Beginner Guide

GLP-1 Lab Tests Before Starting: Basic Checklist

GLP-1 Companion · 8 min read

Baseline laboratory testing before starting a GLP-1 medication serves three distinct purposes: identifying any contraindications or safety concerns that might affect prescribing decisions, ensuring doses are appropriate given your current organ function, and establishing reference values against which your clinical progress can be measured. Some of these tests are required before prescribing; others are strongly recommended. Understanding the purpose of each test helps you advocate for a complete workup and interpret your results meaningfully.

Required vs. Recommended Tests

The distinction between required and recommended tests depends on your specific health history and your prescriber's clinical judgment. In practice, many telehealth GLP-1 programs conduct abbreviated baseline evaluations. A comprehensive workup is the clinical ideal and puts you in the best position for safe, well-monitored treatment.

HbA1c (Glycated Hemoglobin)

HbA1c is the cornerstone of pre-GLP-1 metabolic assessment. It reflects average blood glucose concentration over the previous 2–3 months by measuring the percentage of hemoglobin that has glucose molecules attached to it. Results above 5.7% indicate prediabetes; 6.5% or above indicates diabetes. This test matters for three reasons: it establishes your metabolic baseline for measuring GLP-1 benefit, it helps determine whether the GLP-1 indication is diabetes management or weight management (affecting insurance coverage), and it informs dose selection and monitoring frequency.

Target reference ranges: Normal — below 5.7%. Prediabetes — 5.7–6.4%. Type 2 diabetes — 6.5% and above. GLP-1 medications typically reduce HbA1c by 1–2 percentage points in people with type 2 diabetes, making this a high-value progress marker.

Fasting Glucose

Fasting plasma glucose, measured after a minimum 8-hour fast, provides a snapshot of blood sugar regulation that complements the HbA1c trend. Normal fasting glucose is 70–99 mg/dL. Impaired fasting glucose (prediabetes) is 100–125 mg/dL. Diabetes is defined at 126 mg/dL or above on two separate occasions. Fasting glucose establishes a day-of-test baseline and, combined with HbA1c, gives a more complete picture of glucose homeostasis than either test alone.

Comprehensive Metabolic Panel (CMP)

The CMP is a bundled panel of 14 blood tests that evaluates the function of two organ systems critical to safe GLP-1 use.

Kidney Function Tests

Creatinine, BUN (blood urea nitrogen), and eGFR (estimated glomerular filtration rate) assess kidney filtration capacity. This matters because significant renal impairment may affect how GLP-1 medications are cleared from the body. Patients with severe kidney disease (eGFR below 30 mL/min/1.73m²) require careful prescribing consideration. GLP-1 medications are generally considered protective of kidney function over time, but baseline assessment remains important.

Liver Function Tests

AST (aspartate aminotransferase), ALT (alanine aminotransferase), alkaline phosphatase, and bilirubin assess hepatic health. Non-alcoholic fatty liver disease (NAFLD) is highly prevalent in patients initiating GLP-1 therapy and may present as mildly elevated ALT or AST. Establishing a baseline allows your prescriber to confirm whether any post-treatment enzyme changes represent improvement, stability, or a new concern requiring investigation.

Complete Blood Count (CBC)

The CBC measures the cellular components of blood: red blood cells (and hemoglobin/hematocrit), white blood cells, and platelets. It screens for anemia, which is relevant because GLP-1-associated appetite suppression can reduce dietary iron and B12 intake, potentially worsening pre-existing nutritional deficiencies. It also provides a baseline should any unexpected hematological changes occur during treatment. Anemia at baseline should prompt nutritional assessment and supplementation discussion before starting.

