You sleep for a full night, yet wake feeling depleted. Energy never fully arrives. Caffeine becomes essential rather than optional. Exercise tolerance declines. Concentration becomes inconsistent. Your blood tests return as “normal”, but you do not feel well.

“Normal” and “optimal” are not interchangeable. Laboratory reference intervals are generally calculated from the central 95 per cent of a selected population. They are designed primarily to identify established disease, not to confirm optimal physiological function. The reference population may also include people with undiagnosed nutrient deficiency, chronic inflammation or early disease.

Persistent fatigue requires a structured assessment of patterns. Iron, ferritin and vitamin B12 are important, but they are not the only biomarkers capable of explaining why someone wakes with an empty tank.

“Normal” is not necessarily optimal

A reference interval describes how a particular group tested at a particular laboratory using a particular assay. It does not define the level at which every individual functions best.

The appropriate question is not simply whether a result is inside the printed range. It is whether the result is clinically coherent with the symptoms, history and other biomarkers.

Ferritin: why women can be iron deficient without anaemia

Iron deficiency is not synonymous with iron-deficiency anaemia. Iron stores can become depleted while haemoglobin remains within range.

Symptoms of depleted iron stores can include:

• Persistent fatigue and unrefreshing sleep.
• Reduced exercise tolerance.
• Breathlessness on exertion.
• Hair thinning.
• Restless legs.
• Palpitations.
• Cold intolerance.
• Brain fog.
• Pica, including cravings for ice or non-food substances.

A normal full blood count does not exclude iron deficiency without anaemia. Ferritin and transferrin saturation are required to assess iron availability more accurately.

Why sex-specific ferritin ranges are under scrutiny

Ferritin reference intervals were not simply built from male data. Historically, they were derived from general population samples that included people with undiagnosed iron deficiency.

Because iron deficiency is substantially more prevalent in menstruating women, female reference intervals were often set lower than male intervals. Leading haematologists argue that this difference may reflect statistical normalisation of a disease state rather than a genuine physiological distinction.

The 2024 systematic review in The Lancet Haematology found that ferritin lower limits in published reference intervals were consistently below 30 µg/L. The median lower limit was approximately 8 µg/L for females and 25 µg/L for males. The authors concluded that many existing intervals are too low to identify iron deficiency reliably. [1]

There is no single standardised UK ferritin reference range. Intervals vary between laboratories, assays and populations. The same result can therefore be marked as normal at one laboratory and low at another. [2]

Conventional versus emerging clinical thresholds

The 2026 American Society of Hematology guidance recommends a ferritin threshold of 30 µg/L or lower for adults, including menstruating individuals. It supports a threshold of 50 µg/L or lower in high-risk groups, such as people with heavy menstrual bleeding. Where inflammation is present, ferritin below 100 µg/L or transferrin saturation below 20 per cent supports iron deficiency. [3]

This represents an important upward movement from the conventional 15 µg/L threshold. Women previously told that their iron was “normal” may fall into a deficient category when assessed using newer clinical decision limits.

Ferritin is also an acute-phase reactant. Infection, inflammation and liver disease can raise ferritin independently of iron stores. A full blood count, transferrin saturation and CRP are therefore important when ferritin does not match the clinical picture.

Iron deficiency without anaemia is clinically significant. A 2017 meta-analysis of randomised trials found that iron treatment reduced subjective fatigue in adults with iron deficiency without anaemia, with a pooled effect size of 0.33. A 2018 systematic review of randomised trials reached a similar conclusion, particularly in people with low or borderline ferritin. [4,5]

The 2025 CMAJ review recommends assessing ferritin alongside the full blood count and considering transferrin saturation where inflammation is present. The cause must also be investigated. Heavy menstrual bleeding, coeliac disease, restrictive diets, gastrointestinal blood loss, malabsorption and frequent blood donation are relevant possibilities. [6]

Vitamin B12 and folate: the test that can look fine while you are depleted

Total serum B12 measures circulating cobalamin bound to different proteins. It does not necessarily reflect the amount available to cells.

