GPs order tests that are clinically indicated for the symptom in front of them. That is how the system is designed, and for acute problems it works. But it means a thorough GP panel covers roughly 10 to 15 markers, and leaves out ferritin, vitamin D, B12, folate, Free T3, thyroid antibodies, hs-CRP, fasting insulin and every hormone unless something specific prompts them. This page tells you exactly what to ask for, how to ask, what to do if the answer is no, and when paying privately is the sensible move.
If you have a limited window and want the highest diagnostic yield per request, these are the five to name. Each is cheap, widely available, and commonly omitted.
A full blood count measures haemoglobin. It does not measure your iron stores. You can have entirely normal haemoglobin while your ferritin is depleted, which causes fatigue, breathlessness, palpitations and hair loss. Ask specifically for ferritin, and ask for the number rather than "normal". Iron blood test guide.
Roughly half of UK adults are insufficient over winter. It affects mood, immune function, bone health and muscle strength. Some practices restrict testing on cost grounds and advise supplementing blindly instead. If you have fatigue, low mood or frequent infections, it is worth pressing for.
Ask for both — they work through the same methylation pathway and one without the other is half a picture. Particularly important if you are vegan or vegetarian, over 50, or taking metformin or a proton pump inhibitor, all of which deplete B12.
Most NHS labs run TSH and only add Free T4 if TSH is abnormal. Free T3 is frequently rejected by the lab as outside protocol even when a GP requests it. This is the single most common gap in UK thyroid testing, because TSH alone misses poor T4-to-T3 conversion. Thyroid blood test guide.
Usually only tested once TSH is already abnormal. But antibodies can be raised for years beforehand, which is the window where you would actually want to know. Worth asking for if you have a family history of thyroid or autoimmune disease. Autoimmune blood test guide.
Lead with symptoms, not tests. "I want a ferritin test" invites a judgement call about whether it is indicated. "I have been exhausted for four months, I am breathless on stairs I used to manage, and my periods are heavy" gives the GP a clinical reason to order it. Same outcome, different route.
Bring a written list. A ten-minute appointment disappears fast. Writing down your symptoms, when they started, what makes them worse, and the two or three tests you would like considered keeps the conversation efficient and signals you have thought about it.
Ask for the actual numbers. "Normal" covers an enormous range. A ferritin of 16 is inside the NHS reference range and functionally depleted. Ask for your results with the figures and units, which you are entitled to, then you can interpret them properly rather than relying on a binary.
Name one clear priority. Presenting a list of fifteen requested tests tends to end badly. Pick the one or two that matter most for your symptoms and make the case for those.
If you are declined, ask for it to be recorded. You can say: "I understand. Could you note in my record that I requested this and it was declined, along with the reason?" This is a reasonable request, it creates a documented history, and it occasionally prompts a reconsideration. It is not confrontational — it is just record-keeping.
If you would rather not have the conversation at all, the TrueVitals Ultimate panel tests 114 biomarkers including every marker on this page — ferritin, full iron studies, vitamin D, B12, folate, Free T3, Free T4, thyroid antibodies, hs-CRP, fasting insulin and the full hormone profile. No referral, no justification, results in 48 hours. £349 plus £19 clinic phlebotomy.
GPs have clinical discretion over what they order, guided by NICE guidance and a finite budget held by the practice or the local commissioning group. A test that is not clinically indicated for your presenting symptom is, from the system's point of view, spending that should go elsewhere. That is a defensible position at population scale even when it is frustrating individually.
There is a second constraint most patients never see: the laboratory itself can reject a request. Free T3 is the well-known example — a GP can tick the box and the lab can decline to run it because it falls outside local protocol. So a refusal is not always the GP's decision, and pushing harder in the room will not change it.
Understanding this reframes the choice. The NHS is built to identify and treat disease, and it does that well. It is not built to give you a broad, proactive picture of how your body is functioning while you are still technically well. Those are different jobs. If you want the second one, the NHS was never the right tool, and no amount of asking will make it so. Full NHS vs private comparison.
These are not markers to demand in a ten-minute appointment. They are the ones worth understanding, because they are where proactive testing genuinely adds something the NHS pathway does not offer.
The NHS screens for diabetes with HbA1c. Insulin resistance develops years before HbA1c moves, and it is reversible in that window. Fasting insulin is almost never ordered in primary care. Prediabetes guide.
The 2026 AHA/ACC guidelines recommend ApoB over LDL for risk prediction. Lp(a) is genetic, elevated in about 20% of people, and worth knowing once in your life. The NHS tests neither. Heart health guide.
Testosterone, SHBG, oestradiol, progesterone, LH, FSH, prolactin, DHEA-S and cortisol. Ordered only when there is a specific clinical suspicion, which means most people never see them. Hormone blood test guide.
Standard CRP detects acute infection. High-sensitivity CRP measures the low-grade chronic inflammation associated with cardiovascular and metabolic risk. Different test, different question. Immune health guide.
Coeliac disease affects around 1 in 100 people in the UK and roughly three quarters remain undiagnosed. Many are told they have IBS without ever being screened. Gut health guide.
A kidney marker unaffected by muscle mass or creatine supplementation. Relevant for anyone muscular or training hard, whose creatinine-based eGFR reads misleadingly low. Gym blood test guide.
This page is on a private testing website, so treat the following with appropriate scepticism — but it is still true. There are situations where private testing is the wrong move and the GP is the right one.
Acute or alarming symptoms. Chest pain, unexplained weight loss, blood where it should not be, a lump, severe or sudden pain. These need clinical assessment, not a biomarker panel. Go to your GP, or urgent care.
Anything needing NHS follow-up anyway. Suspected haemochromatosis is the clearest example — the genetic test, the family screening pathway and lifelong treatment are all NHS, so testing privately duplicates rather than accelerates.
Monitoring a diagnosed condition. If you are already on levothyroxine or a statin, your monitoring is part of your NHS care and is free. Use it.
Private testing earns its place when you want breadth rather than depth on one question: a proactive picture across every system, at a point when you are not ill enough to trigger investigation but do not feel right. That is the gap. Is private testing worth it?
114 biomarkers including every marker your GP is unlikely to order. No referral, no justification, no waiting. Results in 48 hours.
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