The Labs Your Doctor Doesn’t Order (But Should)

Written by Erin Porter

I hear it constantly. People share with me a stack of lab results, every value flagged as normal, and tell me they’ve been feeling terrible for years. Their doctor has told them everything looks fine. They’ve been offered antidepressants, or asked if it’s possible that their symptoms are stress-related.

But when I look at what was actually tested, I see the same thing every time: a standard metabolic panel, a TSH, a CBC, maybe a lipid panel. The labs that would actually explain what this person is experiencing were never ordered.

Not because the doctor doesn’t care. But because the standard panel — designed for population-level screening — is not designed to find the conditions most commonly driving chronic illness in health-conscious, motivated patients who are clearly unwell but don’t fit a simple diagnostic box.

There is a different set of labs. They are not experimental or exotic. They are available through any standard laboratory. They are simply not part of the routine order set — and that omission leaves millions of people without answers.

Here are the six I consider essential, and what each one can tell you that standard testing misses entirely

1. ACTH (Adrenocorticotropic Hormone, Plasma)

What it is: ACTH is produced by the pituitary gland and signals the adrenal glands to produce cortisol. It is the messenger in the HPA (hypothalamic-pituitary-adrenal) axis communication chain.

Standard reference range: Approximately 7-63 pg/mL (morning draw; must be collected in the early morning when cortisol and ACTH are at their natural peak)

Why your doctor doesn’t order it: ACTH testing is typically reserved for endocrinology and is associated with diagnosing Addison’s disease or Cushing’s syndrome — conditions at the extreme ends of the spectrum. The vast middle ground — a blunted, low-normal HPA axis from chronic illness, steroid exposure (this happened to me after 5 months of nasal steroids) , or prolonged stress — is not something most primary care physicians are trained to identify or treat.

What it tells you: ACTH reveals the quality of pituitary-adrenal communication. A low-normal ACTH in the context of fatigue, hypoglycemia, poor stress tolerance, and a history of chronic illness or steroid use tells you the pituitary is not adequately signaling the adrenals — not because of structural failure, but because the axis has been downregulated by the cumulative burden of what the body has been through.

For example at the time I got off the nasal steroids, I had an ACTH of 26.8 — technically within the normal range, but at the low end, and clinically consistent with a central HPA downregulation pattern. Without this number, the daily hypoglycemia caused by the nasal steroids had no laboratory explanation. With it, the entire picture made sense.

The critical insight: A normal ACTH does not mean a healthy HPA axis. A low-normal ACTH in someone with chronic illness symptoms is meaningful clinical information that standard testing will never provide.

Optimal: Mid-to-upper range on a morning draw. Low-normal in the context of symptoms warrants investigation.

2. Pregnenolone

What it is: Pregnenolone is the upstream precursor to every steroid hormone in the body — cortisol, DHEA, progesterone, estrogen, testosterone, and aldosterone. It is synthesized in the adrenal glands, gonads, and brain from cholesterol. Without adequate pregnenolone, the entire steroid hormone cascade is compromised.

Standard reference range: Approximately 10-230 ng/dL (varies significantly by age and lab; declines substantially with age and chronic illness)

Why your doctor doesn’t order it: Pregnenolone testing is considered outside the standard of care in conventional medicine. Most physicians are unfamiliar with its clinical significance. It is not included in any standard panel and requires a specific order.

What it tells you: Pregnenolone is the single best upstream indicator of adrenal reserve — the substrate available for hormone production. Chronic illness, prolonged stress, viral infections like EBV, and steroid use all accelerate pregnenolone depletion. When pregnenolone is low, the body must choose which hormone pathways to prioritize — and cortisol, the survival hormone, wins. DHEA, progesterone, and sex hormones are sacrificed. This “pregnenolone steal” explains the constellation of hormonal disruption that follows in chronically ill patients.

I personally had a pregnenolone of almost zero — meaningfully insufficient for a body trying to maintain adequate cortisol counterregulation, DHEA production, and hormonal balance simultaneously. Supplementing conservatively (starting at 5mg) to restore the upstream substrate was one of the most important early interventions in my recovery.

Optimal: Individual and age-dependent, but functional medicine practitioners typically target 75-150 ng/dL for most adults as a functional optimal — significantly higher than many patients present.

Important: Always check DHEA-S alongside pregnenolone. Since pregnenolone converts downstream to DHEA, supplementing pregnenolone in someone with already-high DHEA can push DHEA above optimal range.

3. RBC Magnesium

What it is: Magnesium measured inside red blood cells rather than in the serum (blood plasma). This distinction is far more important than most people realize.

