GLP-1 MEDICATIONS
Blood Tests for Ozempic, Mounjaro & Wegovy Users
GLP-1 receptor agonists like semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro) are the most significant weight-loss drugs in a generation. The STEP 1 trial showed average weight loss of 14.9% with semaglutide, and the SELECT trial demonstrated a 20% reduction in major cardiovascular events. But rapid weight loss changes your body's internal chemistry — and without regular blood testing, you won't know whether those changes are all positive.
This guide covers exactly which biomarkers to monitor, when to test them, and what your results mean — whether you're getting a private prescription, using a weight-loss clinic, or being prescribed through the NHS.
Why do you need blood tests on GLP-1 medication?
GLP-1 drugs work by mimicking a gut hormone that regulates appetite, insulin secretion, and gastric emptying. They are remarkably effective — but they change far more than your appetite. Rapid weight loss of 10-20% of body weight over 12 months affects your liver enzymes, kidney function, nutritional status, hormone levels, and body composition.
The NICE guideline NG246 on obesity management recommends monitoring HbA1c, lipid profiles, and blood pressure during pharmacological weight-loss treatment. But NICE guidelines represent the minimum. If you want to understand how GLP-1 medication is actually affecting your body — not just whether you're losing weight — you need a broader set of biomarkers.
Most weight-loss clinics prescribe GLP-1s with minimal monitoring — sometimes just a questionnaire every few months. That's not enough. Here's what a thorough monitoring plan looks like.
What blood tests should you get before starting?
A baseline blood test before your first injection is essential. Without it, you have no way of knowing whether changes in your results are caused by the medication, the weight loss, or something that was already developing. Your baseline should include:
- Full blood count (FBC) — establishes your red and white cell baselines
- Liver function (ALT, AST, GGT, ALP) — GLP-1s can affect liver enzymes; you need a pre-treatment reference
- Kidney function (creatinine, eGFR) — dehydration and nausea on GLP-1s can stress the kidneys
- Thyroid (TSH) — semaglutide carries a boxed warning about thyroid C-cell tumours in rodent studies
- HbA1c and fasting glucose — your metabolic starting point
- Full lipid panel — cholesterol, LDL, HDL, triglycerides, ApoB
- Vitamin D, B12, folate, ferritin, iron — you need to know if deficiencies exist before you start eating less
- Testosterone (men) / hormonal panel (women) — rapid weight loss affects sex hormones significantly
Think of this as your “before” snapshot. Every subsequent test is measured against it.
Liver function — the most important check
Your liver is at the centre of the metabolic changes happening on GLP-1 medication. When you lose weight rapidly, stored fat is mobilised and processed through the liver. This can temporarily elevate liver enzymes — and in rare cases, GLP-1 drugs have been associated with more serious hepatic events.
The NHS estimates that up to 1 in 3 people in the UK have early-stage non-alcoholic fatty liver disease (NAFLD). If you're starting GLP-1 medication at a higher weight, there's a reasonable chance your liver is already under some strain. The good news: GLP-1s can actually improve NAFLD over time. But you need to track it.
ALT (Alanine Aminotransferase) →
WHY IT MATTERS ON GLP-1s
ALT is the primary marker of liver cell damage. GLP-1 treatment can cause a transient rise in ALT as the liver processes mobilised fat — this is usually benign and resolves. But a sustained rise above 3x the upper limit warrants medical review.
WHAT TO WATCH
Baseline ALT establishes your starting point. A mild rise (up to 2x upper limit) in the first 3 months is common during rapid weight loss. If ALT continues to climb after 3 months, or rises above 120 U/L, speak to your prescriber.
HOW OFTEN TO TEST
Baseline, then every 3 months for the first year. Every 6 months thereafter if stable.
A full liver panel also includes AST, GGT, ALP, and bilirubin. Together, these differentiate between liver cell damage (ALT/AST), bile duct issues (GGT/ALP), and liver processing capacity (bilirubin). Rapid weight loss increases gallstone risk — GGT and ALP help catch gallbladder problems early.
