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If your energy changed somewhere in your mid-thirties and never came back, you are describing something extremely common — and something that usually has an identifiable cause.
That last part matters, because most content on this topic goes straight to a product. We are going to do the opposite and start with what to rule out, because the causes below are common, treatable, and completely invisible to any supplement. Getting checked costs you one appointment. Not getting checked can cost you years.
The Tests Worth Asking For
Bring this list to your appointment. Doctors have limited time, and arriving with specifics changes the conversation.
- Full blood count and ferritin. Ferritin is the one to insist on — see the section below on why. Iron deficiency is the single most common cause of fatigue in menstruating women.
- Thyroid function (TSH, and free T4 if TSH is borderline). Hypothyroidism is several times more common in women than men and frequently presents as fatigue, cold intolerance, weight change and low mood.
- Vitamin B12 and folate. Deficiency causes fatigue and cognitive fog, and is more likely if you eat little meat, take metformin, or use long-term acid-reducing medication.
- Vitamin D. Widespread deficiency at northern latitudes, and correcting a genuine deficiency can make a real difference.
- HbA1c or fasting glucose. Fatigue is often the first noticeable symptom of impaired glucose handling.
- Coeliac screen if you have any digestive symptoms, unexplained anaemia, or a family history.
That set covers the large majority of common, correctable causes. If everything genuinely comes back clear, that is useful information rather than a dead end — it moves the search to sleep, hormones and mood.
Why “Your Bloods Are Normal” Can Still Be the Answer
This is the most useful thing in this article, and it is the reason so many women conclude that medicine has nothing to offer them.
Ask for the actual numbers, not the verdict. “Normal” means “inside the reference range,” and reference ranges are population statistics, not health targets.
Ferritin is the clearest example. You can have a completely normal haemoglobin — no anaemia at all — while your iron stores are almost empty. Many laboratories only flag ferritin below roughly 15 µg/L, yet there is substantial clinical opinion that fatigue symptoms occur well above that threshold, with many practitioners looking for levels above 30–50 µg/L before considering iron stores adequate in a symptomatic woman.
So a result of 18 gets reported as normal, and a genuinely iron-depleted woman is told nothing is wrong. If your ferritin is low-normal and you are exhausted, that is worth a specific conversation rather than acceptance.
The same principle applies to a TSH sitting at the top of the range, or a B12 at the bottom of it. Get your numbers, keep them, and track them over time.
Perimenopause Starts Earlier Than Most Women Expect
Perimenopause — the transition before periods stop — commonly begins in the early-to-mid forties, but can start in the late thirties. It typically lasts several years, and the average age of menopause itself is around 51.
Fatigue during this phase is not vague or imagined. It has concrete mechanisms:
- Sleep fragmentation. Night sweats and hormonal shifts break up sleep architecture. You can spend eight hours in bed and get very little restorative sleep.
- Heavier, more erratic bleeding, which depletes iron — which is exactly why ferritin belongs on the test list above. These two causes compound each other and frequently arrive together.
- Mood and anxiety changes, which independently drain energy.
Worth knowing: perimenopause is diagnosed largely on symptoms and cycle changes rather than a blood test, because hormone levels fluctuate wildly month to month during the transition. A single “normal” hormone panel does not rule it out. If your cycles have changed in length, flow or predictability, say so explicitly — it is often the detail that unlocks the diagnosis.
Sleep Apnoea Looks Different in Women
Obstructive sleep apnoea is substantially under-diagnosed in women, in large part because the stereotype is an overweight middle-aged man who snores loudly.
In women it more often presents as daytime fatigue, insomnia, morning headaches, low mood or brain fog — symptoms that get attributed to stress, depression or hormones instead. It also becomes more common after menopause, and it occurs in women who are not overweight.
If you wake unrefreshed after adequate hours, wake with a dry mouth or headache, or anyone has mentioned that you snore or pause in your breathing, ask specifically about a sleep study. Treatment is genuinely transformative for people who need it.
