Blood Tests Before Starting GLP-1 Medications: What You Actually Need

Before you receive your first GLP-1 prescription, a clinic will run some basic checks. How basic depends entirely on which clinic you use. NHS pathways tend to include the minimum required for prescribing safety. Some private clinics are thorough. Others, particularly the faster-moving telehealth services, run the minimum that enables them to issue a script within 48 hours.

That minimum is not the same as what you'd want if you were thinking carefully about your metabolic health.

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Why Pre-Treatment Testing Matters

There are two distinct reasons to test before starting GLP-1 therapy, and it's worth being clear about both.

Reason 1: Safety screening. Certain conditions are contraindications or cautions for GLP-1 medications. Active pancreatitis, severe kidney impairment, specific thyroid conditions, and significant liver disease all need to be ruled out or managed before starting. This is the testing your prescriber does.

Reason 2: Metabolic baseline. GLP-1 therapy produces significant changes in metabolic markers over time, including HbA1c, fasting insulin, triglycerides, and liver enzymes. Without a pre-treatment baseline, you can't meaningfully interpret those changes 3 or 6 months in. You also can't see what's already being silently optimised by the medication.

The second reason is the one most people don't think about until they're already several months into treatment, at which point the baseline is gone.

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Essential Tests: What Any Responsible Prescriber Should Check

HbA1c (Glycated Haemoglobin)

HbA1c reflects average blood glucose over the preceding 2-3 months. It's the primary screening test for type 2 diabetes and pre-diabetes, and it's essential before GLP-1 prescribing for two reasons.

First, it identifies whether you have undiagnosed diabetes, which significantly changes the clinical picture, appropriate medication choice, and monitoring requirements. Second, it establishes a baseline against which to measure GLP-1 therapy's glucose-lowering effects, which are meaningful even in people without diabetes.

Reference ranges:

• Below 42 mmol/mol: Normal
• 42-47 mmol/mol: Pre-diabetes (impaired glucose regulation)
• 48 mmol/mol and above: Type 2 diabetes diagnosis

For context on how GLP-1 medications affect diabetes versus weight outcomes differently, see the GLP-1 for diabetes vs weight loss guide.

Fasting Glucose

A single fasting glucose reading complements HbA1c and can flag acute glucose elevation that HbA1c might miss (particularly if diabetes is very recent onset). It's a simple, cheap test with meaningful information.

Thyroid Function (TSH, Free T4)

Thyroid dysfunction is common, particularly in women, and significantly affects metabolic rate, energy levels, and the likelihood of responding to GLP-1 therapy. Hypothyroidism causes weight gain, fatigue, and metabolic slowdown. Starting GLP-1 therapy without knowing your thyroid status means you may incorrectly attribute thyroid symptoms to GLP-1 effects (or miss a potentially significant diagnosis).

GLP-1 medications themselves carry a class-label warning regarding thyroid C-cell tumours, which appeared in rodent studies (at doses far higher than clinical use). This warning is a contraindication for anyone with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2, making thyroid history review essential at screening.

Liver Enzymes (ALT, AST, GGT)

Elevated liver enzymes can indicate non-alcoholic fatty liver disease (NAFLD), which is common in overweight individuals and actually responds well to GLP-1 therapy. However, severe liver impairment affects drug metabolism and may require dose adjustment or contraindicate certain medications.

Establishing a baseline is also useful because GLP-1 therapy typically improves fatty liver markers over time. Without a baseline, this improvement is invisible.

Kidney Function (eGFR, Creatinine, Urea)

Kidney function affects drug clearance and the interpretation of other metabolic markers. Severe kidney impairment (eGFR below 30 mL/min) is a caution for semaglutide and a contraindication for some other GLP-1 agents. This needs to be ruled out before prescribing.

Full Blood Count (FBC)

Anaemia causes fatigue and reduced exercise tolerance. On GLP-1 therapy, where appetite is suppressed and dietary intake may be reduced, iron deficiency anaemia can develop or worsen. A baseline FBC identifies pre-existing anaemia and establishes a reference point for monitoring iron status during treatment.

Women are at significantly higher risk of iron deficiency due to menstrual blood loss. This test is essential for pre-menopausal women.

Lipid Panel (Cholesterol, Triglycerides, HDL, LDL)

GLP-1 medications improve lipid profiles, particularly triglycerides and HDL cholesterol, in most people. Knowing your baseline lipid values lets you and your clinician quantify this improvement. A pre-treatment lipid panel also identifies familial hypercholesterolaemia or very high triglycerides that might influence prescribing decisions.

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Optional but Valuable: The Full Metabolic Picture

These tests aren't required for prescribing safety, but they provide meaningful information about your metabolic health that a standard panel misses.

Fasting Insulin and HOMA-IR

Fasting insulin measures how much insulin your pancreas produces to maintain normal fasting glucose. HOMA-IR (Homeostatic Model Assessment of Insulin Resistance) is a calculated index derived from fasting glucose and fasting insulin that estimates insulin resistance.

