Low energy in women most often traces back to one or more treatable causes: iron levels that look “normal” on paper but are actually too low, thyroid function on the slower end, hormonal shifts across the menstrual cycle or into perimenopause, poor sleep quality, or the cumulative toll of chronic stress. The good news is that identifying the right cause usually leads to noticeable improvement. Here’s what to investigate first.
Iron Deficiency Without Anemia
This is the single most overlooked cause of fatigue in women. Standard blood work checks your hemoglobin to see if you’re anemic, but you can have perfectly normal hemoglobin and still be running on depleted iron stores. The key number is ferritin, a protein that reflects how much iron your body has in reserve. Most lab reports flag ferritin as “low” only when it drops below 12 or 15 ng/mL, but growing evidence points to 50 ng/mL as the threshold where the body actually functions well. Three clinical studies have shown that giving iron to women whose ferritin was below 50 ng/mL, even with normal blood counts, significantly improved fatigue.
If you menstruate, you lose iron every month. Heavy periods accelerate the problem. So do plant-based diets that lack easily absorbed forms of iron. The fix can be straightforward: once iron stores are replenished, many women notice better exercise tolerance and sharper concentration within weeks. If your doctor has only checked your hemoglobin, ask specifically for a ferritin level and discuss whether yours is truly optimal, not just within the lab’s printed range.
Thyroid Function on the Slow Side
Your thyroid gland sets the pace for your metabolism, and when it underperforms even slightly, fatigue is often the first symptom. The standard screening test measures TSH (thyroid-stimulating hormone). Most labs consider anything between roughly 0.5 and 4.5 mU/L “normal,” but the median TSH in healthy populations falls between 1.0 and 1.5 mU/L. Up to 30% of people with a TSH above 3.0 mU/L turn out to have early autoimmune thyroid disease when tested further.
Women are far more likely than men to develop thyroid problems, especially during their 30s and 40s. Symptoms beyond fatigue include feeling cold easily, constipation, dry skin, thinning hair, and weight gain that doesn’t respond to diet changes. If your TSH sits in the upper half of the “normal” range and you have several of these symptoms, it’s worth discussing thyroid antibody testing with your provider to check for an autoimmune process like Hashimoto’s thyroiditis.
Menstrual Cycle Energy Shifts
If your energy dips predictably each month, your hormones are a likely explanation. The menstrual cycle divides into two main halves: the follicular phase (from your period through ovulation) and the luteal phase (from ovulation until your next period). Estrogen and progesterone are both higher during the luteal phase, and research shows that mental fatigue increases significantly during this window, even in women without anxiety or mood disorders. Physical fatigue tends to stay more stable across the cycle, but that fog-like mental exhaustion in the week or two before your period is a real, measurable phenomenon.
Tracking your energy alongside your cycle for two or three months can reveal whether this pattern applies to you. If it does, adjusting your schedule to protect demanding cognitive work for your higher-energy follicular phase, and building in more rest during the luteal phase, can make a real difference in how you feel day to day.
Insulin Resistance and Blood Sugar Crashes
Insulin resistance means your cells don’t respond efficiently to insulin, so your body pumps out more of it to keep blood sugar in check. The result is a rollercoaster pattern: blood sugar spikes after meals, followed by crashes that leave you foggy, shaky, or desperate for a nap. This affects the liver, fat tissue, and muscles simultaneously.
Women with polycystic ovary syndrome (PCOS) are especially vulnerable. Research shows that even lean women with PCOS have significantly higher insulin levels two hours after eating compared to women without the condition. But you don’t need a PCOS diagnosis for insulin resistance to affect you. Signs include intense post-meal fatigue, sugar cravings that worsen in the afternoon, weight that clusters around your midsection, and skin tags or patches of darkened skin on the neck or underarms. A fasting insulin test or a glucose tolerance test with insulin levels can help identify the problem, since a standard fasting glucose test often comes back normal in the early stages.
Perimenopause Starts Earlier Than You Think
Perimenopause, the transition period before menopause, typically begins in the mid-40s but can start as early as the mid-30s. It lasts eight to ten years on average. During this time, estrogen levels don’t just steadily decline; they fluctuate unpredictably, sometimes spiking higher than normal before dropping sharply. This hormonal instability disrupts sleep, slows metabolism, and triggers hot flashes that fragment nighttime rest even when you don’t fully wake up.
