Fatigue is one of the most common health complaints among women. The same is true for hair shedding, brain fog, feeling cold, dizziness, reduced exercise tolerance, heavier periods, anxiety-like symptoms, weight changes, and palpitations. The challenge is that these symptoms often overlap: iron deficiency can feel like a thyroid problem, thyroid dysfunction can affect energy, mood, weight, heart rate, and the menstrual cycle, and heavy periods can slowly reduce iron stores for months or years before anemia appears on a standard blood test.
That is why the goal is not to guess the diagnosis. The goal is to understand which symptoms matter, which tests are genuinely useful, and when a “normal” result does not fully answer the question. This article is educational and does not replace diagnosis or treatment. Use it as a starting point for a more focused conversation with a clinician.
Why women are more likely to develop iron deficiency
Iron is needed to make hemoglobin, the protein in red blood cells that carries oxygen. It also supports muscle function, cognitive function, and normal energy metabolism. Women who menstruate lose iron with every cycle. For some, this loss is small and easily replaced through diet. For others, especially with heavy periods, pregnancy, postpartum blood loss, restrictive eating, vegetarian or vegan diets, gastrointestinal symptoms, or frequent blood donation, iron stores may gradually decline.
Iron deficiency can exist before anemia develops. This means hemoglobin may still be “normal” while ferritin, a marker of iron stores, is already low. A 2025 CMAJ review notes that iron deficiency without anemia is associated with fatigue, reduced work performance, and impaired cognitive function. The same review describes ferritin below 30 µg/L in adults as a commonly used diagnostic threshold for iron deficiency.
Large studies show why this matters. In a JAMA Network Open study of 62,685 women from the United States and Canada, the estimated prevalence of iron deficiency varied substantially depending on the ferritin cutoff used. Among women aged 25 to 54, prevalence ranged from 4.46% to 21.23%. Another JAMA Network Open study using NHANES 2017–2020 data estimated that 14% of US adults had absolute iron deficiency and 15% had functional iron deficiency. The main takeaway: iron deficiency is common, and the laboratory or clinical threshold used can strongly influence how often it is detected.
The most useful tests for assessing iron status
A basic evaluation usually starts with a complete blood count and ferritin. A complete blood count shows hemoglobin, hematocrit, red blood cell size, and other parameters. It can detect anemia, but it may miss early iron deficiency. Ferritin reflects iron stores and is one of the key markers of iron status. The World Health Organization describes ferritin as a useful marker of iron stores in people without conditions that distort the result. At the same time, ferritin can rise with inflammation, liver disease, obesity, malignancy, and other conditions, so it should not be interpreted in isolation.
If ferritin is borderline, unexpectedly normal, or difficult to interpret, a clinician may add transferrin saturation, serum iron, and total iron-binding capacity. CRP can sometimes be helpful as well: because ferritin may rise during inflammation, CRP can help explain why ferritin appears “normal” even when symptoms or other markers suggest possible iron deficiency. WHO notes that inflammatory markers such as CRP are often used when interpreting ferritin, and ferritin itself should be interpreted carefully in inflammatory states.
One common mistake is testing only “serum iron.” This marker fluctuates throughout the day and is not enough on its own to diagnose iron deficiency.
Heavy periods are not just a “cycle problem”
Heavy menstrual bleeding is not defined only by the number of milliliters of blood lost. NICE emphasizes that heavy menstrual bleeding should be assessed by its impact on quality of life, not only by measured blood loss. If periods interfere with work, school, sleep, exercise, travel, sex, or planning the day, that has clinical relevance.
Heavy periods can show up in different ways: needing to change pads or tampons very frequently, bleeding through clothes or bedding, passing large clots, periods lasting longer than usual, planning the day around bathroom access, or feeling unusually wiped out during or after menstruation. Dizziness, shortness of breath, palpitations, or unusual weakness during periods should not automatically be dismissed as “normal for you,” especially if these symptoms affect daily life.
NICE recommends a complete blood count for all women with heavy menstrual bleeding. NICE also recommends considering testing for coagulation disorders, such as von Willebrand disease, if heavy periods have been present since the first menstrual cycles and there is a personal or family history suggesting a possible bleeding disorder. For adolescents with heavy menstrual bleeding, ACOG states that evaluation should include assessment for anemia from blood loss, including ferritin, as well as evaluation for bleeding disorders when clinically appropriate.
This matters because heavy periods are sometimes normalized for years. A woman may be told, “That is just your cycle,” while her iron stores are gradually being depleted.