Standard reference range: Approximately 4.2-6.8 mg/dL for RBC magnesium. Optimal functional range: 5.5-6.5 mg/dL.

Why your doctor doesn’t order it: The standard magnesium test ordered in virtually every metabolic panel measures serum magnesium — the magnesium floating in the blood plasma. The body tightly regulates serum magnesium, pulling it from tissues and cells to maintain normal serum levels even when total body magnesium is severely depleted. This means serum magnesium can be completely normal while the patient is profoundly deficient.

RBC magnesium measures magnesium inside the red blood cells — a far more accurate reflection of true magnesium status throughout the body’s tissues. Most conventional physicians are either unaware of this distinction or unfamiliar with the RBC magnesium test.

What it tells you: Magnesium is a cofactor in over 300 enzymatic reactions — including insulin receptor function, cortisol synthesis, ATP production, protein synthesis, and DNA repair. RBC magnesium below optimal range is associated with:

Insulin resistance and impaired glucose metabolism
HPA axis dysfunction and impaired cortisol synthesis
Cardiovascular risk including hypertension and arrhythmia
Sleep disruption and anxiety
Chronic fatigue and muscle cramping
Increased systemic inflammation. If your magnesium is 5.1 — this is within the normal reference range, but below the functional optimal of 5.5-6.5. On a standard serum magnesium test, most would be told their magnesium was fine. The RBC test revealed meaningful room for optimization.
Optimal: 5.5-6.5 mg/dL. The specific form of magnesium matters for absorption — magnesium malate, glycinate, and threonate are among the most bioavailable forms.
4. Fasting Insulin
What it is: Insulin measured after an overnight fast — before any food has been consumed and before any postprandial insulin response has occurred.
Standard reference range: Labs typically report <25 µIU/mL as normal. Functional optimal: below 7 µIU/mL. Acceptable: below 10 µIU/mL.
Why your doctor doesn’t order it: The standard diabetes screening protocol relies on fasting glucose and HbA1c — measures of blood sugar, not of insulin. If fasting glucose is below 100 and HbA1c is below 5.7%, the conventional workup is considered complete. Fasting insulin is almost never included.This is a significant gap. Insulin resistance develops years to decades before fasting glucose rises. The sequence is: insulin rises first (compensatory hyperinsulinemia) → blood sugar stays normal initially as the pancreas works harder → eventually, insulin production can no longer compensate → blood sugar rises → pre-diabetes → diabetes. By ordering only glucose and HbA1c, conventional medicine is waiting for the last stage of a very long process.
What it tells you: Fasting insulin reveals the current effort required by the pancreas to maintain normal fasting glucose. Elevated fasting insulin with normal glucose is the earliest detectable marker of insulin resistance — and it is actionable years before any glucose-based test would show abnormality.
5. Free T3 (Triiodothyronine, Free)
What it is: T3 is the biologically active form of thyroid hormone — the molecule that actually enters cells, binds to thyroid hormone receptors, and drives metabolic function. Free T3 is the unbound, bioavailable fraction.
Standard reference range: 2.0-4.4 pg/mL. Functional optimal: upper third of range (3.2-4.4 pg/mL).
Why your doctor doesn’t order it: Most conventional thyroid screening consists of TSH alone. If TSH is within the 0.4-4.0 mIU/L reference range, the thyroid is considered normal. Free T4 is sometimes added if TSH is abnormal. Free T3 is almost never included in initial or routine thyroid evaluation.
This is a critical gap because the TSH-to-T4-to-T3 pathway has multiple points of potential failure that TSH testing alone cannot detect. A patient can have normal TSH, normal T4, and significantly low Free T3 — meaning the thyroid is producing hormone, the pituitary appears satisfied, but the actual active hormone reaching cells is insufficient. What it tells you: Free T3 reveals whether T4 is being efficiently converted to active T3 — a process that depends on:
Selenium (cofactor for the deiodinase enzyme that converts T4 to T3)
Gut microbiome health (approximately 20% of T4→T3 conversion occurs through gut bacterial deiodinase)
Zinc (T3 receptor activation)
Myo-inositol (TSH receptor signaling efficiency)
Overall inflammatory burden (chronic inflammation shunts T4 toward reverse T3 rather than active T3)

A friend of mine had a Free T3 of 2.8 — in the lower third of the normal range. Her TSH was 2.04 — completely normal. If only TSH had been ordered, her thyroid would have appeared fine. But 2.8 pg/mL of Free T3 meant her cells were working with significantly less than optimal thyroid hormone — contributing to her fatigue, her elevated LDL (thyroid hormones regulate LDL receptor expression), and her impaired glucose counterregulation. If you feel you may have thyroid dysfunction also ask them to test your TPO antibodies for Hashimoto’s.