A high or apparently adequate B12 result can be misleading in the presence of:

• Supplementation.
• Liver disease.
• Inflammation or malignancy.
• Assay interference.
• Macro-B12, where B12 is associated with immunoglobulins and binding proteins.

This does not mean that every high B12 result represents deficiency. It means that total B12 should not be interpreted in isolation when symptoms or other findings raise clinical suspicion. [7,8]

More functional assessment can include:

• Methylmalonic acid (MMA): a sensitive marker of intracellular B12 function.
• Total homocysteine: rises with B12 or folate deficiency, although it is less specific.
• Holotranscobalamin: the fraction of B12 available for cellular uptake.
• Combined B12 indicator (cB12): a composite measure incorporating several B12-related biomarkers.

MMA and homocysteine can also rise with renal impairment, age and other nutrient deficiencies. Interpretation requires clinical context, renal function and folate status. [9]

B12 and folate deficiency can contribute to fatigue, cognitive symptoms, low mood, neuropathy and other neurological features. Risk is increased with long-term metformin, proton pump inhibitors, H2 blockers, vegan or vegetarian diets, previous bariatric surgery, malabsorption and pernicious anaemia.

Homocysteine: a functional marker of B-vitamin status

Homocysteine is an amino acid intermediate in one-carbon metabolism. Its clearance depends on vitamin B12, folate and vitamin B6.

A raised result can prompt assessment of:

• B12 deficiency.
• Folate deficiency.
• B6 deficiency.
• Renal impairment.
• Hypothyroidism.
• Malabsorption.
• Relevant medication use.
• MTHFR variants, where clinically appropriate.

Homocysteine is associated with cardiovascular and cognitive risk, but association does not prove causation. B-vitamin supplementation lowers homocysteine, yet large trials have not consistently demonstrated a reduction in overall cardiovascular events. It should therefore be used to investigate a clinical pattern, not as a standalone cardiovascular screening test. [10]

Thyroid: why TSH alone is not the whole picture

Most UK laboratories use an adult TSH reference interval of approximately 0.4–4.0 mIU/L, although the exact range varies.

Some endocrinologists argue that a narrower upper limit of approximately 2.5 mIU/L better reflects a rigorously thyroid-healthy, antibody-negative population. Others warn that applying this threshold universally could overdiagnose and overtreat people with normal thyroid function.

A meaningful thyroid assessment may include:

• TSH.
• Free T4.
• Free T3, where clinically indicated.
• Thyroid peroxidase antibodies.
• Thyroglobulin antibodies.

Positive antibodies can identify autoimmune thyroid disease and increase the risk of future hypothyroidism, even when TSH remains within range. Free T3 may provide additional context, but its role in diagnosing hypothyroidism remains debated. Levels often remain within range when TSH is abnormal, and measurement is less robust than free T4. [11,12]

Iron and thyroid physiology are also interdependent. Thyroid peroxidase is a haem-containing enzyme, and iron deficiency can compound symptoms associated with reduced thyroid function. Low ferritin can therefore be relevant when fatigue persists despite apparently reassuring thyroid results.

Reverse T3: why it is not the answer you may have been told it is

Sidebar: Reverse T3 (rT3) is an inactive metabolite of T4. It is produced when deiodinase enzymes redirect T4 away from active T3 production, particularly during illness, physiological stress and starvation. This is generally understood as an energy-conserving adaptation rather than a disease process in its own right. A widely promoted claim is that rT3 “blocks” T3 at the receptor and causes hormone “pooling”. That proposition does not hold up well against the evidence. rT3 does not bind nuclear thyroid hormone receptors and, in normal subjects, does not appear to alter pituitary-thyroid function. Current evidence does not support using rT3 to diagnose hypothyroidism or to guide T4 or T3 prescribing. The 2023 American Thyroid Association-commissioned review concluded that rT3 is rarely useful in routine practice, with legitimate exceptions largely limited to rare genetic thyroid syndromes and consumptive hypothyroidism caused by unusual tumours. It is not recommended by professional practice guidelines, and assay agreement between platforms is poor. This matters because rT3-led decision-making often pushes patients towards excessive thyroid hormone doses, which carry real risk. Many women are told that rT3 is the marker their GP will not run. Our preference is to interrogate ferritin and iron status, thyroid antibodies, free T4, the TSH trend over time, symptoms, and the drug and supplement history instead. [13,14]

