Why would a GLP-1 change what my blood work should look at?
These drugs don't usually go after your iron or your thyroid directly. What they do is turn appetite down, sometimes by a lot, and a body that's eating much less gets fewer of the building blocks along the way. A joint advisory from four US obesity and nutrition societies lists nutritional deficiencies from reduced calories among the main challenges of GLP-1 treatment, and a 2026 clinical review points to iron, vitamin B12, vitamin D, calcium, magnesium and zinc as the places signals show up most. Most of what's been reported so far is mild or indirect. The people at higher risk are those with past weight-loss surgery (bariatric surgery), gut conditions, a poor starting diet, older age or long stretches of nausea and vomiting.
What should I write down before the appointment?
Labs only make sense next to the story, so start there. Write down what you've noticed and roughly when it began, whether that's tiredness, a heart that pounds at night, feeling cold, hair shedding, low mood or a crash around 2 PM. Add a rough picture of what you eat on a normal day (it doesn't need to be a food diary), and anything you're taking alongside the drug, including supplements and anything that didn't come from a prescription.
That last part matters more than it feels like it should. A blood test can't tell which vial something came from or how much was in it, so whoever reads your results is working from your description. If you're using a research-grade compound, say so plainly. Our guide to retatrutide side effects and the Straight Answer on a racing heart go further on those symptoms.
Is ferritin the same as the iron number on my regular panel?
No. Hemoglobin, the iron-carrying protein counted on a basic blood count, tells you what's in circulation right now. Ferritin is the storage side, closer to the size of the pantry than what's on the table today, so you can have a normal hemoglobin and still be running low on stores.
Iron deficiency without anemia is underrecognized, according to a 2026 review, because the usual workup focuses on hemoglobin rather than stores. Low stores are linked to fatigue, trouble concentrating, anxiety and lower exercise capacity, and in trials of people who were iron deficient but not anemic, iron replacement brought meaningful drops in fatigue. Ask for ferritin along with a full iron profile. What counts as low varies by lab and by what else is going on in your body, including inflammation, so the number needs a reader. That's a reason to ask for the test by name, not a reason to start taking iron on a hunch.
Should B12 and folate be on the list?
They're on most of the review lists, so it's reasonable to ask. Vitamin B12 and folate are B vitamins that depend on what you eat, which is exactly what's shrinking, and a B12 shortfall can cause anemia and nerve symptoms like tingling. The newer GLP-1 reviews include both in their suggested lab panels, with the broadest panels aimed at higher-risk patients.
An older family-medicine review says screening healthy, average-risk adults for B12 isn't recommended, but may make sense with risk factors such as long-term metformin use, acid-reducing medicines, a vegan or strict vegetarian diet, gut surgery or conditions, and age over 75. Eating very little on a GLP-1 isn't on that list, which predates this class of drug, so it's a question to raise rather than an automatic order. If a result comes back borderline, a follow-up test called methylmalonic acid can help settle it.
Why do people ask for more than TSH on the thyroid?
TSH is the standard thyroid screening number, the signal your brain sends to nudge the thyroid. Free T4 is the thyroid's main output (the "free" means it's not bound to protein), and T3 is the active hormone that T4 gets converted into. During starvation and heavy calorie restriction, a 2026 review explains, the body converts less T4 into T3, leaving T3 low while TSH is normal or even low, and similar changes are increasingly seen with large weight loss and GLP-1 treatment. So TSH alone can look unremarkable while T3 is down, which is why asking for free T3, free T4 and TSH together is a fair request.
The same review adds a useful caution. Low T3 has traditionally been seen as the body adjusting to less fuel rather than a thyroid disease, and as something that doesn't need treating, though the author argues a persistent low alongside other illness may matter. An odd T3 while you're eating very little isn't automatically a thyroid problem.
What about morning cortisol and fasting insulin?
These two are more of a conversation than a checklist item. Cortisol is a stress hormone that peaks in the morning. The Endocrine Society's guideline on adrenal insufficiency, where the adrenal glands underproduce it, uses a morning cortisol (with a companion hormone called ACTH, adrenocorticotropic hormone) as a first screen when the standard stimulation test isn't possible, and calls that stimulation test the gold standard for confirming. So one reading is a first look, not a verdict. We didn't find evidence that GLP-1 drugs cause adrenal problems, so this one is for symptoms that persist.
Fasting insulin shows how much insulin your body needs to keep blood sugar steady after a night without food, and high values can point to insulin resistance, where cells respond poorly to it. The catch is that the test itself is inconsistent. When researchers ran the same samples through a dozen different insulin test kits, the results differed a lot from one kit to the next, and many were well off the lab's reference method. A number from one lab can't be lined up against another's, so any cutoff you've read online deserves a raised eyebrow. A good question to ask is what the result would change about your care.
What do I do with the results?
Three questions get the most out of the visit: what the numbers mean for you in particular, whether anything should change in what you're eating or taking, and when to repeat them while your weight is still moving. The right fix for an off number depends on the number, which is why we don't list amounts of anything here.
If appetite is the underlying issue, our Straight Answer on protein on a GLP-1 covers the food side. If it's tiredness that sent you looking, our guide to peptides for energy and fatigue covers that side. For the bigger picture there's the GLP-1 explainer, and research versus approved if you're sorting out what you're actually taking. The pages for semaglutide, tirzepatide and retatrutide cover each drug.
What can't blood work tell you?
It can't tell you there's a single answer. A 2025 review of nutrition guidance says no formal consensus recommendations exist for people starting GLP-1 drugs, and that its own monitoring advice draws mostly on expert opinion and on what's done after weight-loss surgery rather than on GLP-1 trials. The clinical reviews also say further studies are needed to show how often these shortfalls happen and what monitoring is best.
A normal result doesn't make your symptoms imaginary, because eating too little, too little protein, dehydration and the pace of dose changes can all produce the same tired, jittery, flat feeling without a single abnormal lab. And a lab can't read the label on a vial. It shows what's happening in you, not what you took, so your account of that stays part of the picture.
Sources: joint advisory on nutritional priorities for GLP-1 therapy (2025). micronutrient risk with GLP-1 and dual incretin drugs (2026). nutrition guidance gap for GLP-1 weight loss (2025). medical nutrition in the GLP-1 era (2026). iron deficiency without anemia (2026). vitamin B12 deficiency: recognition and management (2017). non-thyroidal illness syndrome and low T3 (2026). Endocrine Society guideline on primary adrenal insufficiency (2016). comparability of serum insulin test kits (2025).
Semaglutide and tirzepatide have a label and a prescriber behind them, and any vial bought online is sold for research. This is a list of questions to bring, not a diagnosis.