The Reverse T3 connection: When chronic illness, stress, or inflammation is present, the body can shunt T4 toward Reverse T3 (rT3) — an inactive form that blocks T3 receptors. A low Free T3 alongside elevated Reverse T3 identifies this pattern. Her Reverse T3 of 9.7 was actually favorable (low end of normal), suggesting the T3 deficiency was more about inadequate production than conversion problems.

6. Free T4 (Thyroxine, Free)

What it is: T4 is the storage form of thyroid hormone produced directly by the thyroid gland. The thyroid produces predominantly T4, which is then converted peripherally to active T3. Free T4 is the unbound, bioavailable fraction — more clinically relevant than Total T4.

Standard reference range: Approximately 0.82-1.77 ng/dL (LabCorp). Functional optimal: 1.0-1.3 ng/dL (mid-range).

Why your doctor doesn’t order it: TSH is considered the gold standard screening test for thyroid function in conventional medicine. The rationale is that TSH reflects the pituitary’s assessment of thyroid status — if TSH is normal, the pituitary perceives adequate thyroid hormone. Free T4 is often only added when TSH is out of range.

But this logic has a significant flaw: the pituitary’s perception of thyroid adequacy is not the same as cellular thyroid hormone availability. Conditions including chronic illness, HPA axis dysfunction, nutritional deficiencies, and autoimmune damage can result in below-normal Free T4 even when TSH appears normal — because the pituitary-thyroid feedback loop itself is impaired.

What it tells you: Free T4 reveals what the thyroid gland is actually producing. Below-normal Free T4 with a normal TSH — a pattern that TSH-only screening would entirely miss — indicates the thyroid is not meeting the body’s demands, and the pituitary response is blunted.

This was precisely my friends pattern: Free T4 of 0.71 (below the lower limit of normal at most labs) with a TSH of 2.04 (completely normal). Her thyroid was underperforming. Her pituitary was not compensating adequately — consistent with the broader HPA axis blunting we had identified through ACTH and pregnenolone.

The consequences of undetected low Free T4:

Elevated LDL (thyroid hormones upregulate LDL receptors; suboptimal thyroid = fewer receptors = higher LDL)
Fatigue and cognitive fog
Impaired glucose metabolism
Slower gut motility
Reduced T3 available for cellular energy production

All of this was contributing to her clinical picture — and all of it would have been missed with TSH testing alone.

How These Six Labs Connect

What makes this panel truly powerful is how these markers interact and explain each other.

In my good friends bloodwork, the complete picture looked like this:

Low-normal ACTH → pituitary not adequately driving adrenal output
Low pregnenolone → insufficient upstream substrate for cortisol production
Low-normal Free T4 and Free T3 → inadequate thyroid output impairing glucose metabolism and LDL clearance
Elevated fasting insulin / HOMA-IR 2.7 → mild insulin resistance creating postprandial glucose overshoot
Sub-optimal RBC magnesium → reducing the efficiency of insulin signaling, cortisol synthesis, and cellular energy production
No single value explained everything. All six together told a coherent story that standard testing — TSH, fasting glucose, HbA1c, serum magnesium — was completely incapable of telling. This is why she had spent years being told her labs were normal while feeling anything but.
How to Get These Tests Ordered
Through a functional or integrative medicine practitioner: The most straightforward path. Practitioners in this space routinely order these tests and know how to interpret them in clinical context.
Through direct-to-consumer lab services: Companies like Ulta Lab Tests, Any Lab Test Now, and Marek Diagnostics allow patients in most states to order their own labs without a physician order. These are the same LabCorp and Quest panels your doctor would use — at often significantly lower cost.
Asking your primary care physician: Some will order these tests with appropriate clinical justification. Framing the conversation around specific symptoms (fatigue, reactive hypoglycemia, difficulty losing weight despite clean diet) gives the physician clinical rationale.
What to ask for specifically:
- ACTH Plasma (morning draw, collected in chilled EDTA tube)
- Pregnenolone, MS (mass spectrometry — most accurate method)
- RBC Magnesium (not serum magnesium)
- Fasting Insulin (collected after 8-12 hour fast)
- Free T3 (Triiodothyronine, Free)
- Free T4 (Thyroxine, Free, Direct)
- The information in these six tests will tell you more about your metabolic health, adrenal function, and thyroid efficiency than the standard annual panel most people receive. And in my experience, it is where the answers that have been missing finally begin to appear. You deserve to learn the state of your body and health.