Baseline bloods before starting weight management medication — checking safety, and ruling out treatable causes first.
Referred by a GMC-registered GP, results reviewed by the same doctor
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These are the baseline bloods taken before starting weight loss medication, and the reason for them is more interesting than a box-ticking exercise.
They serve two purposes. The first is safety — establishing that liver, kidney and thyroid function are adequate for treatment. The second matters more: establishing why the weight is where it is before assuming the answer is a drug. Several causes of weight gain are reversible, and a BMI figure excludes none of them.
HbA1c and diabetes markers — identifying prediabetes or diabetes, which changes both the treatment choice and how it is monitored.
Thyroid function, in the extended panel — an underactive thyroid causes weight gain and fatigue and is entirely treatable in a different way. Starting a GLP-1 in someone with untreated hypothyroidism treats the symptom and leaves the cause.
Liver function — fatty liver is extremely common alongside excess weight, usually silent, and needs knowing about before treatment starts.
Kidney function — relevant because dehydration from the nausea and vomiting these drugs cause can affect the kidneys.
Lipids with hs-CRP — the cardiovascular picture, which is much of the reason for treating weight in the first place.
Iron status and vitamin D — deficiencies are common and cause the fatigue often attributed to weight alone.
Full blood count and urinalysis complete the baseline.
The extended panel adds cortisol, which screens for Cushing's syndrome — rare, but a genuine and reversible cause of weight gain that is easily missed.
Included: full blood count · liver function · kidney function · heart health with cholesterol, LDL, HDL, triglycerides and hs-CRP · diabetes markers including HbA1c · bone health with vitamin D · iron status including ferritin · urinalysis.
The extended version adds full thyroid function with antibodies, which is worth having before starting weight management treatment — an untreated underactive thyroid is a genuine and easily missed cause of weight gain, and treating it first is both cheaper and safer than prescribing around it.
1. Consultation. A full history, including when the weight changed and what else changed at the same time. That question alone identifies the medication-related cases, which are commoner than people expect.
2. Referral. To a Randox Health clinic at a time that suits you.
3. The sample. A 10 to 12 hour fast is required for accurate lipid and glucose results. Water only.
4. Results review. A GP goes through everything and gives you a straight answer about whether medication is the right route. Where a reversible cause is found, treating that comes first — and where a specialist weight management provider would serve you better for the ongoing programme, we will say so. Structured coaching and weekly support materially improve outcomes on these drugs, and periodic GP review is not a substitute. See our comparison of the two models.
A 10 to 12 hour fast is required for accurate lipid and glucose results. Water only.
Two purposes, and it helps to separate them.
First, safety before starting treatment — liver and kidney function, thyroid, and a check that nothing else explains the picture.
Second, establishing what your weight is actually doing to you. HbA1c, lipids, liver markers and blood pressure describe the metabolic consequences — and those, rather than the number on the scale, are what treatment is trying to change.
Almost everyone wants their thyroid checked, and it is right to check it. But it is worth being straight: an underactive thyroid rarely accounts for more than a few kilograms, and treating it does not usually produce substantial weight loss.
It should still be excluded, because it is treatable and because the fatigue that comes with it makes everything else harder.
Mildly raised liver enzymes are extremely common in people carrying excess weight, and usually reflect fatty liver disease — now the commonest liver condition in the UK.
It matters and it is reversible. Weight loss of around 7 to 10 per cent can substantially improve or resolve it, which makes it one of the more motivating things a blood test can show you.
Not that your weight does not matter, and not that treatment is unnecessary. It means you are starting from a safer baseline.
It also does not mean the weight is your fault. Metabolic bloods describe consequences, not effort.
Most people hoping for a hormonal explanation do not find one, and that can be disheartening.
It is worth saying plainly: an unremarkable set of results does not mean the difficulty is imagined or a matter of willpower. Weight regulation is biological, and the biology is largely not measurable in a blood sample.
Weigh or measure you. Height, weight, waist circumference and blood pressure all need doing — and weight-loss medication cannot safely be prescribed without them, which is why a self-reported figure is not enough.
Below the recognised BMI thresholds, in pregnancy or when trying to conceive, or where an eating disorder is present or suspected. Those are firm.
The NHS Health Check is free for adults aged 40 to 74 every five years, and covers cholesterol, blood pressure, diabetes risk and BMI — much of what is on this page.
NHS tier 2 and tier 3 weight management services are free, and tier 3 includes dietitians, psychological support and access to medication. Referral criteria vary by area but are worth asking about, because the support is more comprehensive than anything a prescription alone provides.
And GP-prescribed weight-loss medication is available on the NHS within specific criteria — at a prescription charge rather than a private price.
Private weight-loss medication is expensive and generally needs to continue long term, because weight is usually regained after stopping.
Ask any provider for the annual cost, not the monthly one, before starting. That is the figure that matters, and it is rarely the one advertised.
It should be checked, but honestly — rarely by much.
An underactive thyroid typically accounts for a few kilograms, and treating it does not usually produce large weight loss. It is still worth excluding, because it is treatable and the fatigue makes everything harder.
Occasionally — PCOS and Cushing's syndrome are the main ones, and both are looked for where the picture fits.
But most people hoping for a hormonal explanation do not find one, and that is worth knowing before testing rather than after.
It does not mean the difficulty is imagined. Weight regulation is biological, and most of that biology does not show up in blood.
A 10 to 12 hour fast, water only, because triglycerides and glucose are both meaningfully affected by recent eating.
Take your usual medication as normal unless told otherwise, and if you have diabetes on insulin or sulfonylureas, speak to us first — fasting needs planning.
Usually it is fatty liver, and it is common and reversible.
Around 7 to 10 per cent weight loss can substantially improve or resolve it, which is one of the more encouraging things these results can show.
Only with proper measurements, which we cannot take remotely. Height, weight, waist and blood pressure are all needed — a self-reported figure is not adequate.
And not below the recognised BMI thresholds, in pregnancy or when trying to conceive, or where an eating disorder is present or suspected.
No, and nothing in a blood sample does — despite how "metabolic health panels" are frequently marketed.
The NHS Health Check for adults aged 40 to 74 covers much of this free, every five years.
And NHS tier 2 and tier 3 weight management services are free, with tier 3 offering dietetic and psychological support alongside medication — more complete than a prescription on its own.
Realistically, for most people, yes, or the weight returns.
That is not a failure of willpower — it reflects how appetite regulation works. Ask for the annual cost before starting, because that is the decision you are actually making.
HbA1c, lipids, liver and kidney function, thyroid and vitamin D — and a serious conversation about sleep, because untreated sleep apnoea both drives weight gain and makes losing weight considerably harder.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 23, 2026
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