Blood Tests You Need Before Starting GLP-1: The Complete UK Panel

Starting GLP-1 medication without a baseline blood panel is like starting a training programme without knowing your current fitness level. You cannot tell whether the medication is working, whether anything is improving, or whether something concerning is developing unless you have numbers to compare against.

This guide covers the exact markers you should test before starting semaglutide or tirzepatide, what you need at 3 months, and what a full 6-month review should include. These are not theoretical recommendations — they are based on the monitoring protocols used in the major clinical trials and current UK prescribing guidance.

Why Baseline Testing Matters

The clinical evidence for GLP-1 is robust. Semaglutide 2.4mg produced 14.9% mean body weight reduction at 68 weeks in STEP 1. Tirzepatide 15mg reached 20.9% at 72 weeks in SURMOUNT-1. But the benefits extend well beyond weight:

Without baseline numbers, you cannot see these improvements. You also cannot catch the handful of cases where markers move in the wrong direction — elevated liver enzymes, declining kidney function, or unexpected glucose changes in people on concurrent diabetes medication.

The Complete Pre-Start Panel

These are the markers you need before your first injection.

Glycaemic Markers

HbA1c (glycated haemoglobin) The 3-month glucose average. Essential for anyone starting GLP-1 for weight loss or diabetes management. Target: below 48 mmol/mol (6.5%) for non-diabetics. If you are already diabetic and on glucose-lowering medication, your prescriber needs this to manage hypoglycaemia risk as GLP-1 reduces blood sugar further.

Fasting glucose Point-in-time blood sugar. Pairs with HbA1c to give a full glucose picture. Target: below 5.6 mmol/L fasting.

Fasting insulin (optional but useful) Not always included in standard panels, but fasting insulin allows calculation of HOMA-IR — an insulin resistance score. Useful context for metabolic health tracking over time.

Lipid Panel

Total cholesterol Target below 5.2 mmol/L for general population risk.

LDL cholesterol The primary cardiovascular risk marker. Target below 2.6 mmol/L; below 1.8 mmol/L if cardiovascular risk is high.

HDL cholesterol Protective cholesterol. Target above 1.0 mmol/L for men, above 1.2 mmol/L for women.

Triglycerides Elevated triglycerides are common in metabolic syndrome. GLP-1 tends to reduce these significantly. Target below 1.7 mmol/L fasting.

Non-HDL cholesterol Sometimes more useful than LDL alone for cardiovascular risk stratification.

Liver Function

ALT (alanine aminotransferase) The key marker for liver health. GLP-1 medications improve non-alcoholic fatty liver disease (NAFLD/MASLD) in most patients, but baseline ALT tells you where you started. Target below 40 IU/L for men, below 35 IU/L for women.

AST (aspartate aminotransferase) Pairs with ALT for liver function assessment. The ALT:AST ratio provides additional information about liver health.

GGT (gamma-glutamyl transferase) Sensitive marker for liver stress and alcohol use. Useful baseline.

Bilirubin Breakdown product of red blood cells processed by the liver. Elevated bilirubin can indicate liver or bile duct problems.

Albumin Liver-synthesised protein. Low albumin suggests poor liver synthetic function or malnutrition — relevant as caloric restriction intensifies on GLP-1.

Kidney Function

eGFR (estimated glomerular filtration rate) The main kidney function marker. Target above 60 mL/min/1.73m². Values below 45 warrant close monitoring; below 15 is kidney failure.

Creatinine Waste product filtered by kidneys. Used to calculate eGFR. Important because nausea and reduced food/fluid intake on GLP-1 can lead to dehydration, which stresses kidneys.

Urea Second kidney marker. Elevated alongside creatinine suggests reduced kidney clearance.

Uric acid Elevated in gout and metabolic syndrome. Can worsen with rapid weight loss; useful baseline.

Thyroid

TSH (thyroid-stimulating hormone) The frontline thyroid test. Target 0.4–4.0 mIU/L. Thyroid monitoring on GLP-1 is discussed in more detail at /guides/glp1-thyroid-monitoring-uk — short version: rodent data showed thyroid C-cell tumours at supra-pharmacological doses; human trial data does not confirm this risk, but baseline TSH is still recommended.

Free T4 Often included with TSH for a complete thyroid picture.

Full Blood Count

Haemoglobin and haematocrit Anaemia is more common with caloric restriction. Baseline before appetite suppression kicks in is useful.

White blood cells General immune status marker.

Platelets General haematological baseline.

Nutritional Markers (Optional but Valuable)

Vitamin B12 Metformin (often co-prescribed with GLP-1 in diabetes) impairs B12 absorption. Rapid dietary restriction can also reduce B12 intake. Baseline before starting.

Ferritin (iron stores) Low ferritin causes fatigue that can be mistaken for GLP-1 side effects. Useful to know your baseline, particularly if menstruating.

Vitamin D Deficiency is extremely common in the UK. Adipose tissue sequesters vitamin D; as fat mass reduces, vitamin D levels can paradoxically increase — a useful trend to track.

What to Test and When: The Full Monitoring Schedule

Where to Get Tested in the UK

NHS via your GP If your GP is managing your GLP-1 prescription, they may run baseline bloods as part of that process. NHS testing is free but can be slow to arrange, and the panel may not cover all markers above — particularly nutritional ones.

Lola Health A UK biomarker testing service offering comprehensive panels. Not a prescriber — purely testing and results. Strong for longevity-style longitudinal tracking. Full review at /guides/lola-health-review-glp1.

Epic Life Mobile nurse comes to your home, takes the blood draw, results come with AI coaching on what they mean. Higher cost than at-home kits but convenient.

Medichecks / Forth Well-established UK private blood testing services. Broad panel coverage, competitive pricing, finger-prick or phlebotomy options.

How to Prepare for Your Blood Test

Most of the markers above require fasting for accurate results:

• Fast for 8–12 hours before the test (water only)
• Avoid intense exercise the day before (can transiently raise ALT and creatinine)
• Avoid alcohol for 48 hours before (affects GGT and liver markers)
• Take prescribed medications as normal unless your GP advises otherwise

The morning slot is usually easiest — fast overnight, test first thing.

What to Do With Your Results

Bring your results to your prescriber or GP. Even if you arranged testing privately, your prescriber needs this data to manage your medication safely.

For markers outside the normal range:

• Elevated ALT (>2x upper limit): Discuss with prescriber before starting or continuing GLP-1
• eGFR below 45: Close kidney monitoring needed; some prescribers will advise against GLP-1 at very low eGFR
• HbA1c above 48 mmol/mol: Confirms pre-diabetes or diabetes; affects management approach
• Low ferritin or B12: Start supplementation before or alongside GLP-1

For ongoing monitoring guidance, see /guides/glp1-monitoring-protocol and the existing /guides/glp1-monitoring-bloodwork-uk.

The Case for Private Testing

NHS GLP-1 monitoring is inconsistent. Some GPs run comprehensive baseline panels; others do not. Private prescribers vary even more widely. The burden often falls on the patient to ensure this happens.

Running your own testing through a service like Lola Health or Epic Life costs less than a month of private GLP-1 prescription in most cases. Given that the entire point of the medication is metabolic improvement, having the data to verify that improvement is happening — and catch anything that is not — makes that cost straightforward to justify.

Start testing before you start the medication. Repeat at 3 and 6 months. The trends are the story.