Blood Tests You Should Get on GLP-1 Medications (Ozempic, Wegovy, Mounjaro)

You've started Ozempic, Wegovy, or Mounjaro and the weight is coming off. But here's something your prescriber may not have emphasised enough: GLP-1 medications need regular blood test monitoring.

These drugs are remarkably effective. They're also powerful pharmaceuticals that alter your metabolism, appetite hormones, and organ function. Rapid weight loss places specific demands on your liver, kidneys, thyroid, and nutrient stores. Without monitoring, problems can develop silently.

Why Monitoring Matters on GLP-1 Medications

GLP-1 receptor agonists like semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro) work by mimicking the incretin hormones your gut naturally produces. They slow gastric emptying, reduce appetite, and improve insulin sensitivity. These are powerful metabolic effects — and they come with responsibilities.

The NHS and NICE both recommend ongoing monitoring for patients on these medications, yet in practice, many people receive a prescription and little follow-up beyond a weight check. That's a missed opportunity.

Practical takeaway: Think of blood tests as your medication's safety net. You can feel fine while your liver enzymes are quietly climbing.

Liver Function Tests

Your liver is the metabolic powerhouse processing the changes GLP-1 medications trigger. Rapid weight loss mobilises fat stored in the liver, and while this is ultimately beneficial (particularly for people with non-alcoholic fatty liver disease), the transition period places extra demands on hepatic function.

The key markers to monitor are:

• ALT (alanine transaminase) — the most sensitive marker for liver cell damage. Normal range is typically below 40 IU/L for men and below 35 IU/L for women
• AST (aspartate transaminase) — elevated alongside ALT, it suggests hepatocellular stress
• GGT (gamma-glutamyl transferase) — particularly relevant if you consume alcohol, even moderately
• ALP (alkaline phosphatase) — helps distinguish between liver and bone causes of enzyme elevation

During the first 3–6 months of GLP-1 therapy, it's common to see a transient rise in liver enzymes as stored fat is mobilised. This usually resolves. However, a sustained or progressive rise — particularly ALT above three times the upper limit — warrants clinical review and possible dose adjustment.

Practical takeaway: Get a baseline liver function test before starting your medication, then recheck at 3 months and 6 months. If enzymes are stable, every 6 months thereafter is reasonable.

Kidney Function

GLP-1 medications can affect your kidneys, particularly during the dose-escalation phase when nausea and reduced fluid intake are most common. Dehydration is the primary risk, and even mild dehydration can stress kidney function.

The essential kidney markers are:

• Creatinine — a waste product filtered by the kidneys. Rising levels suggest declining filtration
• eGFR (estimated glomerular filtration rate) — calculated from creatinine, age, and sex. An eGFR above 90 mL/min/1.73m² is normal; below 60 warrants closer monitoring
• Urea — another waste product that rises with dehydration or kidney impairment

The NICE guidelines for semaglutide note that gastrointestinal side effects (nausea, vomiting, diarrhoea) can lead to dehydration and acute kidney injury, particularly in patients with pre-existing kidney disease. Even if your kidneys were healthy before starting treatment, monitoring is sensible.

Practical takeaway: Stay well-hydrated — at least 2 litres of water daily, more if you're experiencing nausea or vomiting. If you notice significantly reduced urine output or dark urine, check your kidney function promptly.

Blood Sugar and HbA1c

Whether you're taking a GLP-1 for type 2 diabetes or purely for weight management, blood sugar monitoring is essential. These medications are potent glucose-lowering agents, and understanding their effect on your metabolic health helps guide treatment decisions.

Key markers include:

• HbA1c — your average blood sugar over the past 2–3 months. Below 42 mmol/mol (6.0%) is non-diabetic; 42–47 mmol/mol (6.0–6.4%) is pre-diabetic; 48+ mmol/mol (6.5%+) indicates diabetes
• Fasting glucose — a snapshot of your blood sugar after an overnight fast. Normal is below 5.5 mmol/L
• Fasting insulin — not routinely tested by GPs but valuable for assessing insulin resistance. Optimal levels are below 60 pmol/L

For those taking GLP-1 medications for diabetes alongside sulfonylureas or insulin, the risk of hypoglycaemia increases. Monitoring helps determine whether doses of other diabetes medications should be reduced as the GLP-1 takes effect.

For those using GLP-1 medications purely for weight loss, tracking HbA1c and fasting insulin provides powerful evidence that your metabolic health is improving — often one of the most significant long-term benefits of treatment.

