Which Labs to Get Before Starting a GLP-1

Before you start a GLP-1 like Wegovy or Zepbound, your labs should answer two questions: what’s behind your weight, and what has the weight already done to your body? If you can get only one test, make it a comprehensive metabolic panel (CMP), which checks your kidneys, liver and fasting blood sugar. If you’ve had basic labs in the past year, for most people it’s probably safe to start; if you haven’t, get them first. Nobody should start blind.

Question 1: what’s behind the weight?

Sometimes it isn’t only fat. It can be fluid the body is holding because of a heart, liver or kidney problem, a thyroid that’s running low, or hormones. Finding one of these doesn’t always mean you can’t start. You just want to know you’re treating the right thing.

Question 2: what has the weight already done?

• Blood sugar and insulin resistance. A1C is your average blood sugar over three months. Fasting insulin, paired with glucose, can show insulin resistance years before the A1C rises, and it’s usually not on a standard panel.
• Kidneys and liver. Extra weight, high blood pressure and diabetes wear on both over time. A CMP can pick up kidney strain and early signs of fatty liver.
• Heart risk beyond the cholesterol number. See the advanced markers below.

Why the CMP matters for these medicines in particular

The most common side effects of GLP-1s are nausea, vomiting and diarrhea. Get sick enough to become dehydrated and your kidneys can suffer, so you want a starting number to compare against. The CMP alone isn’t enough, though: it says nothing about cholesterol, and it can’t tell you what’s behind the weight.

Five tests a standard panel often skips

A routine physical checks about a dozen things to estimate your 10-year risk. It’s reasonable but incomplete. Five tests worth asking about:

• Fasting insulin. Insulin is the volume knob for blood sugar; paired with glucose, it shows insulin resistance long before diabetes.
• ApoB. LDL measures how much cholesterol is in your blood; ApoB counts the particles carrying it. Think of LDL as the cargo and ApoB as the cars on the road.
• High-sensitivity CRP. A marker of inflammation, which can make plaque in the arteries more likely to rupture.
• Lipoprotein(a). A genetic, “sticky” form of cholesterol. You check it once in your life; about one in five people carries a high level, and if yours is high, every other risk factor matters more.
• A full thyroid panel, not just TSH, plus sex hormones where symptoms point that way.

Other useful markers include vitamin D and uric acid, which is often high in metabolic syndrome.

Sources

• Zepbound prescribing information and Wegovy prescribing information: stomach side effects and kidney injury from dehydration
• National Lipid Association, scientific statement on lipoprotein(a), 2024: testing once in adulthood; about 20% of people have high levels
• American Diabetes Association, Standards of Care: A1C and screening for kidney and liver disease