users of metformin or proton-pump inhibitors. When B12 is borderline, methylmalonic acid (MMA) is a more sensitive confirmation than B12 alone.3
Vitamin D deficiency is widespread in northern latitudes — especially in winter — and is associated with fatigue, muscle weakness and higher infection risk.5 Supplementation is cheap, safe and effective for confirmed deficiency; the standard test is serum 25-hydroxyvitamin D.
Sleep, blood sugar and stress
Two underrated energy drains that don't show up as a 'deficiency': chronic sleep debt and glucose instability. A few short nights measurably lower insulin sensitivity and cognitive performance — over weeks it accumulates into what people describe as general fatigue.6
Blood sugar that swings sharply — typically from refined carbohydrates without protein or fibre — produces an energy crash 90–120 minutes after eating. HbA1c reflects the average over three months and is useful when post-meal slumps are persistent. Chronic stress raises cortisol, disturbs sleep and tightens this cycle — so 'just tired' rarely rests on a single lever.
What to measure, what to do
For persistent fatigue, a baseline blood panel that includes ferritin, Hb, TSH, vitamin B12, vitamin D and HbA1c is the highest-yield first step — it covers around 80% of treatable causes. The Optimize Baseline measures all of these and reads them against optimal ranges instead of just 'normal'.
Practical levers alongside the bloodwork: consistent sleep timing (see the Sleep guide), protein with every meal, regular resistance training (improves insulin sensitivity and mitochondrial function),[7,8] morning daylight, and taking chronic stress seriously (see the Stress guide). Fixing deficits works — but only brings you back to baseline; the lifestyle layer decides whether you stay there.
When to involve a physician
Book a GP appointment when fatigue persists for more than six weeks despite good sleep, with unintended weight loss, heavy menstrual bleeding, palpitations or breathlessness with mild exertion, or clearly abnormal blood values. At Optimize every result is reviewed by our physician and we reach out proactively when something deserves attention.