Most folks who start semaglutide or tirzepatide watch one number, the scale, but I want you to watch a few more. These medicines change your blood work, and most of that change is good. A few results need a closer look, and a few can mislead you if you read them alone.
This guide sorts the common labs into three groups, and the card below is the short version.
Why a Baseline Panel Comes First
A baseline panel is the blood work we draw before your first dose. Without it, a later result has nothing to compare against. An LDL of 124 means one thing if it started at 166 and something else if it started at 110.
The baseline also shows things that change the plan. Kidney function matters because dehydration on these drugs can strain the kidneys. A1c and lipids tell us where your metabolic risk starts. Liver enzymes give us a starting point if fatty liver is part of the picture.
Your history matters as much as your blood. Both FDA labels say anyone with a personal or family history of medullary thyroid cancer should not take these drugs. The same goes for MEN2, a genetic syndrome. For how we read a panel in general, see Reading Your Lab Panel.
What Usually Improves
Most numbers on a standard panel move the right way as weight comes off. The FDA labels report the two main weight trials, both in adults without type 2 diabetes. The semaglutide trial ran 68 weeks in 1,961 adults, and the tirzepatide trial ran 72 weeks in 2,539 adults.
Triglycerides usually fall the most, dropping 21.9% on semaglutide compared with 7.3% on placebo. On tirzepatide they dropped 21.2% to 29.1% across the three doses tested, compared with 5.6% on placebo.
LDL cholesterol moves less than most people expect. It fell 2.5% on semaglutide, while it rose 1.3% on placebo. It fell 4.6% to 7.1% on tirzepatide, compared with 1.7% on placebo. If your LDL is high, plan on a GLP-1 helping only a little.
Blood pressure falls too: systolic pressure, the top number, dropped 6.2 points on semaglutide and 6.6 to 7.7 points on tirzepatide. The placebo groups dropped about 1 point.
A1c measures your average blood sugar over the past few months. It fell 0.4 percentage points in both trials, even though no one in them had type 2 diabetes.
Liver enzymes tend to drift down. One 72-week semaglutide trial enrolled people with MASH, a fatty liver with inflammation and scarring. On biopsy, the inflammation resolved without worse scarring in 63% of people on the drug and 34% on placebo. The label also reports a trend toward lower liver enzymes from week 12 on.
hs-CRP, a blood marker of inflammation, fell more on tirzepatide than on placebo in the sleep apnea trials. Results vary from person to person, and a trial average is not a promise for any one patient.
From our practice: a man in his mid-40s in our weight program started tirzepatide and never moved past the starting dose. He also changed his food and training. He cut two daily energy drinks, ate about 120 grams of protein a day and did resistance training three times a week.
We pulled his panel early, at 5 weeks, though our standard recheck is at 12 weeks.
| Baseline | 5 weeks | |
|---|---|---|
| Weight | 154.8 lb | 139.4 lb (down 9.9%) |
| LDL cholesterol | 166 | 124 (down 42 points) |
| Non-HDL cholesterol | 182 | 133 |
| Triglycerides | 79 | 44 |
Lipid values are in mg/dL. His SHBG also rose during active fat loss, and we now track SHBG in men on these medicines.
This is one person, so read it with its limits. His food and training changed at the same time, so we cannot say how much came from the medicine. His LDL fell about 25%, far more than the trial averages above, and 5 weeks tells us nothing about whether the changes last.
What to Watch
Kidney function can suffer when nausea, vomiting or diarrhea lead to dehydration. Both labels report acute kidney injury after these drugs reached the market, sometimes needing dialysis, mostly in people with those side effects. In the tirzepatide weight trials, acute kidney injury occurred in 0.5% of people on the drug and 0.2% on placebo. The labels tell doctors to check kidney function when side effects could cause dehydration, especially when starting or raising the dose.
Gallbladder problems, including gallstones and inflammation, are more common on these drugs. In the semaglutide weight trials, 1.6% of people on the injection developed gallstones, compared with 0.7% on placebo. Fast weight loss raises gallstone risk on its own, yet the semaglutide group still had more gallbladder problems after accounting for weight lost.
The tirzepatide patient guide lists the symptoms to report: pain in the upper stomach, fever, yellow skin or eyes, and clay-colored stools.
Pancreatitis is uncommon: in the tirzepatide weight trials, it occurred in 0.2% of people on the drug and 0.2% on placebo. The warning sign is severe belly pain that will not go away, sometimes spreading to the back, with or without vomiting.
Thyroid risk carries a boxed warning on both labels. These drugs caused thyroid C-cell tumors in rodents, and no one knows yet whether they do so in humans. The labels also say routine calcitonin blood tests are of uncertain value for early detection of medullary thyroid cancer. So we screen this risk through your personal and family history.
Nutrition markers matter because these drugs cut how much people eat. A 2025 joint advisory from four obesity and nutrition societies reports calorie cuts of 16% to 39% on GLP-1 drugs. It names vitamin D, calcium and B12 as nutrients at risk. It also advises rechecking nutrient levels during treatment, which matters most when appetite falls sharply.
What to Read With Care
Most reports estimate kidney function as eGFR, and labs usually calculate it from creatinine, a waste product that comes from muscle. If you lose muscle, creatinine can fall even when your kidneys have not changed, which can make eGFR look better than it is.
Researchers have seen this after bariatric surgery: in one study, creatinine-based eGFR rose while cystatin C and directly measured kidney function stayed flat. Cystatin C is a second filtration marker that does not depend on muscle mass. When creatinine-based eGFR does not fit the rest of the picture, cystatin C is a useful cross-check.
The good news is that in GLP-1 trials so far, the two markers have mostly agreed. In a 52-week tirzepatide trial in people with type 2 diabetes, both kinds of eGFR told the same story. The 2026 SMART trial studied semaglutide in people with kidney disease. Changes in their muscle and fat did not track with changes in eGFR or measured kidney function.
SHBG is a protein that carries testosterone in the blood, and free testosterone is the small part not attached to any carrier protein. A 2026 review of 10 studies in 639 men found that GLP-1 drugs usually raised total testosterone. Free testosterone results were mixed, often because SHBG rose at the same time.
So a higher total testosterone on your report does not always mean more free testosterone. In men, we read total testosterone, SHBG and free testosterone together and compare each result with that man's own baseline.
Lipase, a pancreas enzyme, rose on average 28% to 35% on tirzepatide and 39% on semaglutide in the weight trials. The labels say the meaning of these rises is unknown when there are no pancreatitis symptoms. We act on symptoms, not on a lone lipase value.
What We Don't Know Yet
Neither FDA label sets a routine lab schedule for people on these drugs, and we know of no trial that tested one. Our 12-week recheck is a clinical judgment, not a proven rule. The kidney marker studies above enrolled people with diabetes or kidney disease, so lean, active patients may not behave the same way.
How We Approach It
We draw a baseline panel before the first dose and recheck at about 12 weeks. After that, we recheck periodically, and sooner if symptoms or results call for it. In men, we now track SHBG along with testosterone. You can see what we measure on our Biomarkers page.
Labs are one part of the plan. Protein and resistance training help protect muscle, and we cover both in GLP-1 and Muscle Loss. Our weight management program builds the panel, the food plan and the training plan together. If you are thinking about coming off a GLP-1, read what happens when you stop.
Between Panels
If vomiting or diarrhea keeps you from holding down fluids, contact us that day. Do the same for belly pain that is severe or will not settle. Do not wait for the next scheduled draw, because a kidney check at the right moment matters more than a perfect panel at week 12.