Practical takeaway: HbA1c every 3 months during the first year gives you a clear trend line. Even if you're not diabetic, watching your HbA1c fall from, say, 44 mmol/mol to 36 mmol/mol is strong evidence that your metabolic health is genuinely improving.

Thyroid Function

This is perhaps the most important monitoring point that gets overlooked. In animal studies, GLP-1 receptor agonists have been associated with thyroid C-cell tumours. While this hasn't been conclusively demonstrated in humans, both the EMA and the manufacturers include thyroid warnings in their prescribing information.

The relevant markers are:

• TSH (thyroid-stimulating hormone) — the primary screening marker. Normal range is 0.4–4.0 mIU/L, though many clinicians prefer to see levels between 0.5 and 2.5 mIU/L
• Free T4 (thyroxine) — the main circulating thyroid hormone. Normal is approximately 12–22 pmol/L
• Free T3 (triiodothyronine) — the active thyroid hormone. Normal is approximately 3.1–6.8 pmol/L

Beyond the C-cell concern, rapid weight loss itself can alter thyroid function. Your thyroid adjusts metabolic rate in response to caloric deficit, and some people develop what's called euthyroid sick syndrome — abnormal thyroid numbers without true thyroid disease — during periods of significant weight change.

GLP-1 medications are contraindicated in people with a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2 (MEN 2). If you have any family history of thyroid cancer, discuss this with your prescriber before starting treatment.

Practical takeaway: Get thyroid function tested before starting treatment and every 6 months thereafter. Report any new neck swelling, difficulty swallowing, or persistent hoarseness to your doctor promptly.

Vitamins and Minerals

Rapid weight loss and reduced food intake can create nutritional deficiencies surprisingly quickly. GLP-1 medications reduce appetite so effectively that many people simply aren't eating enough to maintain adequate vitamin and mineral levels.

The nutrients most at risk include:

• Vitamin B12 — deficiency causes fatigue, neurological symptoms, and anaemia. At particular risk because reduced gastric acid production (a side effect of GLP-1 medications) impairs B12 absorption
• Folate — essential for cell division and DNA synthesis. Low folate alongside B12 deficiency worsens anaemia
• Iron — reduced food intake combined with reduced stomach acid decreases iron absorption. Ferritin below 30 µg/L suggests depletion even if haemoglobin is still normal
• Vitamin D — already widely deficient in the UK population. Weight loss can initially release stored vitamin D from fat tissue, but long-term levels may drop. Aim for levels above 75 nmol/L
• Magnesium — involved in over 300 enzymatic reactions. Deficiency causes muscle cramps, fatigue, and poor sleep — symptoms easily attributed to the medication itself

Practical takeaway: A comprehensive nutritional panel at baseline, 3 months, and then every 6 months helps catch deficiencies early. Supplementation is straightforward once you know what's missing.

How Often to Test

Here's a practical monitoring schedule for anyone on GLP-1 medication:

• Before starting: Baseline bloods covering all markers above — liver, kidney, HbA1c, thyroid, vitamins
• 3 months: Full recheck. This is the critical window where dose escalation, maximum side effects, and rapid metabolic change converge
• 6 months: Full recheck. By now, most people are on a stable dose and initial side effects have settled
• Every 6 months thereafter: Ongoing monitoring for as long as you're on the medication

If you experience significant side effects at any point — persistent vomiting, severe fatigue, unusual bruising, or jaundice — test sooner rather than later.

What Your GP Might Miss

GLP-1 prescribing in the UK has expanded rapidly, and not all GPs have kept pace with the monitoring requirements. Here are the gaps we commonly see:

• No baseline bloods before starting treatment — you can't track change without a starting point
• Liver function not rechecked during the rapid weight loss phase
• Thyroid function overlooked — particularly in patients prescribed via weight management clinics rather than endocrinology
• Nutritional deficiencies ignored — standard NHS blood tests don't include B12, folate, iron, or vitamin D unless specifically requested
• Fasting insulin not tested — one of the best markers for metabolic improvement, but rarely part of routine NHS panels
• No kidney function check during the dose-escalation phase when dehydration risk is highest

This isn't a criticism of GPs — they're working within enormous time and resource constraints. But it means you may need to be proactive about requesting or commissioning the right tests yourself.

Practical takeaway: If your GP isn't offering comprehensive monitoring, consider a private blood test panel specifically designed for GLP-1 medication monitoring. Having the full picture means you and your clinician can make truly informed decisions about your treatment.

Book Your Test

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