Semaglutide · Wegovy and Ozempic
Before your first Wegovy dose you need a baseline, because everything that follows is measured against it: HbA1c and fasting insulin, liver and kidney function, lipids including ApoB, thyroid, and the nutrients a year of eating half as much will run down. Then you retest at three and six months to see what the drug has improved and what the diet has cost. Most UK prescribers ask for none of this. Here is what to test, when, and what each result means.
Venous draw at 103 UK clinics or at home. Report within 48 hours of the lab receiving your sample. No GP referral.
Last reviewed 15 September 2026
The short answer
Before Wegovy, test HbA1c, fasting insulin, liver, kidney, ApoB and lipids, thyroid, ferritin, B12 and vitamin D as your baseline. At three and six months, repeat the same panel: HbA1c, insulin, liver enzymes, triglycerides and hs-CRP should be improving; ferritin, B12, vitamin D and albumin must not be falling. Wegovy and Ozempic are the same drug at different doses, so the same tests apply to both.
Educational, not a substitute for your prescriber. Severe or persistent abdominal pain, vomiting you cannot keep fluids down through, or symptoms of low blood sugar if you also take insulin or a sulfonylurea need a clinical conversation the same day.
Before you start
A baseline does two jobs. It finds anything that should change the plan before you start (an undiagnosed thyroid problem, a fatty liver that needs its own attention, a ferritin already at 20), and it gives every later result something to be compared against. Weight loss without a baseline is a number on a scale. Weight loss with one is a metabolic story you can read.
| Marker | Why it matters before Wegovy | What a result changes | Panel |
|---|---|---|---|
| HbA1c | Your three-month blood sugar; prediabetes (42–47) or undiagnosed diabetes changes the conversation with your prescriber | Diabetes means Ozempic on prescription rather than Wegovy privately, and a medication review | All panels |
| Fasting insulin, HOMA-IR | Insulin resistance is what semaglutide corrects; this is the cleanest measure of how far you are starting from | A high starting HOMA-IR predicts a strong response; the retest shows it happening | Ultimate, Signature |
| Liver (ALT, GGT, ALP, bilirubin) | One in three UK adults has fatty liver; raised ALT or GGT at baseline is common and improves on the drug | Very raised results need investigating before starting; raised ALP or bilirubin later can flag gallstones, which rapid weight loss makes more likely | All panels |
| Kidney (creatinine, eGFR, cystatin C) | A true starting kidney reading before dehydration and muscle loss distort creatinine | Cystatin C at baseline lets you tell muscle loss from kidney change at month six | Creatinine and eGFR in all; cystatin C in Ultimate and Signature |
| ApoB, triglycerides, HDL, Lp(a) | Cardiovascular risk is the reason the NHS funds semaglutide; ApoB is the number that measures it | A high Lp(a) is genetic and will not move with weight loss, which changes how you and your GP manage risk | ApoB in all; Lp(a) in Ultimate and Signature |
| Thyroid (TSH, free T4, free T3, antibodies) | An underactive thyroid makes weight loss harder and mimics GLP-1 fatigue; levothyroxine doses change as weight falls | Treat or adjust thyroid first; otherwise you will blame the drug for a thyroid problem | All panels |
| Ferritin and iron studies | Stores that are already low will be exhausted within months of eating less | Below 50: start iron now rather than at month six when you are exhausted | All panels |
| B12, folate, vitamin D | Semaglutide slows stomach emptying and cuts intake; both reduce absorption over time | Supplement from day one if borderline, and retest at three months | All panels |
| Albumin, hs-CRP | Protein status and background inflammation; both should move in the right direction on the drug | A falling albumin later means protein intake is too low and muscle is going with the fat | All panels |
Guides for each: HbA1c, insulin resistance, liver, kidney, ApoB, Lp(a), thyroid, ferritin, B12, vitamin D.
While you take it
Should be improving
HbA1c falling. Fasting insulin and HOMA-IR falling, usually before the scale catches up. Triglycerides down, HDL up, ApoB falling. ALT and GGT normalising as the liver clears. hs-CRP falling as visceral fat goes. If none of these has moved by month six, the dose or the plan needs a conversation.
Must not be falling
Ferritin, B12, folate and vitamin D drifting down month on month. Albumin dropping, which means protein is too low. Creatinine falling while cystatin C stays flat, which means muscle is going. These are the markers behind the exhaustion, hair shedding and weakness people blame on the drug. Tired on a GLP-1? Why.
