Why Your ‘Normal’ Blood Work Is Lying to You
You’re tired. You’re gaining weight. Your hair is thinning. Your brain feels like it’s running through mud.
So you go to the doctor. Blood work comes back. Everything’s in range. “You’re fine,” they say.
You’re not fine. You’re just not sick enough — yet.
The problem with “normal” ranges
Here’s what nobody explains: lab reference ranges are not health ranges. They’re ranges derived from the average of everyone who’s ever been tested at that lab. Sick people, healthy people, old people, young people — all mixed together.
A 2024 study in Thyroid found that age-specific reference intervals for TSH are significantly different from the standard ranges labs use. Women over 40 have different optimal TSH levels than women under 30 — but the lab gives everyone the same range (Jansen et al., Thyroid, 2024).
A 2025 cross-sectional study in Annals of Internal Medicine confirmed that thyroid function reference intervals vary by age, sex, and race. The one-size-fits-all range misses early thyroid dysfunction in women specifically (Li et al., Ann Intern Med, 2025).
Translation: your “normal” TSH of 4.2 might be flagged as fine, but for your age and sex, it could be the early warning sign of a thyroid that’s slowing down.
Five markers your doctor probably isn’t reading correctly
1. TSH (thyroid-stimulating hormone)
The standard range is 0.4–4.5 mIU/L. Most labs flag anything under 4.5 as normal.
The problem: the upper end of that range includes people with undiagnosed thyroid dysfunction. Research on subclinical hypothyroidism shows that women with TSH levels between 2.5 and 4.5 often have symptoms — fatigue, weight gain, brain fog, hair loss — even though they’re “in range” (Capozzi et al., Gynecol Endocrinol, 2022).
Biologic variation in thyroid testing means a single measurement can be misleading. Studies show that TSH fluctuates by up to 40% within the same person over days and weeks (Andersen et al., Thyroid, 2003). Worse: estrogen directly influences TSH levels, which means your TSH can swing significantly through your menstrual cycle — another reason a single random blood draw is unreliable for women specifically.
What optimal looks like: TSH between 1.0 and 2.5 mIU/L. Above 2.5 with symptoms? Worth investigating further — request Free T3, Free T4, and thyroid antibodies (TPO and TG).
There’s also a hidden pattern that standard tests miss entirely: thyroid hormone resistance. Your TSH and T4 look normal, but your cells can’t actually use the hormone. This often happens in women with PCOS or insulin resistance — you have hypothyroid symptoms (fatigue, weight gain, brain fog) but your blood work says you’re fine. The only clue is high T3 with low T3 uptake, which most labs don’t even test unless you specifically ask.
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2. Ferritin (iron storage)
The standard range is 12–150 ng/mL for women. Labs consider anything above 12 as “normal.”
The problem: ferritin below 30 ng/mL is iron deficiency — even without anemia. Research on iron deficiency in menstruating women found that symptoms like fatigue, hair loss, and brain fog appear well above the 12 ng/mL cutoff (Jepsen et al., Ugeskr Laeger, 2024). A landmark dermatology study confirmed that iron deficiency — even without anemia — is associated with hair loss in women (Trost et al., J Am Acad Dermatol, 2006).
Guidelines from Gut recommend treating iron deficiency when ferritin drops below 30–50 ng/mL, not waiting until it hits the lab’s 12 ng/mL floor (Goddard et al., Gut, 2011).
What optimal looks like: Ferritin between 50–100 ng/mL for women. Below 30? You’re iron deficient, regardless of what the lab says.
3. Vitamin D (25-hydroxyvitamin D)
The standard range is 30–100 ng/mL. Labs flag below 30 as deficient.
The problem: the cutoff of 30 is controversial. Many endocrinologists consider anything below 40 ng/mL as insufficient for optimal hormone production, immune function, and bone health. Vitamin D acts as a hormone precursor — when it’s low, your entire endocrine system underperforms.
What optimal looks like: 40–60 ng/mL. Below 40? Supplement with vitamin D3 + K2. Retest in 8–12 weeks.
4. Morning cortisol
The standard range for 8am cortisol is 6–18 mcg/dL. Labs flag anything within that as normal.
The problem: the range tells you nothing about rhythm. Your cortisol should be highest at 8am and lowest at midnight. A normal morning level with a flattened curve (high at night) explains why you’re wired at bedtime and exhausted in the morning.