When heavy periods require more than blood tests
Heavy menstrual bleeding can have several causes. The FIGO PALM-COEIN system classifies causes of abnormal uterine bleeding in nonpregnant reproductive-aged women into structural and non-structural categories: polyps, adenomyosis, leiomyomas, malignancy or hyperplasia, coagulopathy, ovulatory dysfunction, endometrial causes, iatrogenic causes, and causes not otherwise classified. In simpler terms, heavy bleeding can be related to fibroids or polyps, adenomyosis, ovulation problems, perimenopause, contraception or medications, bleeding disorders, pregnancy-related conditions, thyroid disease, or endometrial changes.
This does not mean everyone immediately needs imaging. NICE recommends taking a history that includes the nature of bleeding, associated symptoms such as persistent intermenstrual bleeding, pelvic pain, pelvic pressure, and the impact on quality of life. Further evaluation depends on the bleeding pattern and associated symptoms.
Symptoms that should be discussed with a clinician promptly include bleeding between periods, bleeding after sex, sudden very heavy bleeding, pelvic pain or pressure, new heavy bleeding after age 40, bleeding after menopause, fainting, chest pain, severe shortness of breath, or signs of an unstable condition.
Thyroid symptoms overlap with iron deficiency symptoms
The thyroid helps regulate metabolic processes throughout the body. When thyroid hormone levels are too low, symptoms may include fatigue, cold intolerance, constipation, dry skin, weight gain, low mood, slower heart rate, and heavier or irregular periods. When thyroid hormone levels are too high, symptoms may include palpitations, anxiety-like feelings, tremor, heat intolerance, sweating, weight loss, diarrhea, sleep disturbance, and lighter or irregular periods.
The problem is that many of these symptoms are nonspecific. Fatigue alone does not prove thyroid disease. Hair shedding alone does not prove thyroid disease. Weight change alone does not prove thyroid disease. NICE recommends testing thyroid function when thyroid dysfunction is clinically suspected. NICE also notes that thyroid disease usually responds well to treatment, and that the goal of treatment is to improve symptoms and bring thyroid function tests into or close to the reference range.
The most important thyroid tests
For most adults, the first test is TSH, or thyroid-stimulating hormone. TSH is usually the best initial test for assessing primary thyroid dysfunction. If TSH is abnormal, free T4 is usually added. Free T4 helps distinguish overt thyroid disease from milder or subclinical patterns.
Sometimes a clinician may add free T3, TPO antibodies, TRAb, or TSI. Free T3 may be useful when hyperthyroidism is suspected, especially if TSH is low and free T4 does not fully explain the clinical picture. TPO antibodies can help identify autoimmune thyroiditis, often called Hashimoto’s thyroiditis, especially when TSH is elevated. TRAb or TSI can help evaluate suspected Graves’ disease.
USPSTF states that evidence is insufficient to recommend routine screening for thyroid dysfunction in nonpregnant adults without symptoms. At the same time, USPSTF describes TSH as the primary screening test and notes that abnormal results are generally confirmed with repeat testing over 3 to 6 months, while T4 helps distinguish subclinical dysfunction from overt disease. In practice, thyroid testing should be symptom-driven or risk-based rather than based on broad random panels for everyone.
Why “normal” does not always mean “nothing is wrong”
Lab interpretation depends on context. A normal complete blood count does not rule out low iron stores. A normal ferritin result may be misleading if inflammation is present. A mildly abnormal TSH may require repeat testing before treatment decisions are made. Thyroid tests may be normal while heavy periods continue to cause iron loss. Low ferritin may explain fatigue, but it does not explain why ferritin became low in the first place.
This is where symptoms need to be assessed as a pattern. If there is fatigue, heavy periods, dizziness during menstruation, hair shedding, and reduced exercise tolerance, iron deficiency should be considered. If there are palpitations, tremor, heat intolerance, unintentional weight loss, and irregular cycles, thyroid overactivity may be worth evaluating. If there is cold intolerance, constipation, dry skin, low energy, weight gain, and heavier periods, thyroid underactivity may be worth checking.
If heavy periods have been present from the very first cycles, especially together with easy bruising, nosebleeds, prolonged bleeding after dental procedures, or a family history of bleeding, a bleeding disorder should be considered. Guidelines on von Willebrand disease describe heavy menstrual bleeding as one of the major mucocutaneous bleeding symptoms of the condition.
A practical testing checklist to discuss with a clinician
For fatigue, weakness, dizziness, hair shedding, brain fog, shortness of breath with exertion, or reduced exercise tolerance, it may be reasonable to discuss a complete blood count, ferritin, transferrin saturation or a full iron panel, CRP if inflammation is possible, TSH, and free T4 if TSH is abnormal or clinical suspicion is high.
