Blood Tests to Get Before Starting GLP-1
Complete guide to baseline blood tests recommended before starting semaglutide or tirzepatide. Learn which labs your provider should order, what they measure, and why each test matters for safe GLP-1 therapy.
More on Blood Work & Labs
Why Baseline Labs Matter
Baseline blood work serves three critical purposes: (1) identifying contraindications that may make GLP-1 therapy unsafe, (2) establishing your starting metabolic profile so improvements can be tracked, and (3) guiding medication selection and dosing decisions. Without baselines, it is impossible to properly monitor your treatment.
Essential Baseline Tests
These are the core tests that every patient should have before beginning GLP-1 therapy. Most providers will order these as a standard pre-treatment panel.
Comprehensive Metabolic Panel (CMP)
A CMP provides a broad snapshot of your metabolic health, including fasting glucose, kidney function (BUN, creatinine, eGFR), liver enzymes (ALT, AST, alkaline phosphatase), electrolytes (sodium, potassium, chloride, CO2), calcium, and total protein/albumin.
Why it matters: Kidney function determines medication safety and dosing. Liver enzymes establish a baseline for monitoring — GLP-1 medications can improve fatty liver, and you want to document the starting point. Fasting glucose reveals prediabetes or diabetes that may affect medication choice.
Hemoglobin A1C
A1C reflects your average blood sugar over the past 2-3 months. Normal is below 5.7%, prediabetes is 5.7-6.4%, and diabetes is 6.5% or higher. GLP-1 medications typically reduce A1C by 1.0-2.0 percentage points.
Why it matters: A1C influences medication selection (tirzepatide may be preferred for higher A1C levels), helps distinguish between weight loss-only and diabetes treatment indications (which can affect insurance coverage), and establishes your glycemic baseline for tracking dramatic improvements.
Lipid Panel
Includes total cholesterol, LDL (bad cholesterol), HDL (good cholesterol), triglycerides, and often non-HDL cholesterol or VLDL. Fasting 10-12 hours is required for accurate triglyceride measurement.
Why it matters: GLP-1 medications typically improve lipid profiles significantly — triglycerides often drop 20-40%, LDL decreases modestly, and HDL may rise. Documenting your starting levels lets you quantify these improvements and may influence whether you need statin therapy.
Thyroid Function (TSH, Free T4)
TSH (thyroid-stimulating hormone) is the primary screening test for thyroid function. Free T4 provides additional detail if TSH is abnormal. Normal TSH is typically 0.4-4.0 mIU/L.
Why it matters: GLP-1 medications carry a boxed warning about medullary thyroid carcinoma (MTC) based on rodent studies. Baseline thyroid function is essential for monitoring. Additionally, undiagnosed hypothyroidism causes weight gain and should be treated alongside or before GLP-1 therapy. A history of MTC or MEN2 syndrome is a contraindication for GLP-1 medications.
Complete Blood Count (CBC)
A CBC measures red blood cells, white blood cells, hemoglobin, hematocrit, and platelets. It screens for anemia, infection, and blood disorders.
Why it matters: Identifies anemia that could be worsened by reduced food intake on GLP-1 therapy. Establishes baseline for monitoring during treatment. Can reveal underlying conditions that affect overall health management.
Recommended Additional Tests
These tests are not universally required but are recommended based on individual risk factors and provide valuable additional information for treatment optimization:
Fasting Insulin
Measures insulin levels directly, revealing insulin resistance before blood sugar levels become abnormal. Most obese patients have elevated fasting insulin (hyperinsulinemia). Tracking insulin improvement alongside A1C provides a more complete picture of metabolic recovery during GLP-1 therapy.
High-Sensitivity CRP (hs-CRP)
A marker of systemic inflammation strongly associated with cardiovascular risk. Obesity causes chronic low-grade inflammation, and hs-CRP is typically elevated in overweight patients. GLP-1 medications can reduce hs-CRP by 20-40%, and tracking this improvement demonstrates reduced cardiovascular risk.
