Blood Tests Before Starting GLP-1: What to Check and Why It Matters

There is one rule I would put above almost any other when choosing a GLP-1 provider: if a clinic is willing to prescribe without any baseline blood work, walk away. Responsible GLP-1 prescribing starts with understanding your body before introducing a powerful metabolic medication. A clinic that skips this is prioritising speed and sales over your safety.

Here is exactly what should be tested before you start, what the results tell you, and how to get it done in the UK.

Why Testing Matters Before You Start

Baseline blood work is not box-ticking. It screens for the specific risks GLP-1 medications carry:

• Thyroid cancer history. GLP-1 medications are contraindicated in people with a personal or family history of medullary thyroid carcinoma (MTC) or MEN2. This must be screened before prescribing.
• Pancreatitis risk. Baseline markers help establish your normal so any later problem is caught quickly.
• Kidney function. GLP-1 side effects like vomiting and dehydration can stress the kidneys. A baseline tells your clinician what is normal for you.
• Undiagnosed diabetes. An HbA1c reading confirms your metabolic starting point and may change which medication and dose is appropriate.

The Essential Panel

A proper baseline should include:

• HbA1c and fasting glucose: your metabolic and blood sugar starting point
• Full thyroid panel: TSH, free T3 and free T4, not just TSH alone
• Kidney function: eGFR and creatinine
• Liver enzymes: ALT and AST
• Full blood count: screens for anaemia and infection
• Lipid panel: cholesterol and triglycerides for cardiovascular context

If a clinic only checks your weight and BMI, that is not adequate assessment.

Ongoing Monitoring

Testing does not stop once you start. Retest at around three months, and then periodically, focusing on:

• Kidney function: particularly if you have had significant vomiting or diarrhoea
• Liver enzymes: to confirm the medication is well tolerated
• Thyroid function: especially in women over 45, where thyroid issues are more common

Some symptoms mean you should test or seek help urgently, not wait for the next scheduled panel: severe abdominal pain (a possible sign of pancreatitis) and a new lump or swelling in the neck (which warrants thyroid investigation). Do not sit on either.

How to Get Tested in the UK

You have two main routes. A clinic-based blood draw, or at-home finger-prick and venous testing kits that you post to an accredited lab. For most people, home testing is the easier and cheaper option, with results typically back within two to three working days.

Vitall Check offer UKAS-accredited at-home panels that cover the full pre-GLP-1 workup, including thyroid, HbA1c, kidney and liver function. Medichecks are another well-established UK option with a wide range of metabolic panels and doctor-reviewed results.

When your results come back, share them with your prescriber rather than self-interpreting. Out-of-range markers do not necessarily rule out treatment, but they change the conversation about dose, monitoring frequency and whether GLP-1 is right for you at all.

The Bottom Line

Blood testing before GLP-1 is the clearest marker of a clinic that takes your safety seriously. Insist on a full baseline panel, retest at three months and then twice a year, and never ignore severe abdominal pain or a neck lump. For the complete schedule, see our GLP-1 monitoring protocol and our breakdown of the GLP-1 blood test panel. Our clinic comparison flags which UK providers include blood testing as standard, which is exactly what you want.