Kidney function — especially with dehydration risk
GLP-1 medications commonly cause nausea, vomiting, and reduced fluid intake — particularly during dose titration. Dehydration is the single biggest risk factor for acute kidney injury on these drugs. The NHS estimates that chronic kidney disease affects around 1 in 10 people in the UK, and many don't know they have it.
Creatinine & eGFR →
WHY IT MATTERS ON GLP-1s
Creatinine is a waste product filtered by the kidneys. eGFR (estimated glomerular filtration rate) calculates how efficiently your kidneys are filtering. On GLP-1s, dehydration from nausea and reduced food/fluid intake can temporarily impair kidney function. If you had any pre-existing kidney weakness, this medication can unmask it.
WHAT TO WATCH
eGFR above 90 is normal. Between 60-89 suggests mild impairment — monitor closely. Below 60 requires medical review and possible dose adjustment. A sudden drop of more than 25% from baseline at any point is a red flag.
HOW OFTEN TO TEST
Baseline, then at 3 months (during peak dose titration), then every 6 months.
Thyroid — a safety requirement, not optional
Semaglutide and tirzepatide carry a boxed warning (the most serious type of FDA/MHRA drug warning) regarding thyroid C-cell tumours. In rodent studies, GLP-1 receptor agonists caused dose-dependent increases in thyroid C-cell tumours, including medullary thyroid carcinoma (MTC). The relevance to humans is uncertain — but it's why these drugs are contraindicated in anyone with a personal or family history of MTC or Multiple Endocrine Neoplasia type 2 (MEN2).
TSH (Thyroid Stimulating Hormone) →
WHY IT MATTERS ON GLP-1s
TSH is your primary thyroid screening marker. Significant weight loss also genuinely affects thyroid function — as body mass decreases, your metabolic rate adjusts, and TSH can shift. Monitoring ensures your thyroid is adapting normally and catches any concerning changes early.
WHAT TO WATCH
TSH between 0.4-4.0 mIU/L is the standard range. A rising TSH may indicate developing hypothyroidism (your thyroid slowing as metabolism adjusts). A dropping TSH below 0.4 with symptoms like tremor, rapid heartbeat, or anxiety needs investigation.
HOW OFTEN TO TEST
Baseline (mandatory before starting). Then every 6 months. Immediately if you develop neck swelling, difficulty swallowing, or hoarseness.
If you want a more complete picture, consider adding Free T4 and Free T3. TSH tells you what your brain is asking the thyroid to do; Free T4 and T3 tell you what the thyroid is actually producing and converting. This matters because weight loss can impair T4-to-T3 conversion.
Metabolic markers — tracking the real progress
The number on the scale is the least interesting outcome of GLP-1 treatment. The metabolic improvements — better insulin sensitivity, lower blood sugar, improved lipids — are what actually reduce your long-term disease risk. These are the markers that tell you whether the medication is doing what it should.
HbA1c (Glycated Haemoglobin) →
WHY IT MATTERS ON GLP-1s
HbA1c measures your average blood sugar over 2-3 months. It's the gold standard for tracking metabolic improvement on GLP-1s. Semaglutide was originally developed as a type 2 diabetes drug — even in non-diabetic users, it typically improves HbA1c by 0.5-1.5 percentage points. Tracking this tells you whether you're genuinely improving your metabolic health, not just losing weight.
WHAT TO WATCH
Below 42 mmol/mol is normal. 42-47 is pre-diabetic. Above 48 is diabetic. If you started in the pre-diabetic range, you want to see this dropping towards 36 mmol/mol or lower. If HbA1c isn't improving despite weight loss, it may indicate ongoing insulin resistance that needs separate attention.
HOW OFTEN TO TEST
Baseline, then every 3 months for the first year. This is also the recommended NICE interval for diabetes monitoring.
Cholesterol & ApoB →
WHY IT MATTERS ON GLP-1s
Weight loss typically improves your lipid profile — lower LDL, lower triglycerides, higher HDL. ApoB is a more accurate predictor of cardiovascular risk than LDL alone because it counts every atherogenic particle in your blood. The SELECT trial showed semaglutide reduced major cardiovascular events by 20%, and lipid improvements are a key mechanism.