Fasting Lipid Panel

A fasting lipid panel measures total cholesterol, LDL cholesterol (low-density lipoprotein), HDL cholesterol (high-density lipoprotein), and triglycerides. Dyslipidemia — particularly elevated triglycerides and LDL with low HDL — is common in patients starting GLP-1 therapy and itself constitutes a cardiovascular risk factor requiring treatment consideration. Establishing lipid baselines allows your team to measure GLP-1-driven improvements and adjust any co-prescribed lipid-lowering medications appropriately.

• LDL — Optimal below 100 mg/dL. High cardiovascular risk targets may be below 70 mg/dL.
• HDL — Protective levels above 60 mg/dL. Below 40 mg/dL in men and 50 mg/dL in women is a risk factor.
• Triglycerides — Normal below 150 mg/dL. High is above 200 mg/dL. Very high above 500 mg/dL raises pancreatitis risk.

Thyroid Function: TSH

TSH (thyroid-stimulating hormone) is the standard screening test for thyroid dysfunction. The prescribing information for all currently approved GLP-1 medications (semaglutide, tirzepatide, liraglutide) includes a contraindication for patients with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia syndrome type 2 (MEN2). This contraindication is based on rodent studies at supratherapeutic doses; clinical epidemiological data have not demonstrated an increased MTC rate in humans treated at therapeutic doses.

Beyond the MTC consideration, hypothyroidism — underactive thyroid — is common in people with obesity and, if untreated, can substantially impair weight loss response to any intervention, including GLP-1 therapy. Identifying and treating hypothyroidism before starting GLP-1 therapy optimizes your response to treatment.

Optional and Supplementary Tests

DEXA Scan

A dual-energy X-ray absorptiometry (DEXA) scan measures body composition with high precision, giving separate values for fat mass, lean mass, and bone mineral density in different body regions. While not routinely ordered before GLP-1 initiation, a baseline DEXA scan is valuable for patients who want to rigorously track body composition changes, particularly fat versus muscle mass, throughout their treatment course. It is the gold standard for detecting muscle preservation or loss that scale weight alone cannot capture.

Fasting Insulin and HOMA-IR

Fasting insulin, combined with fasting glucose, allows calculation of HOMA-IR (Homeostatic Model Assessment of Insulin Resistance), a validated measure of insulin resistance. While not required for GLP-1 prescribing, high insulin resistance at baseline is often associated with stronger metabolic responses to GLP-1 therapy, making this a useful context marker.

Uric Acid

Uric acid may be worth checking at baseline in patients with a history of gout, as rapid weight loss can transiently elevate uric acid levels and precipitate gout flares. If baseline uric acid is already elevated, your prescriber may want a management plan in place before initiating treatment.

How to Request These Tests

If your prescriber has not ordered labs proactively, you can request them directly: "Before I start this medication, I'd like to establish baseline labs including HbA1c, fasting glucose, CMP, CBC, fasting lipid panel, and TSH. Can we order those today?" Most prescribers will accommodate this request without hesitation. If your prescription is through a telehealth platform, check whether the platform has partnered lab options for at-home blood draw or local lab orders, which are increasingly available.

Requesting baseline labs is not a sign of distrust — it is an act of engaged self-care. Prescribers who support informed, proactive patients welcome these conversations.

Key Takeaways

• Core baseline tests: HbA1c, fasting glucose, CMP (kidney and liver function), CBC, fasting lipid panel, and TSH.
• TSH is specifically important to rule out hypothyroidism and assess thyroid context given MTC labeling contraindications.
• Kidney and liver function tests in the CMP inform dose safety and establish a baseline for monitoring.
• A fasting lipid panel documents baseline cardiovascular risk and creates a reference for measuring GLP-1-driven improvement.
• DEXA scan is the gold standard for body composition tracking but is optional and not routinely required.
• You have the right to proactively request baseline labs — asking for them is appropriate and encouraged.

Sources

• Wilding et al. (2021). Once-Weekly Semaglutide in Adults with Overweight or Obesity. NEJM.
• Drucker DJ (2018). Mechanisms of Action and Therapeutic Application of GLP-1. Cell Metabolism.