Most people on a GLP-1 track one number: their weight on the scale. But the SELECT trial hinted that there’s more going on. Semaglutide cut major cardiovascular events by 20% in people with obesity and existing heart disease but no diabetes, and the benefit showed up early, before most of the weight came off. Something other than weight loss was doing the work, and a lot of it is visible in your blood.

GLP-1 receptor agonists (semaglutide, tirzepatide, and the next generation behind them) shift glucose, lipids, inflammation, kidney function, and liver enzymes. Some improve; a few can drift the wrong way, especially if nausea cuts your food and fluid intake or you lose weight quickly.

The most important biomarkers to monitor on a GLP-1 are A1c, ApoB, hs-CRP, IGF-1, and some kidney and liver markers. We’ll walk through each one in turn.

Typical direction of change on a GLP-1 over 6 to 12 months. The left column usually improves. The right column is where side effects and rapid weight loss can cause problems.

Hemoglobin A1c and fasting glucose

GLP-1s were diabetes drugs first, so it is no surprise that they lower blood sugar. Hemoglobin A1c (your average glucose over about three months) and fasting glucose both tend to fall, often within the first few weeks.

The number to watch here is not just whether glucose drops, but whether it drops too far. GLP-1s rarely cause low blood sugar on their own, but the risk climbs when they are combined with insulin or a sulfonylurea. If you take one of those, your A1c and glucose are worth tracking closely so your clinician can adjust the rest of your regimen.

ApoB, triglycerides, and the lipid panel

Weight loss and GLP-1s both improve your lipid panel, and triglycerides move the most, often dropping 20% or more. ApoB, the single best blood marker of how many cholesterol-carrying particles you have, also tends to come down.

hs-CRP and inflammation

hs-CRP is a marker of low-grade inflammation. GLP-1s drop hs-CRP substantially. This effect of GLP-1s on inflamation is part of why the heart protection appeared larger than weight loss alone could explain.

Kidney function: eGFR and creatinine

Over the long run, semaglutide protects the kidneys. But in the short run, GI side effects like Nausea, vomiting, and diarrhea can leave you dehydrated, and dehydration can cause a temporary drop in kidney function that shows up as a rising creatinine and a falling eGFR. This is usually reversible with fluids, but it is exactly the kind of change you want to catch through testing.

Liver enzymes: ALT and AST

Many people who carry extra weight also carry extra fat in the liver, a condition now called MASLD (and, when it inflames, MASH). GLP-1s and the weight loss they drive tend to pull fat out of the liver, and ALT and AST often improve as a result. A 2025 trial found semaglutide resolved liver inflammation in a meaningful share of people with MASH.

If your liver enzymes were mildly elevated before you started, watching them come down is a good sign that fatty liver is improving. It is one of the quieter wins of these drugs.

IGF-1 and the muscle question

The biggest worry with rapid weight loss is that some of what you lose is muscle, not fat. IGF-1 (insulin-like growth factor 1) is the blood marker that tracks your body’s anabolic, muscle-building state, and it tends to fall when you cut calories sharply. A dropping IGF-1, alongside albumin and your actual strength, is an early signal that you may be losing lean mass rather than just fat.

IGF-1 has a U-shaped relationship with health. Both very low and very high levels are linked to higher cardiovascular and mortality risk, so the goal is a healthy middle, not the highest number. On a GLP-1, the direction worth watching is a steep drop, which usually points back to not eating enough protein.

Don’t forget muscle and nutrition: albumin, B12, vitamin D, and ferritin

Rapid weight loss is not all fat. Some of it is muscle, and eating much less means taking in fewer vitamins and minerals. These are the markers people most often overlook. Albumin is a rough read on protein and nutritional status, and keeping it in range is one signal that you are eating enough protein to protect lean mass. Vitamin B12, vitamin D, and ferritin (your iron stores) can all drift down when intake falls, and deficiencies in these show up as fatigue long before anything dramatic. If you are losing weight steadily, these are worth checking periodically so a slow decline does not sneak up on you.

What about lipase and pancreatitis?

Pancreatitis is a known, rare risk with GLP-1s. It is not something you screen for with routine blood work in someone who feels fine. Checking lipase makes sense when there are symptoms (severe, persistent abdominal pain, often radiating to the back), not as a standing monitor. The takeaway is to know the symptom, not to add a monthly test.

How often should you test on a GLP-1?

A reasonable framework is to get a full baseline panel before you start, then recheck the markers that move. The first follow-up around three months catches the early changes in glucose, lipids, kidney function, and electrolytes. A six-month check confirms the trends are holding, and a twelve-month panel gives a full picture, with extra attention to nutrition markers as weight loss continues.

A general framework. Your clinician sets the actual schedule based on your other medications and conditions.

The exact schedule depends on your starting health, your other medications, and how you tolerate the drug. Someone on insulin needs closer glucose monitoring. Someone with a rough run of GI side effects needs an earlier kidney and electrolyte check. The point is that a GLP-1 changes far more than your weight, and the scale is the one number that tells you the least about what is happening inside.