Core Metabolic Labs: The Difference Between Normal and Optimal

When your lab report says a value is "normal," it means you fall within the range of the people the lab tested, not that the number is healthy. Those are different things, and the gap between them is where a lot of metabolic disease hides for years. Here's how I think about the core metabolic labs: what the conventional reference range allows, and where the evidence (and a metabolic-health lens) says the optimal number actually sits. Treat this as a map for a conversation with your physician, not a tool for self-diagnosis.

One note before the numbers. Reference ranges and units vary between labs, "optimal" is better supported for some markers than others, and the right target for any one person depends on the whole picture.

Glucose and insulin

Marker	Conventional "normal"	Optimal target	What it tells you
Fasting insulin	up to ~25 µIU/mL	< 5 µIU/mL	How hard the pancreas works to hold glucose down
Hemoglobin A1c	< 5.7%	< 5.3%	Average blood glucose over about 3 months
Fasting glucose	< 100 mg/dL	70-90 mg/dL	Blood glucose at rest

Lipids and cardiovascular

Marker	Conventional "normal"	Optimal target	What it tells you
Triglycerides	< 150 mg/dL	< 80 mg/dL (ideally 50-70)	Circulating fat, tied to carbohydrate intake and insulin resistance
HDL cholesterol	> 40 mg/dL	> 60 (men), > 70 (women)	The lipoprotein that tracks with metabolic health
Triglyceride/HDL ratio	not reported	< 1.0 (good < 2.0)	A simple stand-in for insulin resistance
Lipoprotein(a)	not routinely run	< 30 mg/dL	A largely genetic, independent cardiovascular risk factor
GGT	up to 40-60 U/L	< 20 U/L	Liver stress and oxidative load

Hormones

Marker	Conventional "normal"	Optimal target	What it tells you
TSH	up to ~4.5 µIU/mL	0.5-2.0 µIU/mL	Thyroid signaling
Free T3	wide	upper half (~3.5-4.5 pg/mL)	The active thyroid hormone that drives metabolism
Reverse T3	wide	< 20 ng/dL	High levels suggest stress or poor conversion
Total testosterone (men)	~300-1000 ng/dL	700-1100 ng/dL	Vitality, muscle, libido
Estradiol (men)	wide	~20-30 pg/mL	Balance with testosterone
Cortisol (AM)	wide	12-18 µg/dL	The morning stress-hormone peak
DHEA-S	wide	> 350 (men), often > 150 (women) µg/dL	Adrenal reserve, age-adjusted

Inflammation and nutrients

Marker	Conventional "normal"	Optimal target	What it tells you
hs-CRP	< 3 mg/L	< 0.5 mg/L (good < 1.0)	Systemic inflammation
Homocysteine	up to ~15 µmol/L	< 8 µmol/L	B-vitamin status and vascular risk
Vitamin D (25-OH)	> 30 ng/mL	~50-80 ng/mL (debated)	Immune and metabolic function
Magnesium (RBC)	serum 1.8-2.6 mg/dL	> 6.0 mg/dL (RBC)	Cellular magnesium, which serum hides
Uric acid	up to ~7 mg/dL	< 5.5 (men), < 4.5 (women)	Tracks with fructose intake and metabolic risk
ALT / AST	up to ~40 U/L	< 20 U/L	Liver health and fatty liver

The handful that matter most

If you track only a few of these, make them the insulin-resistance markers, because insulin resistance is the engine under most metabolic disease and it moves first. Gerald Reaven argued decades ago, in his 1988 Banting Lecture, that insulin resistance sits upstream of the whole metabolic syndrome [1]. You can estimate it cheaply. Fasting insulin paired with fasting glucose gives you HOMA-IR, the validated index Matthews and colleagues described in 1985 [2]. Even simpler is the triglyceride-to-HDL ratio: McLaughlin and colleagues, working in Reaven's group, showed it picks out insulin-resistant people about as well as far more complex testing [3]. A ratio under 1.0 is a good sign; a high one is a red flag even when glucose looks fine.

Two more earn their place. High-sensitivity CRP measures the low-grade inflammation that drives both heart disease and metabolic dysfunction, and Ridker and colleagues showed it predicts cardiovascular events independently of cholesterol [4]. Lipoprotein(a), a largely inherited particle most people never have measured, is an independent cardiovascular risk factor worth checking at least once in your life, as Nordestgaard and colleagues laid out in a 2010 consensus [5].

A word on the hormone and nutrient targets

The hormone and micronutrient "optimal" values deserve more humility than the glucose and lipid markers. They're more individual, more lab-dependent, and less settled in the literature. A personal example: on a strict carnivore diet, my own total testosterone drifted to around 500 ng/dL, comfortably inside the lab's normal range but below where I want it, a reminder that normal and optimal aren't the same thing, and that diet moves these numbers. Use these targets as prompts to investigate, not as rigid goals, and read them with someone who knows your history.

How to use this

The pattern across the whole list is the same. Conventional reference ranges are built to flag obvious disease, not to define health, so a result can read "normal" while a trend has been heading the wrong way for years. That's the argument for measuring these markers early, watching which way they move, and acting while the numbers are still just trends. Bring this list to your physician, ask for the ones missing from a standard panel (fasting insulin, the triglyceride-to-HDL ratio, hs-CRP, and lipoprotein(a) are the usual omissions), and use the results to guide change rather than to label yourself.

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References

1. Reaven GM. Banting lecture 1988. Role of insulin resistance in human disease. Diabetes. 1988. PMID: 3056758
2. Matthews DR, et al. Homeostasis model assessment: insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia. 1985. PMID: 3899825
3. McLaughlin T, et al. Use of metabolic markers to identify overweight individuals who are insulin resistant. Ann Intern Med. 2003. PMID: 14623617
4. Ridker PM, et al. C-reactive protein and other markers of inflammation in the prediction of cardiovascular disease in women. N Engl J Med. 2000. PMID: 10733371
5. Nordestgaard BG, et al. Lipoprotein(a) as a cardiovascular risk factor: current status. Eur Heart J. 2010. PMID: 20965889