Studies on cortisol measurement show that single-point cortisol testing is unreliable for detecting subtle dysregulation. The standard dexamethasone suppression test has limited sensitivity without concurrent free cortisol measurement (Genere et al., J Clin Endocrinol Metab, 2022).
What optimal looks like: Request a 4-point cortisol test (morning, noon, evening, night). The pattern matters more than any single number.
5. Fasting glucose and insulin
The standard range for fasting glucose is 70–99 mg/dL. Labs consider anything under 100 as normal.
The problem: fasting glucose is the last marker to go wrong. By the time your glucose is elevated, insulin resistance has been developing for years. A fasting insulin test reveals the problem 5–10 years earlier.
Fasting insulin above 10 uIU/mL — even with “normal” glucose — means your body is working overtime to keep blood sugar stable. This is early insulin resistance.
What optimal looks like: Fasting glucose under 90 mg/dL. Fasting insulin under 8 uIU/mL. HOMA-IR (glucose × insulin ÷ 405) under 1.5.
How to read your blood work like a functional doctor
Stop looking at the lab’s “normal” range. Start comparing your numbers to these optimal ranges:
| Marker | Lab “Normal” | Optimal Range |
|---|---|---|
| TSH | 0.4–4.5 | 1.0–2.5 |
| Ferritin | 12–150 | 50–100 |
| Vitamin D | 30–100 | 40–60 |
| Fasting insulin | 2–25 | 2–8 |
| Fasting glucose | 70–99 | 70–90 |
| Morning cortisol | 6–18 | 10–18 |
If your numbers are “in range” but below the optimal column, something is off. It might not be a disease — yet. But it’s the beginning of one.
What to ask your doctor
Most doctors order a basic metabolic panel and call it a day. Here’s what to request:
- Full thyroid panel: TSH, Free T3, Free T4, TPO antibodies, TG antibodies
- Iron panel: Ferritin, serum iron, TIBC, transferrin saturation
- Vitamin D: 25-hydroxyvitamin D
- Fasting insulin + HOMA-IR
- Cortisol: 4-point salivary cortisol (not just morning blood draw)
- DHEA-S — your adrenal reserve hormone
If your doctor refuses, find another doctor. Or order these through an at-home testing service — several exist now and cost $150–300 for a comprehensive panel.
The bottom line
“Normal” is not the same as “optimal.” Lab ranges are designed to catch disease, not prevent it. By the time your numbers fall out of the lab’s range, you’ve already been symptomatic for years.
Your body has been telling you something is wrong. The blood work just isn’t listening.
Demand better testing. Compare to optimal ranges. Stop accepting “normal” as a diagnosis.
Coming soon
- Why women need testosterone too (coming May 9) — the hormone everyone thinks is “male only” and why women need it more than they realize
- Omega-3s: the one supplement that actually does something (coming May 8) — which forms work and which are worthless
- Your gut bacteria are running your hormones (coming May 13) — the estrobolome explained
This post contains affiliate links. If you purchase through these links, I may earn a small commission at no extra cost to you. I only recommend products I’ve personally tested or that have strong research backing.
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References:
Age-Specific Reference Intervals for Thyroid-Stimulating Hormones and Free Thyroxine to Optimize Diagnosis of Thyroid Disease. Jansen HI et al., Thyroid (2024). PubMed
Thyroid Function Reference Intervals by Age, Sex, and Race: A Cross-Sectional Study. Li Q et al., Ann Intern Med (2025). PubMed
Subclinical hypothyroidism in women’s health: from pre- to post-menopause. Capozzi A et al., Gynecol Endocrinol (2022). PubMed
Biologic variation is important for interpretation of thyroid function tests. Andersen S et al., Thyroid (2003). PubMed
Treatment of iron deficiency and iron deficiency anaemia in menstruating women. Jepsen TMK et al., Ugeskr Laeger (2024). PubMed
The diagnosis and treatment of iron deficiency and its potential relationship to hair loss. Trost LB et al., J Am Acad Dermatol (2006). PubMed
Guidelines for the management of iron deficiency anaemia. Goddard AF et al., Gut (2011). PubMed
Interpretation of Abnormal Dexamethasone Suppression Test is Enhanced With Use of Synchronous Free Cortisol Assessment. Genere N et al., J Clin Endocrinol Metab (2022). PubMed
Validation of the 1 μg short synacthen test: an assessment of morning cortisol cut-off values and other predictors. Perton FT et al., Neth J Med (2017). PubMed