GLP-1 medications
Blood Tests Before You Start a GLP-1
Reviewed by a qualified clinician · analysed at UKAS-accredited UK labs (ISO 15189)
Last reviewed July 202611 min read
Every Helvy guide is written by our health editors, then checked by a qualified clinician before it goes live and re-checked as the science moves. We name clinical roles, not individuals, until each reviewer has agreed to be credited publicly. This is wellness guidance to help you understand your own data, not a diagnosis.
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Before starting a GLP-1 medicine such as Mounjaro (tirzepatide), Wegovy or Ozempic (semaglutide), it is worth having a baseline blood picture: HbA1c (average blood sugar), liver function, kidney function including eGFR, a full lipid panel, and thyroid. This is general health information, not a substitute for the assessment your prescriber carries out. The reason a baseline helps is simple: it gives you a before-picture, so any change over the following months is easy to read.
Build a baseline test in two minutes →GLP-1 receptor agonists have gone from a diabetes treatment to one of the most talked about weight tools in the country. Whether you are getting one on the NHS, through a regulated online pharmacy, or a private clinic, the medicine works on your metabolism, your appetite and your digestion at the same time. That is a lot of moving parts, and a single blood test taken before you begin turns the whole experience from guesswork into something you can actually track.
This guide is deliberately about the baseline: the markers worth knowing before day one. It does not cover dosing, titration schedules or whether a GLP-1 is right for you. Those are decisions for you and a qualified clinician. What a home blood test can do is give you your own starting numbers so that, three and six months in, you can see what has moved.
1. Why a baseline is worth the trouble
A baseline is just a set of numbers taken while you feel normal, before anything changes. It sounds obvious, but most people start a medicine with no idea where they began. When a symptom appears later, there is nothing to compare it against, so the honest answer to “is this the medication?” is usually a shrug.
GLP-1 medicines are generally well tolerated, but the manufacturer guidance is clear that certain systems are worth keeping an eye on. The reason baseline testing keeps coming up is that some of the important changes, and side effects, act through your kidneys, liver and blood sugar, which are exactly the things a blood test measures well and a mirror does not.
On kidney function specifically, the Mounjaro (tirzepatide) prescribing information advises clinicians to “monitor renal function when initiating or escalating doses ... in patients with renal impairment reporting severe adverse gastrointestinal reactions”. The logic is that vomiting or diarrhoea can dehydrate you, and dehydration is hard on the kidneys. Knowing your kidney numbers before you start is what makes that kind of monitoring meaningful.
2. HbA1c and blood sugar
HbA1c is your average blood sugar over roughly the last three months. It is the single most useful metabolic baseline before a GLP-1, because lowering blood sugar is one of the things these medicines were originally designed to do. If your starting HbA1c sits in the pre-diabetes or diabetes range, a follow-up test is the clean way to see the medicine working on the number that matters, rather than relying on how you feel.
Our HbA1c blood test guide explains the UK ranges in mmol/mol, and the pre-diabetes guide covers where the thresholds sit. A one-off fasting glucose is a snapshot; HbA1c is the trend, which is why it is the better before-and-after marker.
3. Liver function
Liver enzymes such as ALT and ALP are a routine baseline. Fatty change in the liver is common alongside excess weight, and rapid weight loss can shift liver markers in either direction, so a starting reference makes any later result far easier to interpret. A single value on its own tells you little; the same value read against where you began tells you a great deal.
The liver function test guide walks through ALT, ALP, albumin and what a raised result can and cannot mean. As always, an unexpected liver result is a conversation for a qualified clinician, not a reason to change anything on your own.
4. Kidney function and eGFR
Kidney markers, creatinine, urea and the calculated eGFR, are the baseline the prescribing guidance leans on most. Because the most common GLP-1 side effects are gastrointestinal, and because dehydration from persistent vomiting or diarrhoea is what can strain the kidneys, a before-picture of your renal function is genuinely useful.
