GLP‑1 Blood Work: What to Test Before Ozempic or Mounjaro — and What to Monitor After

GLP‑1GLP‑1 (glucagon-like peptide-1) A gut hormone released after eating that boosts insulin, slows the stomach, and signals fullness. Drugs like semaglutide mimic it. receptor agonistsreceptor agonist A drug that switches on a specific receptor, copying the body’s own signal — e.g. a GLP‑1 receptor agonist mimics the GLP‑1 hormone. — semaglutide (Ozempic, Wegovy), tirzepatide (Mounjaro, Zepbound), and now orforglipron (Foundayo) — are being prescribed at scale. Telehealth platforms have made access faster and easier than ever. That access is largely a good thing. But speed has a cost. A meaningful share of patients starting GLP‑1 therapy have never had the baseline workup that would catch reasons not to take it, flag underlying conditions, or set the numbers they’ll need to track their progress.

This article lays out exactly what to order, when, and why — so you can walk into any appointment with your prescriber knowing what a complete pre-treatment assessment looks like.

Why Pre-Treatment Labs Get Skipped — and Why They Shouldn’t

A 2025 retrospective study published in Healthcare reviewed 715 patients prescribed semaglutide for obesity or weight management. Despite all patients having at least one relevant medical condition warranting pre-treatment assessment, only 1.8% received a complete pre-treatment workup. Lipase, amylase, and calcitonin — three tests directly relevant to known GLP‑1 risks — were checked in zero patients.

This isn’t a critique of individual prescribers. It reflects a systemic pattern. In fast-moving telehealth workflows, ordering extra labs adds friction and delay. And the culture around GLP‑1 prescribing hasn’t settled on standard pre-treatment protocols the way it has for, say, starting a statin or an antidepressant.

The practical consequence is that patients start therapy without a baseline — no pre-treatment HbA1cHbA1c (hemoglobin A1c) A blood test that shows your average blood sugar over the past ~3 months. to compare against, no kidney function documented, no thyroid history verified. If something changes during treatment, there’s nothing to anchor to.

Contraindications to Know Before You Start

Two conditions are absolute contraindications to all GLP‑1 receptor agonists based on FDA labeling:

Personal or family history of medullary thyroid carcinoma (MTC). GLP‑1 receptors are expressed on thyroid C-cells. In rodent studies, GLP‑1 agonists caused dose-dependent thyroid C-cell tumors. Human data hasn’t confirmed the risk carries over, but the FDA keeps the contraindication because it’s biologically plausible and can’t be ruled out.

Multiple endocrine neoplasia type 2 (MEN2MEN2 (multiple endocrine neoplasia type 2) A rare inherited syndrome that raises thyroid-cancer risk — a reason GLP‑1 drugs are avoided in some families.). MEN2 is a genetic syndrome that includes MTC as a defining feature. Any patient with a known or suspected MEN2 diagnosis should not be started on a GLP‑1 agonist.

Both of these require a family history conversation before prescribing — not just a lab. If you’re going through a telehealth platform, make sure this question is being asked explicitly.

Active pancreatitispancreatitis Inflammation of the pancreas — a rare but serious potential side effect. is not a hard contraindication in all labeling, but a personal history of pancreatitis warrants serious consideration and a risk-benefit discussion before starting. GLP‑1 agonists have been associated with acute pancreatitis in post-marketing surveillance, though causation remains debated in the literature.

The Baseline Panel: What to Order and Why

These are the labs that should be drawn before your first dose — not because they’re all required, but because they establish your clinical baseline, identify conditions that change how you’re monitored, and catch contraindications that lab reports rather than history alone.

HbA1c and fasting glucose Your most important baseline. HbA1c tells you where your 3-month average blood glucose sits before therapy starts — essential for documenting whether the drug is improving glycemic control over time. In patients without diagnosed diabetes starting for weight loss, this is still worth establishing.

Comprehensive metabolic panel (CMP) Covers liver enzymes (ALT, AST) and kidney function (creatinine, eGFReGFR (estimated glomerular filtration rate) A calculation from a blood creatinine test that estimates how well the kidneys are filtering waste. A declining eGFR is the primary indicator of worsening kidney function.) in a single draw. The kidney piece matters because GI side effects like vomiting and diarrhea can cause dehydration that temporarily worsens kidney function — and you need a baseline eGFR (a kidney-function score) to interpret any change. Liver enzymes set a baseline too, because big weight loss (and the fatty-liver improvement that often comes with it) can move them.