Fasting insulin and HOMA-IR: metabolic dysfunction before abnormal glucose

Glucose and HbA1c can remain within range while insulin is rising. The pancreas can compensate for declining insulin sensitivity by producing more insulin, maintaining normal glucose for a period of time.

Fasting insulin and HOMA-IR can help identify this earlier pattern.

HOMA-IR is calculated from fasting glucose and fasting insulin:

HOMA-IR = fasting insulin × fasting glucose ÷ 22.5

Published thresholds vary substantially. A Brazilian laboratory proposal reported a 95 per cent reference interval of approximately 2.52–13.14 µU/mL for fasting insulin and 0.39–2.86 for HOMA-IR. A prospective Chinese cohort identified cut-offs of approximately 1.4 for dysglycaemia and 2.0 for type 2 diabetes risk. A value of 2.5 is commonly used in US research as a marker of insulin resistance. These thresholds are population-specific, not universal diagnostic boundaries. [15–17]

The symptom pattern can include:

• Central weight gain.
• Post-meal energy crashes.
• Sugar or carbohydrate cravings.
• Night waking.
• Difficulty changing body composition despite training.
• Reduced recovery.
• Increasing fatigue after exercise.

Perimenopause and menopause can alter sleep, body composition and insulin sensitivity. Fasting insulin is therefore relevant to women over 40, particularly when glucose and HbA1c appear reassuring.

Vitamin D: a threshold established primarily for bones

Current UK guidance identifies serum 25-hydroxyvitamin D below 25 nmol/L as the level at which the risk of deficiency and poor musculoskeletal health increases. Levels between 25 and 50 nmol/L are commonly described as insufficient in NHS clinical guidance, while levels above 50 nmol/L are often considered sufficient.

SACN established the 25 nmol/L threshold primarily for population-level musculoskeletal protection. It was not designed to define an optimal energy level.

A 2026 Frontiers in Endocrinology commentary calls for the UK threshold to be reconsidered and proposes 50 nmol/L as a more clinically meaningful deficiency threshold. This is a recommendation for policy change, not an implemented national redefinition. [18,19]

Vitamin D deficiency can contribute to muscle weakness, reduced physical function and fatigue. Limited cutaneous synthesis between October and March makes low levels common in the UK. Sunbeds are not a safe solution.

Magnesium: why a normal serum result may not be reassuring

Less than 1 per cent of total body magnesium is present in the blood. Serum magnesium is tightly regulated by renal and bone mechanisms, so intracellular depletion can exist alongside a normal serum result.

Symptoms and risk factors may include:

• Fatigue.
• Muscle cramps.
• Poor sleep.
• Palpitations.
• Tension headaches.
• Long-term proton pump inhibitor use.
• Diuretic use.
• Alcohol excess.
• Diabetes.
• Gastrointestinal disease or malabsorption.

Red-cell magnesium may provide additional information about intracellular stores, but its clinical utility remains debated. Methods and reference intervals are not sufficiently standardised for universal interpretation.

Some researchers propose 0.85 mmol/L as a more clinically useful lower threshold for serum magnesium. This is not yet a universally accepted diagnostic cut-off. Magnesium should be interpreted alongside symptoms, medication, kidney function, diet and gastrointestinal history. [20–22]

Cortisol and the cortisol awakening response

The cortisol awakening response is the normal rise in cortisol during the first 30–45 minutes after waking. It contributes to morning alertness and physiological readiness.

