Approximately 1.6 million UK adults used a GLP-1 weight loss medication in the past year. Nearly 7 in 10 paid privately, and many started without a single blood test. That is a safety gap. A comprehensive baseline before you start, and monitoring every 3 months after, is how you use these medications safely.
Jackson et al. (BMC Medicine, 2026) found that 2.9% of UK adults, approximately 1.6 million people, used a GLP-1 or GLP-1/GIP medication for weight loss in the past year. Nearly one in ten had used one or were interested in starting. By mid-2025, IQVIA estimated over 2 million were paying privately versus roughly 290,000 on the NHS, a ratio of approximately 7 to 1.
Mounjaro (tirzepatide) became eligible for NHS primary-care prescription from 23 June 2025 under NICE TA1026, initially for patients with BMI 40+ and four or more related health conditions. South Yorkshire ICB's patient guidance confirms: "If you choose to start tirzepatide, you will need up-to-date blood tests to check kidney, liver, and thyroid function."
But the private market, where the vast majority of users buy, is less regulated. Many online pharmacies prescribe after a questionnaire only. No blood test. No baseline. No monitoring. When Eli Lilly raised the UK list price of the highest-dose Mounjaro from £122 to £330 per month in September 2025, it pushed more users toward unregulated sources with even less clinical oversight.
Tirzepatide produces average weight loss of 15 to 21% of body weight (SURMOUNT programme). That is dramatic metabolic change. Rapid weight loss stresses the liver, increases gallstone risk, can cause dehydration that damages kidneys, depletes nutritional stores as dietary variety narrows, and shifts thyroid and hormonal markers. Without baseline bloods, you have no reference point to detect problems. Without monitoring, you won't catch them until symptoms appear, and by then the damage may be harder to reverse.
This is the monitoring protocol that responsible clinics follow. If your prescriber hasn't mentioned blood tests, that should concern you.
Your baseline establishes what "normal" looks like for you. Without it, there is no way to distinguish a medication side effect from a pre-existing condition, and no reference point to track improvement. This is the most important single test in the entire GLP-1 journey.
HbA1c (baseline glycaemic control and eligibility marker), fasting glucose, fasting insulin, HOMA-IR (insulin resistance score). These track the metabolic improvement that GLP-1 medications are designed to produce. Without a baseline, you cannot quantify how much your insulin sensitivity has improved.
ALT, AST, GGT, ALP, bilirubin, albumin. Rapid weight loss mobilises fat from the liver, which can temporarily elevate liver enzymes. Pre-existing fatty liver disease (NAFLD, affecting 25 to 30% of UK adults) needs identifying before treatment, because GLP-1 medications can both improve NAFLD long-term and cause transient liver stress short-term. GGT and ALP are early warning markers for gallstone formation, which is increased during rapid weight loss.
Creatinine, eGFR, urea, electrolytes (sodium, potassium, chloride). GLP-1 side effects (nausea, vomiting, diarrhoea) cause dehydration, which stresses the kidneys. Pre-existing kidney impairment needs identifying first. Monitoring eGFR during treatment catches acute kidney stress from dehydration before it causes lasting damage.
TSH, Free T4, Free T3. GLP-1 receptor agonists carry a boxed warning for thyroid C-cell tumours in animal studies (rodents). While the human relevance is uncertain, thyroid monitoring is recommended. Additionally, weight loss itself alters thyroid function, and pre-existing subclinical thyroid dysfunction (affecting 8 to 10% of the population) should be identified before starting.
Ferritin, vitamin B12, vitamin D, folate, zinc. Appetite suppression on GLP-1 medications dramatically reduces food intake. Patients commonly eat 800 to 1,200 calories per day during titration. Dietary variety narrows. Nutritional deficiencies that were borderline before treatment can become clinically significant within weeks. Ferritin and B12 are the first to drop. Without a baseline, the decline is invisible until symptoms appear.
Total cholesterol, LDL, HDL, triglycerides, ApoB, Lp(a). GLP-1 medications produce significant lipid improvements. A baseline allows you to quantify the cardiovascular benefit and share it with your GP. ApoB and Lp(a) are not routinely tested but are the best predictors of cardiovascular risk, which is the main reason most people on GLP-1 medications need monitoring in the first place.