Lipid panel GLP‑1 therapy typically improves lipid profiles — triglyceridestriglycerides The main form of fat carried in the blood and stored in fat tissue. High levels travel with insulin resistance and cardiovascular risk, and they respond strongly to diet, alcohol, and omega-3 dose. fall meaningfully, HDLHDL (high-density lipoprotein) Often called "good cholesterol." HDL particles ferry cholesterol away from artery walls back to the liver. Higher levels are generally linked to lower cardiovascular risk, though HDL function matters more than the number alone. improves, and LDL often modestly decreases. Without a baseline, you lose the ability to document this cardiovascular benefit over time.

TSHTSH (thyroid-stimulating hormone) A pituitary hormone measured to check how well the thyroid is working. Thyroid function affects weight, energy, and metabolic rate — all variables relevant to how you respond to GLP‑1 therapy. A baseline TSH catches undiagnosed hypothyroidism or hyperthyroidism before attributing any sluggish response to the drug. It also establishes a reference given the theoretical thyroid C-cell concern.

Lipase Establishes a pancreatic baseline given the pancreatitis association in post-marketing data. The 2025 retrospective study found this was ordered in zero out of 715 patients — which is a gap, particularly in patients with any history of gallstones, heavy alcohol use, or prior pancreatic issues.

CBC (complete blood count) General baseline. Catches anemia or other conditions that might influence symptoms during the GI adjustment period of dose titrationtitration Stepping a drug’s dose up slowly to reach the right level while limiting side effects..

Fasting insulin and HOMA-IRHOMA-IR A simple score from fasting glucose and insulin that estimates how insulin-resistant you are. Not in every standard pre-treatment protocol, but worth adding. HOMA-IR quantifies the insulin resistance your GLP‑1 is specifically designed to address. Without a baseline, you can’t measure the metabolic improvement that matters most. See our insulin resistance article for how to interpret the number.

Clinical note: This entire panel can be ordered through a direct-to-consumer lab service without a doctor’s order if your prescriber doesn’t include it. Getting your own baseline before your first appointment gives you leverage in the conversation — and documentation that won’t disappear if you switch providers.

Monitoring During Treatment: What to Check and When

Labs during treatment serve two purposes: safety monitoring and progress tracking. Here’s a practical schedule:

At 3 months:

• HbA1c and fasting glucose — first meaningful checkpoint for glycemic response
• CMP — kidney function and liver enzymes, particularly during active weight loss phase
• HOMA-IR — early signal on whether insulin resistance is improving
• Weight and waist circumference — not labs, but essential data points

At 6 months:

• Full repeat of baseline panel
• Lipid panel — typically when the first meaningful lipid changes become apparent
• B12 — GI side effects can impair absorption; worth checking if nausea or reduced food intake has been significant

Annually:

• Full metabolic panel repeat
• Lipid panel
• HbA1c (or more frequently if managing diabetes)
• HOMA-IR to track ongoing insulin resistance trajectory

If GI symptoms are persistent or severe:

• Lipase (pancreatitis screen)
• CMP (dehydration can transiently impair kidney function with significant vomiting)

Labs That Tell You If It’s Actually Working

Weight on the scale is the metric most people track. It’s valid — but it captures only part of the metabolic picture. These are the numbers that tell a more complete story:

HOMA-IR reduction. A meaningful drop in HOMA-IR (from, say, 3.5 to 1.8) indicates genuine insulin sensitization — not just caloric restriction. This is the metabolic outcome that predicts long-term diabetes prevention and cardiovascular risk reduction.

Triglyceride reduction. Triglycerides respond quickly to GLP‑1 therapy and to dietary change. A baseline of 200+ dropping to under 100 in 6 months is a meaningful cardiovascular benefit beyond the weight loss itself.

HbA1c trajectory in prediabetes. If your baseline HbA1c was 5.8–6.4% and it drops below 5.7% after 6 months of therapy, you’ve achieved something clinically significant — reversal of prediabetic glycemia. Without the baseline, you’d never know.

hsCRPhs-CRP (high-sensitivity C-reactive protein) A blood marker of low-grade inflammation in the body, used to refine heart-disease risk.. Not always included, but high-sensitivity CRPCRP (C-reactive protein) A blood marker of inflammation made by the liver. The high-sensitivity version (hs-CRP) is used for heart-risk assessment. is a marker of chronic low-grade inflammation tightly linked to insulin resistance and metabolic disease. GLP‑1 therapy reduces hsCRP in many patients. Documenting it before and after adds another dimension to the metabolic picture.