The most common causes of fatigue in women
Iron deficiency without anaemia (ferritin under ~30) is wildly common in menstruating women and a top cause of fatigue, brain fog and exercise intolerance. Subclinical hypothyroidism is the second-most common — and routinely missed when only TSH is tested. Perimenopause, with disrupted sleep from night sweats and falling progesterone, is the third. HPA-axis dysregulation (high or flat cortisol) is the fourth.
What to test
Ferritin, full blood count, TSH with free T4 and free T3 and TPO antibodies, morning cortisol with DHEA-S, vitamin D, B12, HbA1c, and — if you're over 38 — estradiol, FSH and progesterone. A celiac screen and fasting iron studies are sensible add-ons if symptoms are severe.
Reading the results in context
'Normal' often means 'not deficient enough to flag' — not 'optimal for energy'. Ferritin 30 may technically be in range but most women feel meaningfully better above 70. TSH 3.5 is in range but symptomatic — most thyroid-aware doctors treat to 1–2 in symptomatic women.
What to do next
Replete iron if ferritin is low — by mouth for mild cases, infusion for severe or malabsorptive ones. Treat thyroid disease properly (not just TSH). Address sleep, perimenopause symptoms and stress recovery in parallel — fixing one without the others rarely holds.
HPA-axis dysregulation — the 'wired but tired' picture that overlaps perimenopause and thyroid disease.
Frequently asked
The most common single cause in menstruating women is low ferritin (iron stores). The most common single cause from late 30s onward is thyroid disease or perimenopause. A blood panel resolves which.
Yes — ferritin can be low enough to cause symptoms (fatigue, hair shedding, restless legs, brain fog) long before haemoglobin drops into anaemia range.
Most doctors aim for ferritin 70–100 ng/mL for energy and hair, not just the lab minimum of 15–30. 'In range' is not the same as optimal.
Yes. Free T3 can be low and TPO antibodies can be positive with a normal TSH — both worth treating in a symptomatic woman.
Notoriously. Night sweats, low progesterone (which is sedating) and HPA-axis disruption combine to wreck sleep quality and daytime energy.
If it's affecting work, relationships or driving safety — and especially if it's accompanied by weight change, breathlessness, bleeding changes or low mood — see a doctor within weeks, not months.