FBC, hs-CRP. Baseline haemoglobin, white cells, and platelets establish your starting point. CRP tracks inflammation, which should improve as weight reduces. A declining CRP alongside weight loss is one of the clearest signals that the medication is producing genuine metabolic benefit, not just scale-weight reduction.
This is when changes are most likely to appear. You're in the dose escalation phase, eating significantly less, and your body is adapting to rapid metabolic change. Repeat the full baseline panel. Compare every marker to your starting values. Key things to watch: liver enzymes rising (gallstone or liver stress), eGFR declining (dehydration), ferritin or B12 dropping (nutritional depletion), HbA1c improving (confirms efficacy).
Once your dose is stable and weight loss plateaus, the metabolic shifts slow down but monitoring remains important. Nutritional depletion is cumulative. Hormonal changes from significant weight loss (particularly testosterone, oestradiol, and thyroid markers) can take months to manifest. Six-monthly retesting catches slow-developing trends before they become symptomatic.
Severe or persistent abdominal pain (gallstones or pancreatitis risk). Yellowing of skin or eyes (liver stress). Persistent vomiting or diarrhoea lasting more than 48 hours (dehydration and kidney risk). Unusual fatigue, weakness, or cognitive changes (nutritional depletion). Do not wait for your next scheduled test. These symptoms warrant immediate investigation.
Specialist GLP-1 monitoring panels from UK providers typically cover 15 to 25 markers: liver, kidney, lipids, HbA1c, thyroid, FBC. This is the minimum. It catches the safety-critical side effects but misses the metabolic optimisation that comprehensive testing reveals.
Losing 15 to 21% of your body weight is one of the most significant metabolic events your body will experience. It changes your insulin sensitivity, thyroid function, hormonal balance, inflammatory status, and nutritional reserves simultaneously. Testing only liver and kidney function during this process is like monitoring a car engine by checking only the oil level while ignoring the coolant, battery, brakes, and tyres.
The TrueVitals Ultimate panel tests 114 biomarkers. That includes every recommended GLP-1 monitoring marker plus full hormonal profiling (testosterone, SHBG, oestradiol, cortisol, DHEA-S), complete thyroid function with antibodies, advanced lipids (ApoB, Lp(a), homocysteine), insulin resistance markers (fasting insulin, HOMA-IR, C-peptide), immunoglobulins, and inflammatory markers. The AI-powered report tracks changes over time with directional arrows and percentage shifts, showing you exactly how your body is responding to treatment.
The cost argument works in your favour. A specialist GLP-1 monitoring panel costs £80 to £120 for 15 to 25 markers. The TrueVitals Ultimate costs £349 for 114 markers. Per marker, comprehensive testing is significantly cheaper, and you get the metabolic depth that minimum monitoring structurally cannot provide. If you're spending £150 to £330 per month on the medication itself, spending £349 every 3 to 6 months on comprehensive monitoring is a fraction of the total investment and by far the most informative part.
GLP-1 medications do not selectively target fat. Research suggests that 25 to 40% of weight lost on semaglutide and tirzepatide is lean mass (muscle). The clinical trials report average figures, but individual variation is enormous. Some people preserve muscle well. Others lose disproportionate amounts, particularly if they're not resistance training and protein intake is inadequate.
Blood markers cannot directly measure muscle mass, but they reveal the hormonal and nutritional environment that determines whether you preserve it. Testosterone supports muscle protein synthesis. Vitamin D acts like a hormone in muscle tissue. Ferritin fuels the oxygen delivery that recovery requires. Cortisol, when chronically elevated from the stress of severe caloric restriction, promotes muscle catabolism. A comprehensive panel tracks all of these, giving you the data to adjust your protein intake, training, and supplementation to protect your lean mass while losing fat.
This is the difference between losing weight and losing weight well. The scale doesn't distinguish between the two. Your blood does.
114 biomarkers. Every marker recommended for GLP-1 monitoring plus the full metabolic, hormonal, and inflammatory picture. Results in 48 hours.
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