WHAT TO WATCH
Total cholesterol below 5 mmol/L, LDL below 3 mmol/L, and ApoB below 1.0 g/L are standard targets. If you're losing weight but your lipids aren't improving (or are getting worse), it may indicate a genetic component like familial hypercholesterolaemia that needs specific treatment.
HOW OFTEN TO TEST
Baseline, then every 6 months.
Nutritional deficiencies — the hidden cost of eating less
This is the section most weight-loss clinics skip entirely. GLP-1 drugs work partly by suppressing appetite and slowing gastric emptying. You eat significantly less — often 30-50% fewer calories. That means 30-50% fewer vitamins and minerals from food. Over months, this creates genuine nutritional deficiencies that cause fatigue, hair loss, brain fog, poor recovery, and weakened immunity.
Ferritin & Iron →
WHY IT MATTERS ON GLP-1s
Iron deficiency is already the most common nutritional deficiency worldwide. Eating less means absorbing less. Red meat intake — the best dietary iron source — often drops on GLP-1s because it becomes harder to tolerate. Women are at particular risk due to menstrual losses combined with reduced intake.
WHAT TO WATCH
Ferritin below 30 µg/L is associated with fatigue even when haemoglobin is normal. Optimal: 50-150 µg/L. If ferritin drops below 30 on treatment, supplementation with iron bisglycinate (better tolerated on a sensitive stomach) is recommended.
HOW OFTEN TO TEST
Baseline, then every 3-6 months.
Vitamin D →
WHY IT MATTERS ON GLP-1s
Vitamin D is fat-soluble — it's stored in body fat. When you lose fat rapidly, stored vitamin D is released into the bloodstream (temporarily raising levels) and then excreted. Long-term GLP-1 users often see vitamin D levels drop after the initial weight-loss phase. In the UK, where 1 in 5 adults are already deficient, this is a real concern.
WHAT TO WATCH
Below 25 nmol/L is deficient. Below 50 nmol/L is insufficient. Optimal: 75-150 nmol/L. Supplement with 2,000-4,000 IU daily as maintenance; more if levels are low.
HOW OFTEN TO TEST
Baseline, then every 6 months.
Vitamin B12 →
WHY IT MATTERS ON GLP-1s
GLP-1 drugs slow gastric emptying, which can impair B12 absorption over time. If you're also taking metformin (commonly co-prescribed with GLP-1s for type 2 diabetes), the risk is compounded — metformin is well-documented to cause B12 depletion. Symptoms include fatigue, numbness, tingling, and cognitive issues.
WHAT TO WATCH
Serum B12 below 200 pg/mL is deficient. Below 400 pg/mL is suboptimal and may cause symptoms. If you're on both a GLP-1 and metformin, monitor B12 closely and supplement with methylcobalamin (1,000mcg sublingual) if levels drop.
HOW OFTEN TO TEST
Baseline, then every 6 months. Every 3 months if co-prescribed metformin.
Folate (Vitamin B9) →
WHY IT MATTERS ON GLP-1s
Folate works alongside B12 for red blood cell production and DNA synthesis. Reduced food intake — particularly of leafy greens and legumes — directly reduces folate intake. Deficiency causes fatigue, irritability, and megaloblastic anaemia.
WHAT TO WATCH
Serum folate below 7 nmol/L is deficient. Optimal: 20-45 nmol/L. Supplement with methylfolate (5-MTHF) 400-800mcg if levels are low.
HOW OFTEN TO TEST
Baseline, then every 6 months.
Muscle mass — the problem nobody talks about
Here's what the weight-loss headlines don't tell you: up to 40% of the weight lost on GLP-1 drugs can be lean mass (muscle), not just fat. A 2024 analysis in The Lancet Diabetes & Endocrinology reported that semaglutide users lost approximately 40% lean mass and 60% fat mass. This matters enormously — muscle is your metabolic engine, your injury protection, and your long-term independence as you age.
Blood tests alone can't measure muscle mass directly, but several biomarkers act as proxy indicators of whether you're preserving muscle or losing it.