For heavy menstrual bleeding, clinicians commonly consider a complete blood count, ferritin when fatigue or symptoms of iron deficiency are present, a pregnancy test if bleeding is unusual and pregnancy is possible, testing for coagulation disorders or von Willebrand disease when history suggests it, and pelvic ultrasound or other gynecologic evaluation when symptoms suggest fibroids, polyps, adenomyosis, or endometrial pathology.
For symptoms suggestive of thyroid disease, testing usually starts with TSH. Free T4 is added if TSH is abnormal. Free T3 may be useful when hyperthyroidism is suspected. TPO antibodies may help when autoimmune hypothyroidism is suspected. TRAb or TSI may help when Graves’ disease is suspected.
NICE specifically states that thyroid hormone testing should not be done routinely for heavy menstrual bleeding unless there are other signs or symptoms of thyroid disease. NICE also does not recommend routine testing of female hormones for heavy menstrual bleeding. This is an important point: more testing is not always better. The right tests should match the symptom pattern.
Pregnancy, postpartum, and perimenopause need separate attention
Pregnancy and the postpartum period change the interpretation of both iron and thyroid markers. Iron requirements increase during pregnancy, and postpartum blood loss can worsen iron deficiency. Thyroid disease also requires more careful management before conception, during pregnancy, and after birth.
The 2025 RCOG Green-top Guideline notes that thyroid disease is common in women of childbearing age and that both undertreatment and overtreatment can affect pregnancy and fetal outcomes. Care should be optimized before pregnancy, during pregnancy, and after delivery.
Perimenopause also changes the picture. Cycles may become irregular, and bleeding may become heavier for some women. New heavy bleeding, bleeding between periods, or bleeding after menopause should not be dismissed as “just hormones” without evaluation.
What to prepare before a medical appointment
A short symptom diary can be more useful than a vague description. Over two or three cycles, it can help to track period start and end dates, the heaviest days, how often menstrual products need to be changed, whether there are clots, leaks, or sudden episodes of heavy bleeding, and whether dizziness, palpitations, shortness of breath, fatigue, pelvic pain, or pelvic pressure occur.
It is also useful to note hair shedding, cold intolerance or heat intolerance, bowel changes, sleep changes, and all supplements being taken, especially iron, biotin, thyroid supplements, and multivitamins. Biotin matters because it can interfere with some laboratory methods, including thyroid tests. Tell your clinician if you take it.
The bottom line
For women with fatigue, heavy periods, hair shedding, dizziness, or unexplained changes in energy, the basics matter. First, identify the symptom pattern. Then choose the tests.
A complete blood count can detect anemia, but ferritin helps identify depleted iron stores. Ferritin is useful, but inflammation can distort its value. TSH is usually the first test for thyroid assessment, but thyroid screening in people without symptoms does not have clear evidence support. Heavy menstrual bleeding deserves evaluation because it can affect quality of life, iron stores, and sometimes point to gynecologic disease or a bleeding disorder.
Good evaluation often starts with one simple question: “What pattern connects my symptoms, my cycle, and my lab results?” This question can help prevent fatigue from being dismissed as a lifestyle issue for months or years when the body has already been sending measurable signals.
Sources
- CMAJ — Diagnosis and management of iron deficiency in females: CMAJ review
- JAMA Network Open — Prevalence of iron deficiency using 3 definitions among women in the US and Canada: JAMA Network Open iron deficiency definitions study
- JAMA Network Open — Absolute and functional iron deficiency in the US, NHANES 2017–2020: JAMA Network Open NHANES iron deficiency study
- WHO — Guideline on use of ferritin concentrations to assess iron status: WHO ferritin guideline
- NICE NG88 — Heavy menstrual bleeding: assessment and management: NICE NG88
- ACOG — Screening and management of bleeding disorders in adolescents with heavy menstrual bleeding: ACOG Committee Opinion
- FIGO — PALM-COEIN classification of abnormal uterine bleeding: FIGO PALM-COEIN classification
- NICE NG145 — Thyroid disease: assessment and management: NICE NG145
- USPSTF — Thyroid dysfunction screening recommendation: USPSTF thyroid screening recommendation
- ASH/ISTH/NHF/WFH — 2021 guidelines on the management of von Willebrand disease: von Willebrand disease guideline
- RCOG — Green-top Guideline No. 76: Management of thyroid disorders in pregnancy: RCOG Green-top Guideline No. 76