The kidney function test guide explains eGFR and what the stages mean. The practical takeaway is undramatic: know your starting eGFR, stay well hydrated, and if severe gut symptoms set in, that baseline is what lets a clinician judge whether anything has shifted.
5. Cholesterol and the full lipid panel
Weight loss often improves the lipid picture, and a full lipid panel, total, LDL, HDL, triglycerides and non-HDL, is one of the more rewarding things to track because it tends to move in the right direction. Measuring before you start means you get to watch it improve rather than assuming it did.
The cholesterol blood test guide covers the full set, and non-HDL cholesterol is the number UK guidelines increasingly favour as an overall risk marker.
6. Thyroid and other useful markers
Thyroid markers, TSH and Free T4, are worth a baseline because an under-active thyroid can quietly drive weight, energy and mood, and it is the kind of thing you want ruled in or out before you attribute everything to the medication. A tired, sluggish few weeks could be the GLP-1 settling in, or it could be a thyroid that was never checked.
Beyond thyroid, a broad wellness baseline that includes vitamin D, B12 and inflammation (CRP) rounds out the picture, since appetite-reducing medicines can shrink how much you eat and, over time, how many nutrients you take in. Our companion piece on nutrient monitoring on a GLP-1 goes deeper on that side.
7. How to get your baseline
No single off-the-shelf panel maps perfectly onto a GLP-1 baseline, which is why the honest route is either a custom marker list or a pair of complementary panels. The build-my-test tool lets you assemble exactly the markers above.
If you prefer ready-made panels, the Advanced Heart Health panel (£159) measures HbA1c, ApoB, Lp(a), hs-CRP and the full lipid panel, the metabolic and cardiovascular half of a baseline. The Thyroid & Vital Organs panel (£159) covers your thyroid, liver and kidney markers alongside the cholesterol panel, the organ-function half. Both are home finger-prick tests processed by UKAS-accredited UK laboratories, with results in around 5 working days.
Whichever route you take, the value is in repeating it. Test before you start, then again at three to six months, under the same conditions, and you turn a prescription into your own tracked experiment.
READY TO TEST?
Get your before-picture in place
A home finger-prick baseline gives you your own starting numbers, so the months ahead are something you can read rather than guess at. Results in around 5 working days from UKAS-accredited UK laboratories.
Frequently asked questions
What blood tests should I get before starting Mounjaro or Wegovy?
A useful baseline covers HbA1c (average blood sugar), liver function, kidney function including eGFR, a full lipid panel, and thyroid (TSH and Free T4). This is general information; your prescriber decides what they need for their own assessment. The point of a baseline is a before-picture that makes any later change easy to read.
Do I have to test before starting a GLP-1?
A regulated prescriber will carry out their own assessment before prescribing. A personal baseline blood test is optional and is about giving you your own tracked starting numbers, not about replacing clinical care. Many people find it reassuring to know where they began.
Why does kidney function matter on a GLP-1?
The most common side effects are gastrointestinal, and prolonged vomiting or diarrhoea can dehydrate you, which is hard on the kidneys. The Mounjaro prescribing information advises monitoring renal function in people with renal impairment who have severe gut symptoms. A baseline eGFR is what makes that monitoring meaningful.
When should I retest after starting?
A common approach is to retest at around three months, when changes are most likely to show, and again at six months, keeping the conditions the same each time. Anything that concerns you, or that your prescriber has asked you to watch, should be discussed with a qualified clinician.
Related guides
GLP-1 Testing Hub
Every guide on blood tests around Mounjaro, Wegovy and Ozempic in one place.
Blood Tests for Ozempic & Mounjaro
What to monitor while you are on a GLP-1, and how often.
GLP-1 & Nutrient Deficiencies
Which nutrient markers can drift when appetite drops, and how to watch them.
HbA1c Blood Test UK
Your three-month average blood sugar, the UK ranges, and what they mean.