Testosterone →
WHY IT MATTERS ON GLP-1s
Testosterone is critical for muscle protein synthesis — in both men and women. Rapid weight loss and severe caloric restriction can suppress testosterone production. Paradoxically, losing body fat usually increases testosterone (fat tissue converts testosterone to oestrogen via aromatase). The net effect depends on how much muscle you're preserving versus losing.
WHAT TO WATCH
Men: total testosterone below 12 nmol/L is clinically low. Below 15 nmol/L with symptoms (fatigue, low mood, reduced strength) is suboptimal. If testosterone drops rather than rises as you lose weight, it suggests you may be losing too much lean mass or eating too little.
HOW OFTEN TO TEST
Baseline, then every 3-6 months. Compare the trend against your weight loss trajectory.
DHEA-S →
WHY IT MATTERS ON GLP-1s
DHEA-S is an adrenal hormone that serves as a precursor to both testosterone and oestrogen. It's also a marker of physiological stress. Rapid weight loss and prolonged caloric restriction are a form of physical stress — DHEA-S can decline as your adrenal system adapts. Declining DHEA-S alongside declining testosterone is a signal that your body is under significant metabolic strain.
WHAT TO WATCH
DHEA-S varies widely by age and sex. The trend matters more than any single reading. A decline of more than 30% from baseline warrants attention to protein intake, training intensity, and overall caloric adequacy.
HOW OFTEN TO TEST
Baseline, then every 6 months.
Practical advice: The best defence against muscle loss on GLP-1s is resistance training (3-4 sessions per week) and adequate protein intake (1.6-2.2g per kg of target body weight daily). Blood tests help you monitor whether these interventions are working.
Cardiovascular markers — beyond the weight loss
The SELECT trial was a landmark moment for GLP-1 drugs. Published in The New England Journal of Medicine, it showed that semaglutide reduced major adverse cardiovascular events by 20% in overweight adults with existing cardiovascular disease — independent of weight loss. Tracking your cardiovascular biomarkers lets you see this benefit in your own data.
hs-CRP (High-Sensitivity C-Reactive Protein) →
WHY IT MATTERS ON GLP-1s
hs-CRP measures systemic inflammation — a key driver of atherosclerosis and cardiovascular disease. Excess body fat is a major source of chronic inflammation. As you lose weight on GLP-1s, hs-CRP typically falls significantly. This is one of the clearest markers that your cardiovascular risk is genuinely improving, not just your appearance.
WHAT TO WATCH
Below 1.0 mg/L is low risk. 1.0-3.0 is moderate. Above 3.0 is high (excluding acute infection). Most people on GLP-1 treatment see hs-CRP drop from the moderate/high range to low risk within 6-12 months.
HOW OFTEN TO TEST
Baseline, then every 6 months.
Lp(a) — Lipoprotein(a) →
WHY IT MATTERS ON GLP-1s
Lp(a) is a genetically determined cardiovascular risk factor that affects approximately 1 in 5 people. Unlike LDL, it doesn't respond to diet or weight loss — it's almost entirely genetic. Testing Lp(a) once gives you a lifelong risk assessment. If it's elevated (above 75 nmol/L), you may need more aggressive management of other risk factors regardless of your weight.
WHAT TO WATCH
Below 75 nmol/L (or 30 mg/dL) is considered low risk. Above 125 nmol/L is high risk. A single test is usually sufficient — Lp(a) doesn't change significantly over time.
HOW OFTEN TO TEST
Once (at baseline). No need to retest unless clinical circumstances change.
What to test when you stop GLP-1 medication
Discontinuation is the part no one prepares for. Studies show that most people regain two-thirds of lost weight within a year of stopping GLP-1 treatment. But it's not just weight — your metabolic markers, hormones, and inflammatory markers can all shift as your body readjusts.
When you stop (or are planning to stop), get a comprehensive blood test that includes:
- HbA1c — your blood sugar regulation will change as the drug's glucose-lowering effect wears off
- Full lipid panel + ApoB — lipid improvements may partially reverse
- hs-CRP — inflammation may return if weight is regained
- Thyroid (TSH) — metabolic rate readjusts
- Testosterone / hormonal panel — hormone levels should stabilise at a new set point
- Full nutritional panel — identify any deficiencies that developed during treatment
Test at the point of stopping, then again at 3 months and 6 months post-discontinuation. This gives you a clear picture of what's rebounding and what's holding.
