GLP-1 Medication Side Effects: What to Watch For in Your Blood Tests

Millions of people across the UK are now taking GLP-1 receptor agonists — medications like semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro). The weight-loss results have been remarkable. But here's what the headlines often skip: these are powerful drugs that change how your body metabolises nutrients, processes fat, and regulates hormones. And some of those changes show up in your blood long before you notice symptoms.

If you're on a GLP-1 medication — or considering one — understanding what to watch for in your blood tests isn't optional. It's essential.

The Rise of GLP-1 Medications

GLP-1 receptor agonists were originally developed for type 2 diabetes. They mimic a natural gut hormone called glucagon-like peptide-1, which stimulates insulin release, slows gastric emptying, and reduces appetite. The result: lower blood sugar and significant weight loss.

Semaglutide (marketed as Ozempic for diabetes and Wegovy for weight management) and tirzepatide (Mounjaro) have become some of the most prescribed medications in the UK. The NHS now offers semaglutide through specialist weight management services, and private prescriptions have surged.

The efficacy is well-documented — average weight loss of 10–15% of body weight over 68 weeks in clinical trials. But efficacy and safety are two separate conversations.

Common Side Effects You'll Feel

Most GLP-1 side effects are gastrointestinal. You'll likely notice these yourself:

• Nausea — the most common side effect, especially during dose escalation
• Vomiting and diarrhoea — typically settle within weeks
• Constipation — affects roughly 1 in 5 users
• Abdominal pain — usually mild and transient
• Reduced appetite — the intended therapeutic effect, but it can become excessive

These symptoms are uncomfortable but generally manageable. The side effects that should concern you more are the ones you can't feel — the ones that only blood tests reveal.

What Blood Tests Reveal

Your blood tells a story that symptoms alone cannot. On GLP-1 medications, several key markers deserve close monitoring.

Liver function

This is arguably the most important area to watch. GLP-1 medications cause rapid fat mobilisation — your body breaks down stored fat at an accelerated rate. That fat has to go somewhere, and much of it passes through your liver for processing.

The result? Your liver works significantly harder than usual. In some patients, this leads to elevated liver enzymes — specifically ALT (alanine aminotransferase) and AST (aspartate aminotransferase). The MHRA has received reports of liver injury associated with semaglutide, and the European Medicines Agency's safety assessment includes hepatic events as an area of ongoing monitoring.

Mildly elevated ALT (up to 2–3 times the upper limit of normal) may be an expected consequence of rapid fat loss and fatty liver improvement. But significantly elevated levels — or a rising trend over time — could indicate drug-induced liver injury and require immediate medical attention.

What to watch: ALT, AST, GGT, bilirubin, and albumin. A full liver function panel, not just one marker.

Kidney health

GLP-1 medications can affect kidney function, particularly in the early stages of treatment. The most common mechanism is dehydration — if nausea and vomiting reduce your fluid intake, your kidneys bear the brunt.

Elevated creatinine and reduced eGFR (estimated glomerular filtration rate) are warning signs. The European Medicines Agency has noted reports of acute kidney injury in patients taking GLP-1 receptor agonists, especially those who become significantly dehydrated.

If you already have reduced kidney function, this monitoring becomes even more critical. Your prescriber should be checking urea and electrolytes (U&Es) regularly.

What to watch: Creatinine, eGFR, urea, sodium, and potassium. Dehydration also shows up as elevated urea-to-creatinine ratio.

Thyroid markers

This is the side effect that receives the least public attention but carries genuine significance. In animal studies, GLP-1 receptor agonists have been associated with thyroid C-cell tumours (medullary thyroid carcinoma). While this hasn't been definitively established in humans, the risk was serious enough for regulators to include a boxed warning on semaglutide prescribing information.

GLP-1 medications are contraindicated in patients with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia syndrome type 2 (MEN 2).

Routine monitoring should include TSH (thyroid-stimulating hormone) as a minimum, with Free T4 and calcitonin if there's any clinical suspicion. A rising calcitonin level on a GLP-1 medication warrants urgent investigation.

What to watch: TSH, Free T4, and calcitonin — especially if you have any thyroid nodules or family history of thyroid cancer.

Pancreatic markers

There has been ongoing debate about whether GLP-1 medications increase the risk of pancreatitis and, more controversially, pancreatic cancer. The European Medicines Agency's safety assessment noted reports of acute pancreatitis, and patients should be advised to seek immediate medical attention for severe abdominal pain.

Blood tests can help detect early pancreatic stress. Elevated lipase and amylase levels may indicate pancreatic inflammation even before symptoms develop.

What to watch: Lipase and amylase levels. Persistent elevation warrants further investigation.

Nutritional Deficiencies

This is the area most commonly overlooked. When you eat 40–60% less food — which is typical on GLP-1 medications — you inevitably consume fewer vitamins and minerals. Rapid weight loss compounds the issue.

Key deficiencies to monitor include:

• Iron and ferritin — reduced food intake means less dietary iron; symptoms include fatigue, breathlessness, and hair loss
• Vitamin B12 — GLP-1 medications slow gastric emptying, which may impair B12 absorption over time
• Vitamin D — already deficient in much of the UK population; weight loss can temporarily worsen levels as fat-stored vitamin D is redistributed
• Folate — essential for cell division and DNA repair; deficiency causes fatigue and cognitive fog
• Magnesium and zinc — commonly depleted during rapid weight loss phases

Many patients attribute fatigue and hair loss on GLP-1 medications to the drug itself — when the real culprit is nutritional deficiency that a simple blood test would identify.

How Often to Monitor

There is no universal guideline, but based on MHRA recommendations and prescribing information, a sensible monitoring schedule looks like this:

• Baseline: Full blood panel before starting the medication — liver function, kidney function, thyroid, HbA1c, lipids, full blood count, iron studies, vitamin D, B12, and folate
• 4–6 weeks after starting (or after each dose increase): Liver function, kidney function, and electrolytes
• Every 3 months during active weight loss: Comprehensive panel including nutritional markers
• Every 6 months once weight-stable: Ongoing liver, kidney, thyroid, and nutritional monitoring

If you're on a GLP-1 medication and haven't had blood tests in the last three months, you're overdue.

When to Speak to Your Doctor

Certain blood test results require prompt medical attention:

• ALT or AST more than 3 times the upper limit of normal — possible liver injury
• Rising creatinine or falling eGFR — potential kidney impairment
• Elevated calcitonin — needs thyroid investigation
• Significantly elevated lipase (more than 3 times normal) — risk of pancreatitis
• Very low ferritin (below 15 µg/L) — iron deficiency requiring treatment
• Low vitamin B12 (below 200 ng/L) — needs supplementation and possibly injection therapy

Don't wait for symptoms. The entire point of monitoring is to catch problems when they're still biochemical findings — not clinical emergencies.

The Bottom Line

GLP-1 medications represent a genuine breakthrough in weight management and diabetes treatment. But "breakthrough" doesn't mean "risk-free." Your liver, kidneys, thyroid, pancreas, and nutritional status all deserve attention while you're on these drugs.

Regular blood monitoring isn't about looking for reasons to stop your medication. It's about making sure the medication is working safely — and catching any issues early enough to address them without harm. The NHS recommends ongoing monitoring for anyone taking semaglutide, and your blood tests are the most reliable window into how your body is truly responding.

Take the medication. Lose the weight. But keep your eyes on the blood work.

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