There is a reasonable set of blood tests to have before starting one of these drugs, a much shorter list worth repeating, and a handful that get ordered routinely without supporting evidence. Few people leave the prescribing appointment with any of that written down.
Before the first dose
| Test | What it is for |
|---|---|
| HbA1c | Establishes where you are starting — normal, prediabetic or diabetic — and gives you something to measure response against. |
| Fasting glucose | Flags hypoglycemia risk if insulin or a sulfonylurea is already on board. |
| Lipid panel | These drugs move triglycerides, so a baseline lets you tell drug effect from everything else you changed at the same time. |
| Liver panel | Fatty liver disease is common in this population. Without a starting ALT and AST, a later number means very little. |
| Creatinine and eGFR | The most useful one on the list. Dehydration is the main kidney risk on these drugs, and you cannot detect a meaningful fall without knowing where you began. |
| TSH | An underactive thyroid looks exactly like resistance to weight loss. Worth excluding before blaming a plateau on the drug. |
| Vitamin B12, if you take metformin | Metformin depletes B12 in a substantial minority of long-term users, and eating less compounds it. |
| Calcitonin — only with a family history of MTC | Not a screening test. Without a nodule or a family history it produces far more confusion than information. |
While you are titrating
Less than people expect. No label requires routine bloods during the climb.
- Around week 4, a conversation rather than a blood test — how the nausea is, whether you are keeping fluids down, blood pressure, and whether fasting glucose is drifting low enough to warrant cutting the insulin or sulfonylurea.
- Around week 12, repeat the A1C if you are treating diabetes. Three months should show movement; a flat result at an adequate dose is a prompt to ask why.
- Liver tests only if something is wrong — new right-sided pain, yellowing, or nausea still going strong past the first month.
- Repeat creatinine after any bad episode of vomiting or diarrhea, roughly a week or two afterwards. This is the recheck most worth remembering.
- Pregnancy testing before each step up for anyone who could become pregnant — particularly given that tirzepatide's label flags reduced oral-contraceptive reliability for four weeks after every escalation.
- Lipase only if pancreatitis is genuinely on the table — severe pain reaching through to the back. Not as a routine.
Once you are settled
- A1C every three months until you are at target, then twice a year.
- Lipids once a year, more often only around a statin change.
- Liver panel once a year, more often with fatty liver disease or new symptoms.
- Creatinine once a year — every three to six months if your eGFR is under 60, or you take an ACE inhibitor, an ARB or a diuretic.
- TSH only if something suggests it. A normal baseline does not need repeating annually.
- B12 every year or two alongside metformin, sooner with pins and needles or anemia.
- Calcitonin: no routine repeat. The boxed warning is managed by counseling and examining your neck, not by surveillance bloods.
⛔ What not to order
This half gets left out of most guidance, and it costs people money and anxiety.
- Lipase or amylase as a screen. In someone without symptoms the false-positive rate is high and a mildly raised result predicts very little. The labels direct testing when there are clinical signs — not before.
- Calcitonin as a screen. Without a family history or a nodule it is not endorsed as surveillance by anyone.
- Tumor markers — CEA, CA 19-9, CA 125. No indication whatsoever here. Ordering them generates expensive investigation of false positives.
- Scheduled DEXA scans. Genuinely useful in research and in selected patients; not something everyone on a GLP-1 needs on a timetable.
Results that need acting on
- ALT or AST more than three times the upper limit. Hold, repeat in a week or two, and look for other causes — alcohol, paracetamol, viral, other drugs.
- Creatinine up more than about a quarter from baseline, or eGFR falling into a new band. Hold the dose, get fluids in, look again at NSAIDs and blood-pressure medication, then repeat the test.
- Fasting glucose under 70, or any symptomatic hypo on insulin or a sulfonylurea. ★ The thing to reduce is the insulin or sulfonylurea — not the GLP-1.
- Weight climbing or A1C rising at a steady dose. Before concluding the drug has stopped working, check the boring explanations: storage, missed doses, injection technique.
- Lipase over three times the upper limit with pain radiating to the back. Stop, image, and do not restart until it is cleared.
What this does not cover
Type 1 diabetes monitoring, which is a different discipline and outside these drugs' approved use. Children, where the product labels add growth, puberty and mood screening. And bariatric protocols, which overlap only partly with this.
Where to go next
- Diabetes medication interactions — what to cut when glucose runs low.
- Sick-day guide — when a bad week means rechecking your kidneys.
- Contraindications and cautions — what the label forbids versus what it merely warns about.
- All cheat sheets — the full reference library.