Testing schedule — baseline, 3 months, 6 months, ongoing
| BIOMARKER | BASELINE | 3 MONTHS | 6 MONTHS | ONGOING |
|---|---|---|---|---|
| Liver function (ALT, AST, GGT) | Yes | Yes | Yes | Every 6 months |
| Kidney (creatinine, eGFR) | Yes | Yes | Yes | Every 6 months |
| Thyroid (TSH) | Yes | — | Yes | Every 6 months |
| HbA1c | Yes | Yes | Yes | Every 3-6 months |
| Lipids + ApoB | Yes | — | Yes | Every 6 months |
| Ferritin, B12, folate, vitamin D | Yes | Yes | Yes | Every 6 months |
| Testosterone / DHEA-S | Yes | — | Yes | Every 6 months |
| hs-CRP | Yes | — | Yes | Every 6 months |
| FBC | Yes | — | Yes | Annually |
| Lp(a) | Yes | — | — | Once only |
Frequently asked questions
Does Ozempic show up on a blood test?
Standard blood tests do not detect semaglutide or other GLP-1 drugs. These medications don't appear on routine blood panels, drug screens, or workplace testing. However, the metabolic effects of GLP-1 treatment — improved HbA1c, lower triglycerides, changes in liver enzymes — will be visible in your results.
Can I get a blood test on the NHS while taking Ozempic?
If you're prescribed GLP-1 medication through the NHS (for type 2 diabetes or through the upcoming obesity services), your GP should arrange monitoring blood tests. If you're getting a private prescription through a weight-loss clinic, most clinics don't provide comprehensive blood monitoring — you'll need to arrange your own. A private blood test gives you a fuller picture than the limited panel most clinics check.
Do I need to fast before my blood test?
For the most accurate results, fast for 10-12 hours before your blood test (water is fine). This is particularly important for lipid panels and glucose measurements. HbA1c, thyroid, and nutritional markers are not significantly affected by fasting, but doing it all in one fasted draw is simplest.
I'm on a compound semaglutide — do I need different tests?
Compounded semaglutide (from compounding pharmacies, not branded Novo Nordisk) works through the same mechanism and carries the same monitoring requirements. The biomarkers in this guide apply regardless of whether your semaglutide is branded or compounded. Note: the MHRA recommends only using licensed medications from regulated pharmacies.
How soon after starting should I get my first follow-up test?
Three months after starting — or 6-8 weeks after reaching your maintenance dose, whichever is later. The titration period (gradually increasing your dose over 4-16 weeks) is when side effects peak and your body is adapting. Testing too early won't give you meaningful trend data.
What's the difference between testing for Ozempic vs Mounjaro users?
Semaglutide (Ozempic/Wegovy) and tirzepatide (Mounjaro) are both GLP-1 receptor agonists, but tirzepatide also activates GIP receptors. The monitoring biomarkers are essentially the same. Tirzepatide tends to produce greater weight loss (up to 22% in the SURMOUNT-1 trial), so nutritional monitoring and muscle-mass preservation may be even more important.
RELATED GUIDE
Blood Tests for Women on GLP-1 Drugs During Perimenopause →
Bone loss, muscle depletion, thyroid disruption — the compounded risks specific to perimenopausal women on Ozempic, Wegovy or Mounjaro, and the biomarkers that catch problems early.
Monitor your health on GLP-1 medication
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Medical disclaimer: This content is for informational purposes only and does not constitute medical advice, diagnosis, or treatment. GLP-1 receptor agonists are prescription medications — always follow your prescriber's guidance regarding monitoring and dosage. Do not start, stop, or change the dosage of any medication based on information in this article without consulting your doctor. If you experience severe side effects (persistent vomiting, severe abdominal pain, signs of pancreatitis), seek medical attention immediately. All Helvy blood tests are processed by UKAS-accredited NHS laboratories and reviewed by a GMC-registered doctor.
Last updated: April 2026 · By Helvy · Medically reviewed by a GMC-registered doctor