C-peptide

When your pancreas makes insulin, it first builds a larger molecule called proinsulin and then snips it into two pieces: insulin itself and a leftover connecting fragment called C-peptide. One molecule of each, every time. Injected insulin contains no C-peptide, so measuring C-peptide in blood or urine tells a clinician how much insulin your own pancreas is still producing — a question an insulin level alone cannot answer once someone is on injections.

That distinction drives real decisions. A low C-peptide alongside high blood sugar points toward type 1 diabetes or a long-burned-out type 2, where the insulin-producing cells are largely gone. A normal or high C-peptide with high blood sugar points toward insulin resistance, the classic type 2 picture. MedlinePlus, the NIH consumer health library, describes the test in exactly these terms.

It matters for GLP-1 therapy because of how these drugs work on blood sugar. A GLP-1 receptor agonist amplifies the insulin your pancreas releases in response to a meal — it does not supply insulin. If there is very little left to amplify, the glucose-lowering effect has little to work with. That is one reason every approved GLP-1 in this class is labeled for type 2 diabetes, weight, or both, and none is approved as a replacement for insulin in type 1 diabetes.

The common confusion is assuming a low C-peptide rules out GLP-1 therapy for weight. It does not. Appetite suppression and weight loss run through receptors in the brain and gut, not through the pancreas, which is why Wegovy and Zepbound are approved for weight in people without diabetes at all. What a low C-peptide predicts is a weaker effect on A1c, not a weaker effect on the scale.

Worth asking your clinician: whether a C-peptide test would clarify your diabetes type, whether it should be drawn fasting or with a paired glucose, and how the result should change your medication plan. Our STEP 2 trial page shows how semaglutide performed in people who did have type 2 diabetes, and the drug interaction checker flags the insulin and sulfonylurea combinations that need dose attention.