How reference ranges are actually built
Most lab reference intervals are constructed using a method called the central 95%, the lab takes a sample of "presumed healthy" people who walk into draw stations, throws out the top 2.5% and bottom 2.5%, and calls everything in between "normal." The CLSI (Clinical and Laboratory Standards Institute) guideline EP28-A3c codifies this approach.
The problem is the input population. The people walking into Quest and LabCorp are disproportionately older, heavier, and metabolically unwell. The 2017 NHANES analysis found that fewer than 12% of U.S. adults are metabolically healthy by basic criteria (Araújo et al., Metabolic Syndrome and Related Disorders, 2018). A range built from this population is, by definition, a range that includes a lot of disease.
This is why the same testosterone level, say 320 ng/dL, can be called "normal" by Quest and "clearly suboptimal" by an experienced endocrinologist. Both are right within their respective frames. Statistical normal and clinical optimal are different concepts.
The four most consequential gaps
Total testosterone
Total testosterone (men, ng/dL)
The Endocrine Society's clinical practice guideline acknowledges that men may experience symptoms of hypogonadism at levels above the lab's lower limit (Bhasin et al., J Clin Endocrinol Metab, 2018). The HIM study (Hypogonadism in Males), which followed 2,162 men, found symptomatic improvement plateaus around 500-600 ng/dL, meaning men under that level often feel hypogonadal even though they're "normal." Optimal, where men report best mood, libido, recovery, and body composition, is typically 700-900.
Vitamin D (25-hydroxyvitamin D)
Vitamin D 25-OH (ng/mL)
Vitamin D <30 ng/mL is officially "deficient." But large meta-analyses show diminishing all-cause mortality risk all the way up to ~50 ng/mL (Garland et al., Am J Public Health, 2014), with bone mineral density, immune function, and mood markers all favoring 50-80 over 30-50. The lab calling 32 ng/mL "normal" misses that this person is missing out on substantial benefit.
Thyroid (TSH)
TSH (mIU/L)
The NACB (National Academy of Clinical Biochemistry) recommended in 2003 that the upper limit of TSH be lowered to ~2.5 mIU/L based on data showing higher cardiovascular and metabolic risk above that level (Surks et al., JAMA, 2004). Most labs never updated. A TSH of 3.8 is technically normal, often symptomatic (fatigue, weight gain, cold intolerance), and would warrant investigation in a functional medicine setting.
HbA1c
The diabetes diagnostic threshold is 6.5%. Pre-diabetes starts at 5.7%. Most people get told their A1C of 5.5 is "fine." But large prospective cohorts show cardiovascular risk begins to climb above 5.4% (Selvin et al., NEJM, 2010). Optimal is <5.4%; ideally <5.2%. Catching elevation at 5.5-5.6 is far easier to reverse than at 6.0+.
The other ranges where the gap matters
| Marker | Lab "normal" | Optimal |
|---|---|---|
| Free testosterone (men) | 4.0-28.0 ng/dL | 15-25 ng/dL |
| Estradiol (men) | 10-40 pg/mL | 20-35 pg/mL |
| SHBG (men) | 10-80 nmol/L | 20-45 nmol/L |
| Estradiol (women, premenopause) | 15-350 pg/mL | 50-250 (cycle-dependent) |
| Progesterone (women, luteal) | 1.8-24 ng/mL | 10-20 |
| Free T3 | 2.0-4.4 pg/mL | 3.2-4.2 |
| Reverse T3 | 9.2-24.1 ng/dL | <15 |
| Fasting insulin | 2.6-24.9 µIU/mL | <7 |
| Triglycerides | <150 mg/dL | <80 |
| HDL (men) | >40 mg/dL | >55 |
| HDL (women) | >50 mg/dL | >65 |
| ApoB | <130 mg/dL | <80 |
| hs-CRP | <3.0 mg/L | <1.0 |
| Homocysteine | <15 µmol/L | <8 |
| Ferritin (men) | 30-400 ng/mL | 70-150 |
| Ferritin (women) | 13-150 ng/mL | 50-100 |
| Vitamin B12 | >200 pg/mL | >500 |
| Magnesium (RBC) | 4.2-6.8 mg/dL | 5.5-6.8 |
The pattern: Optimal ranges are usually narrower and shifted toward the upper end of "normal" for hormones and nutrients, and toward the lower end for inflammatory and metabolic markers. The body works best when biomarkers cluster in tight, healthy ranges, not just inside the wide statistical fence.
Why your doctor doesn't always use optimal ranges
Three reasons:
- Insurance coverage. Treatment is reimbursed when patients fall outside lab "normal," not when they're suboptimal. There's no billing code for "vitamin D 35 with brain fog."
- Liability. Following the lab's flagged values is the path of least medical-legal risk. Diagnosing and treating off the lab range requires more clinical judgment.
- Volume medicine. A 12-minute appointment doesn't allow for nuanced lab interpretation. The flagged-or-not heuristic is faster.
This is also why preventive and longevity-focused medicine, telehealth or in person, has emerged as a separate model. The job there is to help patients reach optimal, not just stay above the floor.
Reference ranges that actually move with age
One nuance worth understanding: some labs have started publishing age-stratified ranges (e.g., LabCorp's age-specific testosterone tiers, released 2017). These are better than a single 264-916 number, but they're still descriptive, they show what's common at each age, not what's optimal. A 60-year-old man with testosterone of 350 is "normal for his age" but absolutely could function much better at 700.
The concept "normal for your age" is descriptive. "Optimal for any age" is a different, and more useful, frame.
How to interpret your own results
- Compare each marker against optimal, not just the lab's range.
- Look at patterns, not just individual numbers. Low-normal testosterone + high-normal SHBG + low-normal vitamin D is a different picture than any one of those alone.
- Match results to symptoms. A 35-year-old with testosterone of 480 and no symptoms is different from one with the same number who's exhausted.
- Track trends over time. A single value is a snapshot; a series across 6-12 months is the actual signal.
- Use a clinician who reads optimally, not just normally.
What this looks like at OPTML
Every panel includes a physician interpretation that compares your results to optimal ranges, identifies the patterns, and recommends specific next steps when biomarkers fall outside healthy windows. The Foundation, Optimized Health, and Longevity panels are designed to give you the markers that matter, not just the ones reimbursed.
The bottom line
"Normal" labs in 2026 represent the middle of a population that is largely metabolically unwell. Treating "normal" as the goal is treating "average" as the goal, and the average American is overweight, pre-diabetic, vitamin D deficient, and sleeping six hours. Optimal is the standard worth aiming for, and the gap between the two is precisely where the unexplained fatigue, weight gain, brain fog, low libido, and slow recovery live.