Perimenopausal vasomotor symptoms and sleep fragmentation can affect cortisol rhythms. In a 2023 experimental model of menopause, sleep fragmentation increased bedtime cortisol by approximately 27 per cent and reduced the cortisol awakening response by approximately 57 per cent. Estradiol suppression alone did not significantly alter the awakening response. [23]

This supports a clinically important distinction: waking empty may reflect fragmented sleep and altered stress physiology rather than insufficient time in bed.

Cortisol awakening response testing is not a routine diagnostic test for fatigue or menopause. Salivary cortisol patterns require careful collection and clinical interpretation. A single cortisol value cannot diagnose “adrenal fatigue”.

Sex hormones, SHBG and what not to test routinely

Oestradiol, FSH, LH, testosterone and SHBG can be relevant in selected cases. Testosterone should not be interpreted without SHBG and appropriate clinical context.

The interpretation depends on:

• Menstrual cycle stage.
• Contraception.
• HRT.
• Symptoms.
• Age.
• Pregnancy status.
• Suspected premature ovarian insufficiency.

NICE states that women aged 45 and over with typical menopausal symptoms are generally diagnosed clinically rather than through routine hormone testing. FSH may be appropriate between 40 and 45 where menopause is suspected, or under 40 where premature ovarian insufficiency is being assessed. Hormone testing is not routinely required to monitor HRT. [24,25]

This is why comprehensive testing should be targeted rather than indiscriminate.

Markers that are often missed

Depending on the clinical picture, additional tests may include:

• Full blood count and blood film: anaemia, red-cell abnormalities and other haematological patterns.
• CRP or high-sensitivity CRP: inflammation affecting ferritin and broader interpretation.
• Coeliac serology: a commonly missed cause of iron and B-vitamin malabsorption.
• Vitamin B6: relevant to homocysteine metabolism.
• Renal and liver function: affects MMA, homocysteine, ferritin and magnesium interpretation.
• Fasting lipids: triglycerides and HDL add context to insulin resistance.
• Gut health markers: only where clinically indicated and interpreted appropriately.

Patterns matter more than single numbers

Functional medicine uses a pattern-based approach. It does not mean treating every borderline result or pursuing an unlimited list of tests.

Useful patterns can include:

• Low ferritin alongside reduced exercise tolerance and heavy menstrual bleeding.
• B12 or folate insufficiency with raised homocysteine.
• High-normal fasting insulin with post-meal crashes, increased triglycerides and reduced recovery.
• Positive thyroid antibodies with rising TSH and low-normal free T4.
• Low vitamin D with muscle weakness and reduced physical function.
• Sleep fragmentation with a flattened morning cortisol response.
• Gastrointestinal symptoms alongside recurrent iron or B-vitamin deficiency.

Trends, ratios and interactions can be more informative than one isolated result.

A realistic approach when you cannot do everything

1. Start with a baseline assessment. Persistent fatigue usually warrants more than a full blood count alone. Ferritin, transferrin saturation, B12, folate, thyroid markers, vitamin D, renal function, liver function, CRP, glucose and HbA1c may be considered according to the clinical history.
2. Test appropriately. Fasting insulin requires a fasting sample. Hormone results must be timed appropriately where cycle stage is relevant. Always disclose supplements and medication.
3. Correct likely contributors first. Iron, B12 or folate deficiency, vitamin D deficiency and thyroid dysfunction should be assessed and managed before pursuing less established explanations.
4. Make one meaningful change at a time. This makes symptom response and retesting easier to interpret.
5. Retest at an appropriate interval. Three to six months is reasonable for many nutritional and metabolic trends, although anaemia or significant deficiency may require earlier clinical review.
6. Do not start iron without appropriate assessment. Ferritin, transferrin saturation and the underlying cause of deficiency should be considered before supplementation. Iron can be harmful when taken unnecessarily.

What happens at Formula Health?

Formula Health provides an award-winning, multidisciplinary approach to personalised healthcare.

The process begins with a 90-minute consultation covering your symptoms, menstrual and hormonal history, medication, training load, sleep, diet and previous results. Blood testing is targeted to the clinical picture rather than delivered as a one-size-fits-all panel.

