Normal vs optimal: 5 blood test ranges explained

Adapted from Charmaine’s free guide. Rewritten for the website mockup; the edits are awaiting her final approval.

You have done everything right. You saw your GP, had blood tests, and the answer came back: normal. Yet you still feel exhausted, foggy, or as if your hormones or weight are working against you.

You are not imagining it.

What’s the difference between normal and optimal?

NHS blood panels are designed to detect disease, such as anaemia, diabetes or an underactive thyroid at a level that can be diagnosed. They aren’t designed to identify the early stages of dysfunction.

Reference ranges describe where most of the population sits. Population average isn’t the same as optimal health. Many people whose results are technically in range have real, measurable problems.

In functional medicine, I use narrower target ranges, based on research into where markers sit when people feel and function well. I also test markers the standard panel leaves out.

“The goal is not to find out what disease you have. It’s to understand why your body isn’t functioning the way it should, and what we can do about it.”

The five markers at a glance

Ranges vary between laboratories, so always read your own results against the range printed on your report.

1. Ferritin: why your iron stores matter

Ferritin is your iron storage protein, the fuel tank rather than the fuel line. Iron is essential for carrying oxygen, energy production, thyroid hormone conversion and making dopamine and serotonin.

Ferritin isn’t always included in a routine panel, and a result of 20 µg/L may still be reported as normal. Many people feel the effects of low iron well before anaemia develops.

Signs include persistent tiredness, especially in the afternoon, hair thinning, brain fog, restless legs, breathlessness on mild exertion and brittle nails.

Ferritin also rises with inflammation, so it should be read alongside CRP and transferrin saturation. I explain the full iron panel in Ferritin “normal” but still exhausted? How to read your iron results.

2. Vitamin D: more like a hormone than a vitamin

Vitamin D acts on receptors in almost every tissue, including the brain, immune system, gut, thyroid, ovaries and heart. It is involved in immunity, mood, bone and muscle health and hormone production.

In the UK, the NHS advises that from October to early March we can’t make enough vitamin D from sunlight. It recommends a daily 10 microgram (400 IU) supplement for everyone over the autumn and winter, and all year for some people.

The NHS treats levels below 25 nmol/L as deficient. In my practice, I generally aim for 100 to 150 nmol/L. I prefer to test first, then set a dose: adults shouldn’t take more than 100 micrograms (4,000 IU) a day unless advised by a clinician. I retest after about three months.

Signs of low vitamin D include frequent infections, low mood in winter, bone or muscle aches, fatigue and poor sleep.

3. Thyroid: why TSH alone can miss the picture

Your thyroid makes mainly T4, which is converted into the active hormone T3 in the liver, gut and other tissues. TSH is the signal from the pituitary gland asking the thyroid to work harder.

NHS testing usually starts with TSH, and adds free T4 if TSH is abnormal. That is a reasonable screen, but it doesn’t show how well you convert T4 to T3. Thyroid antibodies, which identify Hashimoto’s thyroiditis, aren’t routinely tested either.

Signs include persistent fatigue, feeling cold, constipation, weight gain, brain fog, low mood, dry skin and thinning hair, including the outer third of the eyebrows.

To support healthy thyroid function, I look at selenium, zinc, iodine and iron status, gut health and stress. Iodine supplements aren’t right for everyone, particularly with Hashimoto’s, so test before you take them. Read more in Is TSH enough? and Thyroid or perimenopause? How to tell the difference.

4. hs-CRP: your inflammation thermometer

C-reactive protein (CRP) is made by the liver in response to inflammation. The high-sensitivity test (hs-CRP) detects much lower levels: the quiet, long-term inflammation linked with heart disease, type 2 diabetes, joint pain, fatigue, brain fog and low mood.

Standard CRP is designed to detect infection or major inflammation and is usually reported as normal below 5 mg/L. For heart risk, the American Heart Association uses hs-CRP bands: below 1 mg/L is low risk, 1 to 3 mg/L average and above 3 mg/L high. I aim for below 1.0 mg/L.

What helps: a Mediterranean-style diet, omega-3 fats from oily fish, regular movement, good sleep and stress management. I also look for hidden drivers, such as gum disease, gut inflammation, food reactions and excess abdominal fat.

5. Fasting insulin and HOMA-IR: catching insulin resistance early

Insulin moves glucose into your cells. When cells stop responding well, the pancreas makes more insulin to compensate. Blood glucose can look normal for many years while insulin creeps up.

NHS screening uses fasting glucose and HbA1c, which rise later in the process. Fasting insulin is rarely tested. HOMA-IR is a simple calculation from fasting insulin and glucose that estimates insulin resistance.

Signs include energy crashes after meals, sugar cravings, weight gain around the middle, afternoon brain fog, irregular periods or PCOS, high triglycerides and skin tags.

What helps: fewer refined carbohydrates, a protein-rich breakfast, strength training two or three times a week, better sleep, stress management and, for some people, a shorter eating window. Supplements such as magnesium, inositol and berberine have evidence, but berberine interacts with several medicines, so take advice first.

What should you do with your results?

Start by asking for copies of your results, with the reference ranges. Then look at the five markers above. Are any missing? Are any at the edge of the range? Have they changed over time?

In my consultations, I run an advanced blood panel that includes all five markers, read against functional target ranges and alongside your symptoms, history and goals. See Laboratory testing and Why are my blood tests normal but I still feel unwell?

If you have new or worrying symptoms, see your GP first. Functional testing works alongside NHS care, not instead of it.

Common questions

What blood tests should I ask for if I’m always tired?

A useful starting point is a full blood count, ferritin and iron studies, vitamin D, B12 and folate, TSH with free T4 and free T3, hs-CRP, fasting glucose and insulin or HbA1c. Your GP may test some of these; others are available privately.

What is a good vitamin D level in the UK?

The NHS treats below 25 nmol/L as deficient and 25 to 50 nmol/L as possibly inadequate. In my practice I generally aim for 100 to 150 nmol/L, using a dose based on testing and no more than 4,000 IU a day without clinical advice.

What is a normal hs-CRP level?

For heart risk, below 1 mg/L is considered low risk, 1 to 3 mg/L average and above 3 mg/L higher risk. A one-off high result can reflect a recent infection, so it is often worth repeating.

Can I get fasting insulin tested in the UK?

It is rarely included in NHS tests unless there is a specific reason, but it is widely available privately. It’s measured after an overnight fast, ideally with fasting glucose so that HOMA-IR can be calculated.

Does a normal TSH mean my thyroid is fine?

Not always. TSH is a good screening test, but it doesn’t show free T3 levels or whether thyroid antibodies are present. If you have symptoms, it may be worth looking at the full thyroid panel.

Sources

• NHS: Vitamin D
• NHS inform: Vitamin D (upper limit for supplements)
• NICE NG145: Thyroid disease, assessment and management
• British Thyroid Foundation: Thyroid function tests
• Newcastle upon Tyne Hospitals: Adult haematology guidelines for GPs (ferritin thresholds)
• American Heart Association, Circulation: C-reactive protein and heart risk