Depression and Anxiety Cause Physical Exhaustion
Fatigue is a core diagnostic symptom of depression, not a side note. Anxiety is metabolically expensive in a way people consistently underestimate — a nervous system held in low-grade alert all day is genuinely tiring.
This deserves saying plainly: if that is the cause, it is a medical explanation, not a personal failing, and it responds to treatment. It is also frequently missed in women who are functioning — holding down a job, running a household — because functioning is mistaken for being well.
What Actually Helps Once Causes Are Ruled Out
If the investigations come back clear, the levers with real evidence behind them are unglamorous and effective:
- Regular aerobic movement. Counterintuitive when you are exhausted, but exercise is the best-evidenced intervention for both fatigue and mitochondrial capacity. Start smaller than feels worthwhile — ten minutes daily beats an ambitious plan you abandon.
- Protecting sleep timing, which matters as much as duration. A consistent wake time is the strongest single anchor.
- Eating enough. Under-fuelling is a genuinely common and overlooked cause of fatigue in women, particularly those cycling through restrictive diets. Persistent low energy availability disrupts hormones, mood and metabolic function.
- Protein and iron-containing foods at adequate levels, especially if you menstruate heavily.
- Caffeine timing. Not elimination — just keeping it out of the eight hours before bed, since it degrades sleep quality even when it does not stop you falling asleep.
Where a Supplement Honestly Fits
Last, and only after the above.
If a specific deficiency is found, the correct supplement is the one that corrects it — iron, B12, vitamin D — ideally at a dose your doctor specifies and with follow-up testing. That is where supplementation has the strongest evidence by a wide margin.
Beyond that, botanical formulas marketed for mitochondrial support contain ingredients with early and mostly preliminary research. Rhodiola has the most credible human data for stress-related fatigue, and the effects observed are modest. These products may add something at the margin for someone whose foundations are already in place. They will not correct iron deficiency, treat a thyroid disorder, resolve sleep apnoea or substitute for sleep — and any marketing implying otherwise is the reason to distrust it.
Frequently Asked Questions
My doctor said I’m fine but I still feel terrible. What now?
Request a printed copy of your results and look at the actual figures, particularly ferritin, TSH and B12. Low-normal is not the same as optimal in a symptomatic person. If the numbers genuinely look good, ask specifically about sleep assessment and perimenopause, which bloodwork does not capture well. Asking for a second opinion is reasonable and not rude.
Should I just take iron to be safe?
No. Test first. Excess iron accumulates and causes harm, and some people carry haemochromatosis without knowing. Iron is one supplement that genuinely should not be taken speculatively.
How long should I give a change before judging it?
Iron repletion typically takes around three months to restore stores even after blood counts normalise. Thyroid treatment needs six to eight weeks to assess. Exercise adaptations show within weeks. Almost nothing here works in days, which is precisely why anything promising a transformation in a week deserves suspicion.
Is it just normal ageing?
Energy does decline gradually with age. A distinct change you can date — “I was fine until about two years ago” — is not typical ageing and warrants investigation.
The Bottom Line
You are not imagining it, and you are not lacking willpower. But the most likely explanations are things a blood test and an honest conversation can find — not something invisible that only a product can reach.
Book the appointment. Take the list. Get the numbers. Then, if everything is clear and your sleep and movement are handled, look at whether a supplement adds anything for you.
If you have already been through that process and want to assess the supplement side properly, our detailed ingredient breakdown is here:
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Related reading: The “Mitochondria Hack” Explained · How to Balance Your Diet with Superfoods
Disclaimer: This content is for informational purposes only and is not medical advice, and the author is not a licensed clinician. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. Always consult your healthcare provider about persistent fatigue and before starting any new supplement.
Written by Julia Wayne, Nutrition & Supplement Research Specialist at BioBoost Living.