Why am I always tired? Fatigue in women, explained
Tiredness in women is often a fixable nutrient or hormone problem. The common causes of fatigue, the tests that find them, and when to see a doctor.
Reviewed by Emma Carter, Senior Health & Nutrition Editor
Feeling tired now and then is normal. Feeling exhausted week after week, no matter how much you sleep, is not, and it is one of the most common reasons women see their doctor. The good news: much of the time, persistent fatigue in women traces back to something specific and treatable.
This guide is the diagnostic one: what causes fatigue in women, how to tell the causes apart, and which tests to ask for. If your energy dips are more everyday than exhausting, our guide to low energy covers the sleep, food and stress side. If you already know what you are dealing with and want a plan, see how to have more energy.
The usual causes, and how to tell them apart
Women are prone to a handful of hidden causes that build up slowly. The pattern of your other symptoms often points to which one to check first.
| Likely cause | Other clues | The test |
|---|---|---|
| Iron deficiency | Heavy periods, hair shedding, breathlessness, looking pale | Ferritin |
| Underactive thyroid | Weight gain, feeling cold, dry skin, low mood | TSH and free T4 |
| Low vitamin B12 | Tingling, sore tongue, a plant-based diet, taking metformin | B12, with folate |
| Low vitamin D | Low mood, aches, little sun exposure | Vitamin D |
| Sleep, stress, blood sugar | Worse after poor sleep or skipped, carb-heavy meals | None; lifestyle first |
Iron is the one to suspect first in menstruating women, because periods cause a monthly loss and low stores leave your blood carrying less oxygen. Ask for ferritin by name rather than relying on a standard blood count: anemia is a late sign of iron deficiency, so stores can be low while a full blood count still reads normal (Clinical Medicine, 2021). Our guides to iron deficiency and the ferritin test explain how it is found. An underactive thyroid slows everything down, and low B12 is easy to miss on a plant-based diet or with age.
The tests worth asking for
If tiredness has lasted more than a couple of weeks, a blood panel is the fastest route to an answer. A reasonable starting set:
- Complete blood count, to check for anemia
- Ferritin, your iron stores
- Thyroid function: TSH and free T4
- Vitamin B12 and folate
- Vitamin D
Ask your doctor to read the results together. A normal result on one marker does not rule out a problem on another.
What actually helps
The fix depends entirely on the cause:
- If iron is low, more dietary iron plus a supplement if your doctor advises one, rechecked after a few months. Our iron guide covers gentler forms.
- If your thyroid is underactive, this is treated with medication, not supplements.
- If B12 or vitamin D are low, targeted supplementation guided by your levels.
- For everyone, protein at each meal, enough fiber, consistent sleep and movement give your energy a stable base.
If your bloods come back clear, the cause is often everyday habits. Our low energy guide covers the sleep, blood-sugar and stress side.
When to see a doctor
Book an appointment if fatigue lasts beyond two weeks despite rest, or straight away if it comes with:
- Heavy periods
- Breathlessness or heart palpitations
- Noticeable hair loss
- Unexplained weight change
These point to causes worth checking promptly.
Frequently asked questions
What is the most common cause of fatigue in women?
Iron deficiency is one of the most common and most missed causes, largely because of monthly blood loss from periods. Low thyroid function, low vitamin B12 or vitamin D, poor sleep, and stress are also frequent contributors. A simple blood panel can identify most of these.
What blood tests should I ask for if I'm always tired?
A sensible starting panel includes a complete blood count, ferritin (iron stores), thyroid function (TSH and free T4), vitamin B12, folate and vitamin D. Your doctor may add others based on your history.
When should I see a doctor about tiredness?
See a doctor if fatigue lasts more than two weeks despite adequate rest, or sooner if it comes with heavy periods, breathlessness, heart palpitations, noticeable hair loss, or unexplained weight change.
Can supplements fix tiredness?
Supplements only help when they correct a genuine deficiency, for example, iron for confirmed iron deficiency. Taking them blindly rarely helps and can occasionally cause harm, so it's best to test first.
References
- Iron deficiency without anaemia: a diagnosis that matters · Al-Naseem A, Clinical Medicine (London), 2021;21(2):107-113
- Fatigue · Mayo Clinic
- Tiredness and fatigue · NHS
- Underactive thyroid (hypothyroidism) · NHS