Results are interpreted in context. The women’s health, nutrition, medical and performance practitioners coordinate the plan as one team: your health pitstop crew.

Follow-up appointments are 60 minutes, allowing the team to review trends, symptoms, treatment response and next steps. Blood testing and functional medicine sit alongside the wider Formula Health service, including women’s health, nutrition, performance and advanced medical care. Formula Health services

Medical disclaimer: This article provides general information and does not replace individual medical advice. Reference ranges vary between laboratories and must be interpreted by an appropriately qualified clinician. Do not start, stop or change prescribed medication or supplementation without clinical advice. Persistent or severe fatigue requires appropriate medical assessment.

References

1. Truong J, Naveed K, Beriault D, et al. The origin of ferritin reference intervals: a systematic review. The Lancet Haematology. 2024;11:e530–e539.
2. Rodgers S, Woolley T, Smith J, et al. Updated adult ferritin reference intervals based on a large healthy UK sample. Annals of Clinical Biochemistry. 2024;61:365–371.
3. American Society of Hematology. Clinical practice guideline for the diagnosis of iron deficiency. 2026. See also the ASH diagnostic guidance summary.
4. Yokoi K, Konomi A. Iron deficiency without anaemia is a potential cause of fatigue. British Journal of Nutrition. 2017.
5. Houston BL, et al. Efficacy of iron supplementation on fatigue and physical capacity in non-anaemic iron-deficient adults. BMJ Open. 2018.
6. Sholzberg M, et al. Diagnosis and management of iron deficiency in females. CMAJ. 2025.
7. Nexo E, Parkner T. Vitamin B12-related biomarkers. Annals of Clinical Biochemistry. 2024.
8. High serum total vitamin B12 may mask biologically active B12 deficiency. Annals of Clinical Biochemistry. 2024.
9. Fedosov SN. Biomarkers and algorithms for the diagnosis of vitamin B12 deficiency. Frontiers in Molecular Biosciences. 2016.
10. Homocysteine, B vitamins and cardiovascular risk.
11. Subclinical hypothyroidism: when to treat. Cleveland Clinic Journal of Medicine. 2019.
12. The optimal healthy ranges of thyroid function defined by cardiovascular disease and mortality risk. The Lancet Diabetes & Endocrinology. 2023.
13. Jonklaas J, Bianco AC, Cappola AR, et al. Guidance for reverse triiodothyronine testing and free thyroxine and free triiodothyronine measurement: an American Thyroid Association clinical practice review. Thyroid. 2023.
14. Burmeister LA. Reverse T3 or perverse T3? Still puzzling after 40 years. Cleveland Clinic Journal of Medicine. 2018;85(6):450–455.
15. Proposal for fasting insulin and HOMA-IR reference intervals. Archives of Endocrinology and Metabolism.
16. Optimal cut-offs of HOMA-IR to identify dysglycaemia and type 2 diabetes. PLOS ONE. 2016.
17. Early insulin resistance in normoglycaemic low-risk individuals and subclinical atherosclerosis. Cardiovascular Diabetology. 2023.
18. Scientific Advisory Committee on Nutrition. Vitamin D and health report. UK Government.
19. Christie F, et al. Vitamin D in the UK: an urgent call to redefine the threshold for deficiency. Frontiers in Endocrinology. 2026.
20. Workinger JL, Doyle RP, Bortz J. Challenges in the diagnosis of magnesium status. Nutrients. 2018.
21. Costello RB, et al. Recommendation on an updated standardisation of serum magnesium reference ranges. European Journal of Nutrition. 2022.
22. Colaneri-Day S, Rosanoff A. Clinical guideline for detection and management of magnesium deficiency in ambulatory care. Nutrients. 2025.
23. Effects of sleep fragmentation and estradiol decline on cortisol in a human experimental model of menopause. Journal of Clinical Endocrinology & Metabolism. 2023.
24. NICE. Menopause: diagnosis and management, NG23.
25. British Menopause Society and Women’s Health Concern. Hormone blood tests in the menopause transition. 2026.