Many people have normal fasting glucose but significantly elevated fasting insulin, a pattern called compensated insulin resistance. This means the pancreas is working overtime to keep glucose in range. GLP-1 therapy reduces both insulin resistance and fasting insulin substantially in most people. Knowing where you start makes the improvement measurable.

This is particularly relevant if you have PCOS, where insulin resistance is a core mechanism. For detail on GLP-1 and PCOS, see the GLP-1 for PCOS guide.

Uric Acid

Elevated uric acid (hyperuricaemia) is associated with metabolic syndrome and insulin resistance, and it predisposes to gout. Rapid weight loss, as occurs on GLP-1 therapy, can temporarily increase uric acid levels and precipitate a gout flare in susceptible individuals. Knowing your baseline uric acid helps contextualise any joint pain or gout episodes that arise during early treatment.

Vitamin D (25-OH Vitamin D)

Vitamin D deficiency is extremely common in the UK population, and deficiency is associated with reduced metabolic rate, insulin resistance, and poorer weight loss outcomes in some studies. On a restricted diet during GLP-1 therapy, vitamin D intake from food is likely to decrease further.

A baseline reading allows for targeted supplementation rather than guesswork.

Ferritin

As discussed above, iron deficiency is common in women. However, haemoglobin can be normal even when ferritin (stored iron) is depleted. The threshold for hair loss and fatigue from low iron is a ferritin below approximately 30-40 ng/mL, which is well within the "normal" range for most lab reference ranges.

For women concerned about hair thinning on GLP-1 therapy, a ferritin measurement is essential. See the GLP-1 hair loss prevention guide for the full picture.

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Specific Safety Tests: Amylase, Lipase, and Calcitonin

Amylase and Lipase

These are pancreatic enzymes elevated in pancreatitis. Active pancreatitis is a contraindication for GLP-1 therapy. A history of pancreatitis is a significant caution. While routine testing is not standard unless symptoms suggest it, anyone with a history of abdominal pain, gallstones, or heavy alcohol use should have these checked before starting.

Calcitonin

Elevated calcitonin is a marker for medullary thyroid carcinoma, the specific thyroid cancer mentioned in GLP-1 class warnings. Routine screening is not mandated, but some endocrinologists and more thorough clinics include it as part of pre-treatment evaluation, particularly where there is any thyroid-related family history.

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How to Get Tested in the UK

Via Your GP

Your GP can request many of these tests on the NHS, but not always as a comprehensive pre-GLP-1 panel. You may need to frame the request specifically, and some less common tests (fasting insulin, HOMA-IR, calcitonin) are unlikely to be available without a specific clinical reason.

What NHS GPs will typically test: HbA1c, fasting glucose, thyroid function, liver enzymes, kidney function, FBC, lipid panel.

What is harder to access on the NHS: fasting insulin, HOMA-IR, uric acid, ferritin (without confirmed anaemia), calcitonin.

Waiting times for results can be 1-2 weeks or longer depending on your surgery's turnaround.

Via Private Testing: Medichecks

Medichecks is the most widely used private blood testing service in the UK and offers specific GLP-1 pre-treatment panels as well as comprehensive metabolic health tests. You can order online, choose a local phlebotomy appointment or home finger-prick kit, and receive results within 1-3 days.

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Interpreting Your Results

What to Take to Your Prescriber

Print your results, or export them as a PDF, and bring them to your prescribing appointment. A good clinic will want to see them. If they don't ask, share them anyway and ask for their interpretation in the context of your GLP-1 prescription.

Red Flags That Need Investigation First

• HbA1c of 48 mmol/mol or above: Warrants discussion about diabetes management, not just weight loss
• eGFR below 45 mL/min: Significant renal impairment, requires specialist input
• Severely elevated liver enzymes (ALT or AST more than 3x upper limit of normal): Needs hepatology review
• TSH outside range: Thyroid dysfunction needs treatment before or alongside GLP-1 therapy
• Very high triglycerides (above 10 mmol/L): Risk for pancreatitis on GLP-1 therapy; specialist input needed

What Good Results Look Like

For most women starting GLP-1 therapy, the results will show mild to moderate abnormalities in metabolic markers, which is expected given the population this medication is prescribed to. Pre-diabetes HbA1c, modestly elevated triglycerides, and mildly elevated liver enzymes are common findings that do not prevent prescribing but do provide a baseline for monitoring improvement.

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Monitoring During Treatment

Pre-treatment testing is the start, not the end. GLP-1 therapy should include periodic monitoring throughout treatment.

Suggested testing schedule:

• Baseline: Before starting (as above)
• 3 months: HbA1c, liver enzymes, kidney function, weight and blood pressure
• 6 months: Full metabolic panel including lipids and thyroid
• Annually: Comprehensive repeat of baseline panel

For information on which clinics include monitoring as standard versus which require you to arrange it yourself, see the best GLP-1 clinic UK guide.

For guidance on NHS eligibility for GLP-1 medications including Wegovy, and how testing requirements differ on the NHS pathway, see the NHS Wegovy eligibility guide.