Vitamin D Level (25-hydroxyvitamin D)
Vitamin D deficiency is present in up to 80% of obese individuals (fat tissue sequesters vitamin D). Low vitamin D impairs muscle function, bone health, and immune function. With reduced food intake on GLP-1 medications, supplementation is often necessary, and baseline levels guide dosing.
Lipase and Amylase
Pancreatic enzymes that establish baseline pancreatic function. GLP-1 medications carry a rare risk of pancreatitis, and having pre-treatment levels is essential for comparison if a patient develops abdominal pain during treatment. A history of pancreatitis is a relative contraindication.
Testosterone (Men Only)
Total and free testosterone, SHBG, and estradiol. Up to 40% of obese men have low testosterone. Establishing baseline levels allows tracking of hormonal recovery during weight loss, which can be significant — many men see testosterone increases of 100-200+ ng/dL with substantial weight loss.
Tests That May Identify Contraindications
Certain lab results may indicate that GLP-1 therapy needs to be approached with caution or is contraindicated:
Elevated calcitonin: Calcitonin above 50 pg/mL requires evaluation for medullary thyroid carcinoma before starting GLP-1 medications. MTC is a contraindication for all GLP-1 receptor agonists.
Severely elevated liver enzymes: ALT or AST greater than 3x the upper limit of normal warrants investigation before starting therapy. Moderate elevations from fatty liver are common and may actually improve with treatment.
Severely impaired kidney function: eGFR below 15 mL/min contraindicates some GLP-1 medications. Mild to moderate kidney impairment is generally safe but may require dose adjustments.
Elevated lipase/amylase: May indicate current or recent pancreatitis, which is a contraindication for GLP-1 therapy. Requires gastroenterology evaluation before proceeding.
Preparing for Your Blood Draw
Fast for 10-12 hours before your blood draw. Water, black coffee, and plain tea are acceptable during the fast.
Schedule early morning appointments. Testosterone peaks in the morning, and fasting is easiest overnight. Aim for 7-9 AM.
Stay hydrated — drink water the evening before and morning of. Dehydration makes veins harder to access and can affect some lab values.
List all medications and supplements you are currently taking. Some can affect lab results and need to be considered in interpretation.
Avoid strenuous exercise for 24 hours before the draw. Intense exercise can temporarily elevate liver enzymes and affect other markers.
Medical Disclaimer: This article is for educational purposes only and is not a substitute for professional medical advice. Lab test interpretation should always be done by a qualified healthcare provider in the context of your complete medical history. Do not self-diagnose or change medications based on lab results without consulting your provider.
Frequently Asked Questions
Do I need blood tests before starting GLP-1 medication?
Yes, baseline blood work is strongly recommended before starting any GLP-1 medication. These tests help identify contraindications, establish your starting metabolic profile, and create reference points to track improvements. Most providers require at minimum a metabolic panel, A1C, lipid panel, and thyroid function before prescribing.
How much do pre-GLP-1 blood tests cost?
With insurance, these lab panels typically cost $0-50 in copays. Without insurance, a comprehensive metabolic panel with lipids, A1C, thyroid, and liver function through direct-to-consumer labs costs $150-300. Many telehealth GLP-1 providers include baseline labs in their program fee or can order discounted lab work.
Do I need to fast before these blood tests?
A 10-12 hour fast is required for accurate fasting glucose and lipid panel results. Most providers recommend scheduling early morning blood draws after an overnight fast. Water is fine during the fasting period. If testing testosterone, morning draws (before 10 AM) are essential as levels peak in the morning.
What if my blood tests show abnormal results?
Abnormal results do not necessarily prevent GLP-1 therapy — in many cases, they strengthen the indication for treatment. Elevated A1C, high cholesterol, or fatty liver are conditions GLP-1 medications can improve. However, some findings (like very low thyroid function, pancreatitis history, or severely elevated liver enzymes) may require evaluation before starting treatment.
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- Wilding JPH et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. NEJM 2021;384:989-1002.
- Jastreboff AM et al. Tirzepatide Once Weekly for the Treatment of Obesity. NEJM 2022;387:205-216.
- Lincoff AM et al. Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes. NEJM 2023;389:2221-2232.
- FDA Prescribing Information for Wegovy (semaglutide) and Zepbound (tirzepatide).