Watch
A steep HbA1c fall if you also take insulin or a sulfonylurea (hypo risk). Raised ALP or bilirubin with pain under the right ribs (gallstones). TSH shifting after a large weight loss on levothyroxine (dose review). Your report tells you which results to take and why.
The full monitoring schedule, marker by marker, is on the GLP-1 monitoring page. It applies to semaglutide exactly as it applies to tirzepatide.
Which panel
Advanced
74 biomarkers · £269
HbA1c, ApoB and full lipids, liver, kidney, ferritin with iron studies, B12, folate, vitamin D, full thyroid with antibodies, hs-CRP, albumin. Everything on the baseline list except fasting insulin, Lp(a) and cystatin C.
View AdvancedThe Wegovy baseline
Ultimate
114 biomarkers · £349
Adds fasting insulin and HOMA-IR, Lp(a), cystatin C, complete hormones including testosterone and oestradiol, and tumour markers. The three markers that matter most for a GLP-1 (insulin, Lp(a), cystatin C) are only here.
View UltimateHealth plans
2, 3 or 4 tests · from £706
Baseline plus the month 3 and month 6 retests bought together at a lower price per test, each booked when it is due, each report compared against the last.
View health plansClinic draw £19. At-home £39. Fast from 10pm the night before; take Wegovy on its normal day. Pay monthly available.
Honest comparison
| Wegovy | Ozempic | Mounjaro | |
|---|---|---|---|
| Drug | Semaglutide, up to 2.4 mg weekly | Semaglutide, up to 2 mg weekly | Tirzepatide, up to 15 mg weekly |
| UK licence | Weight management | Type 2 diabetes | Both |
| Mechanism | GLP-1 only | GLP-1 only | GLP-1 and GIP |
| Baseline tests | Identical | Identical, plus whatever your diabetes team already runs | Identical |
| Monitoring | 3, 6, then every 6 months | Same; NHS HbA1c checks cover only the diabetes side | Same |
| What the NHS or prescriber checks | Usually weight only | HbA1c, annual kidney and liver | Usually weight only |
If your GP has already run an HbA1c, a kidney and liver panel and a lipid profile in the last three months, you have part of a baseline. The pieces they will almost certainly not have run are fasting insulin, ApoB, Lp(a), cystatin C, ferritin, B12 and vitamin D, and those are the ones that explain how you feel six months in. What to ask your GP for.
After your result
Every marker explained in plain English, the ones to fix before you start flagged, and the ones to watch identified. Reviewed by a medical professional before it reaches you.
Protein at 1.2 to 1.6 g per kg, resistance training, and any supplements the baseline showed you need, from day one rather than month six.
The second report compares every result against the first. You see the insulin falling and the liver clearing, and you catch a ferritin or B12 drifting before it becomes exhaustion. The monitoring schedule.
FAQs
Most UK prescribers do not require one, but you should have one. A baseline finds thyroid problems, fatty liver, diabetes and low iron before you start, and gives every later result something to be compared against. HbA1c, fasting insulin, liver, kidney, ApoB, thyroid, ferritin, B12 and vitamin D is the list.
Yes. They are the same drug, semaglutide, at slightly different maximum doses and with different licences. The baseline and the monitoring schedule are identical. If you are on Ozempic for diabetes your NHS team will run HbA1c and an annual kidney and liver check, which covers the diabetes side but not the nutrient, muscle or cardiovascular side.
Baseline before starting, then at three months (usually when you reach the 2.4 mg dose), six months, and every six months after that while you remain on it. Once about three months after stopping shows whether the metabolic gains held.
Not directly. The thyroid warning on the label concerns a rare tumour type seen in rodent studies, and routine calcitonin testing is not recommended. What does change is levothyroxine requirement after significant weight loss, and an undiagnosed underactive thyroid is a common reason weight loss stalls or fatigue persists, which is why a full thyroid panel is on the baseline.
The pattern can. Creatinine falling month on month while cystatin C is unchanged means muscle mass is dropping rather than kidney function changing, and a low or falling albumin means protein intake is too low. Both are in the Ultimate panel.
Do not stop Wegovy; take it on its normal day. Fast from 10pm the night before with a morning appointment so that fasting insulin, HbA1c and lipids are accurate. Full preparation guide.
Advanced and Ultimate reports are delivered within 48 hours of the laboratory receiving your sample, which is usually the same day as your draw.
One venous draw, 114 biomarkers, a report within 48 hours that tells you what to fix before you start and what to watch as you go. Clinic or at home, no GP referral.
Mounjaro blood tests · GLP-1 monitoring · Tired on a GLP-1 · Weight loss blood test · Find a clinic
.png)