Thinking About a Weight-Loss Jab? Here's the Blood Test to Have First

If you're starting Mounjaro, Wegovy, or another GLP-1 weight-loss injection, a baseline blood test matters more than the number on the scale. It checks kidney, liver, thyroid, and blood sugar markers before the drug changes them, so any future shift can be caught early. One honest limitation: a blood test can't tell you whether the medication is right for you long-term, only whether it's safe to start.

Why this is on my mind this week

Recent survey data puts current GLP-1 use at 6.3% of households in Great Britain in 2026, up from 4.1% in 2025 and 2.3% in 2024. That's roughly 1.9 million adults currently taking a weight-loss injection, and women make up around 77% of users, with the highest uptake among those aged 30 to 49.

The MHRA has logged several hundred adverse event reports linked to these medications since 2024, including two deaths currently being investigated for a possible connection. Around 15% of users are getting these drugs off-label, often through online prescribers with limited clinical oversight.

None of this means the drugs themselves are unsafe when used properly. It does mean a lot of people are starting with nothing more than a questionnaire and a BMI figure: no blood test, no baseline, nothing to compare against if something changes six months in.

What a baseline blood test actually checks

Picture two people eight months into treatment, both flagged for slightly raised liver enzymes on a routine check. One had blood taken before their first injection. The other didn't. For the first, we can usually tell within a day or two whether that change is new or was quietly there all along. For the second, there's no way to know, and that gap is the difference between a reassuring conversation and a referral for further tests.

GLP-1 medications are cleared through the kidneys and processed by the liver, so knowing both are working properly beforehand isn't optional. It's basic safety, and it's the only honest way to tell a genuine change from normal variation later on.

A proper pre-treatment panel covers kidney function, liver function, HbA1c and fasting glucose, a lipid profile, and thyroid function. Thyroid checks matter specifically because GLP-1 drugs carry a caution around certain thyroid conditions, something a photo and a weight reading were never going to pick up.

	Online prescriber (questionnaire only)	Baseline blood test first
What's assessed	Self-reported weight, BMI, symptoms	Kidney, liver, thyroid, glucose, lipids
Catches early kidney or liver strain	No	Yes
Flags thyroid-related caution	No	Yes
Gives a comparison point for 3-6 month follow-up	No	Yes

What helps before you start

The most important thing to know about starting a GLP-1 medication is that your baseline numbers become the yardstick for everything that follows. Without them, neither you nor your prescriber can tell whether a symptom three months in is the drug working as expected or something that needs attention.

if you're already a few weeks into treatment and only just booking your first blood test, mention your start date when you book. Early nausea and reduced appetite can temporarily affect hydration, and dehydration on its own can nudge kidney readings higher, so a test taken during that adjustment period needs reading differently to one taken before day one or once things have settled.

Getting all of this from a single visit is where a purpose-built panel earns its keep, rather than piecing results together from several separate tests. Our GLP-1 Health Monitoring Panel was built for exactly this: 38 biomarkers for £89, covering liver, kidney, thyroid, cholesterol and blood sugar together. It's a private blood test UK-wide, so a wait for a GP slot doesn't have to hold up your baseline, you can get a blood test without GP involvement and have results back quickly.

Where there's more going on, family history, existing conditions, other medications, we sometimes build a Bespoke Wellness Profile around the GLP-1 panel instead, so it reflects your actual history rather than a generic list. It's still a home blood test UK service either way. For women, adding hormone markers through a women's hormone blood test can also flag whether rapid weight change is affecting cycle regularity, something GLP-1 use has been linked to in some patients. For men trying to conceive during treatment, a male fertility test at home through our Male Fertility Panel can check whether reproductive hormones have shifted too, rather than assuming they haven't.

FAQ

Do you need a blood test before starting Mounjaro or Wegovy?

It isn't a legal requirement everywhere it's prescribed, but it's strongly advisable. Checking kidney, liver, and thyroid function beforehand gives a safety baseline and something to compare against if symptoms appear later, which is why we built a dedicated GLP-1 Health Monitoring Panel covering 38 biomarkers for exactly this purpose.

Can weight-loss injections affect your kidneys or liver?

These medications are processed through the kidneys and liver, so pre-existing issues in either can affect how the drug behaves in your body. This is worth discussing with a healthcare professional before starting, particularly if you have any history of kidney or liver problems.

Are online weight-loss jab prescriptions safe without a blood test?

Many online prescribers rely on a questionnaire rather than lab results, which means some risk factors go unchecked. A private blood test alongside the prescription gives a fuller safety picture than self-reported symptoms alone.

How often should you get blood tests while on a GLP-1 medication?

There's no single rule, but many clinicians suggest rechecking core markers around three to six months after starting, then periodically after that. This is worth agreeing with whoever is prescribing your medication.

Can weight-loss drugs affect fertility or periods?

Rapid weight change linked to GLP-1 use has been associated with menstrual cycle changes in some women, and hormone shifts in some men. This is worth checking through a hormone or fertility blood test rather than assuming it isn't relevant to you.

Weight-loss injections are having a real moment in the UK, and for a lot of people they're genuinely helpful. But a prescription without a blood test is a bit like starting a journey without checking the fuel gauge first, it might be fine, but you won't know until it isn't.

Ready to take control of your health? Book your test today at kuonhealthcare.co.uk