Many women in early perimenopause don’t connect their fatigue to hormonal changes because their periods still seem regular. But subtle shifts like shorter cycles, heavier or lighter flow, new onset of night sweats, or waking at 3 a.m. with a racing heart can all signal that the transition has begun. If you’re in your late 30s or 40s and your fatigue arrived alongside any of these changes, a conversation about hormone levels is worthwhile.
Sleep Apnea Looks Different in Women
Sleep apnea is dramatically underdiagnosed in women because it doesn’t always present the classic way. In men, the hallmark symptoms are loud snoring and witnessed pauses in breathing. Women with sleep apnea are more likely to report fatigue, morning headaches, insomnia, and depression. A study of over 1,100 patients found that fatigue, headaches upon waking, and use of sleep aids were all significantly more common in women with the condition than in men.
Because the symptom profile overlaps so heavily with depression and general stress, women with sleep apnea often receive antidepressants or anxiety medication instead of a sleep study. If you wake up feeling unrefreshed no matter how many hours you sleep, get morning headaches, or your partner mentions any irregular breathing at night, a home sleep test can rule this in or out relatively easily.
Vitamin D and B12 Gaps
Both of these vitamins play direct roles in energy production, and deficiencies in either can cause fatigue that feels disproportionate to your activity level. Vitamin D deficiency is extremely common, particularly if you live in a northern climate, have darker skin, or spend most of your day indoors. The recommended daily intake for adults under 70 is 600 IU, though many practitioners suggest higher doses when blood levels are low. A simple blood test called 25(OH)D measures your vitamin D status.
B12 deficiency tends to show up in women who eat little or no meat, take certain acid-reducing medications, or have digestive conditions that impair absorption. Symptoms beyond fatigue include tingling in the hands or feet, difficulty with balance, and memory problems. Both deficiencies are inexpensive to test for and straightforward to correct with supplementation.
Chronic Stress and the “Tired but Wired” State
Ongoing stress keeps your body in a low-grade fight-or-flight state that burns through energy reserves without you realizing it. Women who function well on the surface while running on anxiety underneath often experience a paradox: exhausted during the day, wired at night. Physical symptoms of this pattern include muscle tension, digestive problems like diarrhea or stomach upset, a racing heart rate, headaches, and sleep that never feels deep enough. Over time, neglecting sleep, exercise, and nutrition under the pressure of keeping everything together compounds the problem.
This type of fatigue doesn’t show up on any blood test, which can be frustrating when everything comes back “normal.” But if your fatigue coexists with a persistent internal sense of urgency, difficulty relaxing, or physical tension you can’t shake, the stress response itself may be what’s draining you.
Not Enough Protein for Your Activity Level
Many women undereat protein without realizing it, and the impact on energy is real. The minimum recommendation for sedentary adults is 0.8 grams of protein per kilogram of body weight per day. For a 140-pound (64 kg) woman, that’s about 51 grams. But if you exercise regularly, that baseline isn’t enough. The International Society of Sports Nutrition recommends 1.4 to 2.0 grams per kilogram daily for active individuals, which would put that same woman at 90 to 128 grams per day.
Falling short on protein means your muscles can’t fully repair after activity, your blood sugar is less stable between meals, and you’re more likely to feel run down by mid-afternoon. Front-loading protein at breakfast and lunch, rather than eating most of it at dinner, tends to have the biggest effect on daytime energy.
When Fatigue Becomes Something More
Most causes of low energy in women are fixable once identified. But if your fatigue is severe, lasts longer than six months, doesn’t improve with rest, and gets worse after physical or mental exertion, it may meet the criteria for myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). The hallmark symptom is post-exertional malaise: a crash in energy and function after effort that wouldn’t have been a problem before you got sick. Other required features include unrefreshing sleep and either cognitive difficulties (trouble with memory, focus, or processing information) or symptoms that worsen when you stand up.
ME/CFS is a distinct medical condition, not a catch-all label for being tired. It often begins after a viral infection and requires a specific diagnostic approach. If your fatigue follows this pattern, particularly the worsening after exertion, bring up ME/CFS by name with your provider so the evaluation goes in the right direction.