Blood Tests on Mounjaro: What Your Doctor Should Monitor

If you are taking Mounjaro (tirzepatide) for weight loss, routine blood tests are not optional. They are a core part of safe, supervised treatment. Tirzepatide affects multiple metabolic pathways, including glucose regulation, lipid metabolism, liver enzyme activity, and nutrient absorption. Without regular monitoring, clinically significant changes can go undetected until symptoms appear.

At CutKilo, every patient receives a structured blood test schedule as part of their treatment plan. This article explains what your doctor should be checking, why each marker matters, and how often testing should happen while you are on Mounjaro.

Tirzepatide is a dual GIP/GLP-1 receptor agonist originally developed for type 2 diabetes. It lowers blood glucose by enhancing insulin secretion and suppressing glucagon. Even in patients without diabetes, this glucose-lowering effect is clinically relevant.

Your doctor should check HbA1c at baseline and every three to six months during treatment. Fasting glucose may also be useful between HbA1c tests, particularly during dose escalation. If your HbA1c drops below 35 mmol/mol or you experience symptoms such as shakiness, sweating, or confusion, your doctor should assess for hypoglycaemia. This is especially important if you are also taking sulfonylureas or insulin.

Liver function tests are essential for anyone on Mounjaro. Rapid weight loss, which tirzepatide often produces, mobilises stored fat from the liver. In patients with pre-existing fatty liver disease, this can cause transient rises in ALT and AST before levels improve as hepatic fat clears.

Your doctor should request a full liver panel (ALT, AST, ALP, GGT, bilirubin, and albumin) at baseline, at three months, and at six-monthly intervals thereafter. A sustained rise in ALT above three times the upper limit of normal warrants further investigation, including liver ultrasound. Most patients see their liver enzymes improve over time, but monitoring is how we distinguish normal fluctuation from a problem that needs attention.

Tirzepatide can cause gastrointestinal side effects, particularly nausea, vomiting, and reduced fluid intake during dose titration. These effects increase the risk of dehydration, which directly impacts kidney function.

Your doctor should check eGFR and serum creatinine at baseline and every three to six months. Urea and electrolytes (sodium, potassium) should be included to give a fuller picture of renal health. If your eGFR drops by more than 15% from baseline, or you develop signs of dehydration such as dark urine, dizziness on standing, or significantly reduced urine output, this should prompt clinical review and possible dose adjustment.

One of the well-documented benefits of tirzepatide is its effect on the lipid profile. Clinical trials have shown reductions in total cholesterol, LDL cholesterol, and triglycerides, alongside modest increases in HDL cholesterol.

A fasting lipid panel should be checked at baseline and repeated at six months. If you are already taking statins, your doctor may use the improving lipid profile as an opportunity to review whether your statin dose needs adjusting. The key markers to track are total cholesterol, LDL, HDL, triglycerides, and the total cholesterol to HDL ratio.

GLP-1 receptor agonists carry a boxed warning regarding medullary thyroid carcinoma based on rodent studies. While this has not been demonstrated in humans, thyroid monitoring remains a sensible precaution, particularly for patients with a personal or family history of thyroid disease.

TSH and free T4 should be checked at baseline and annually during treatment. If you are already taking levothyroxine, be aware that weight loss can alter your thyroid hormone requirements. Your doctor may need to recheck levels more frequently and adjust your dose. Any new neck swelling or difficulty swallowing should be reported and investigated promptly.

Significant calorie reduction while on Mounjaro increases the risk of nutritional deficiencies. The most common are iron, vitamin B12, folate, vitamin D, calcium, and magnesium. These deficiencies can cause fatigue, hair loss, muscle weakness, and mood changes, all of which patients may mistakenly attribute to the medication itself.

A full blood count should be checked at baseline and every six months. Vitamin D, B12, folate, ferritin, calcium, and magnesium should be included. If you are losing weight rapidly (more than 1 kg per week sustained), or experiencing symptoms such as tingling, extreme fatigue, or hair thinning, additional testing may be needed sooner. Adequate protein intake is also important for preserving lean mass and bone density during treatment.

As a general guide, the following schedule applies to most patients:

Baseline (before starting treatment): HbA1c, fasting glucose, full liver panel, eGFR, creatinine, electrolytes, fasting lipids, TSH, free T4, full blood count, ferritin, B12, folate, vitamin D, calcium, magnesium.

Three months: HbA1c, liver function, kidney function, electrolytes, fasting glucose.

Six months: Repeat full panel including lipids and nutritional markers.

Ongoing (every six months): Full panel. TSH annually unless on levothyroxine or symptomatic.

Your doctor may adjust this schedule based on your individual risk factors, dose changes, and any symptoms that arise during treatment.

At CutKilo, blood test monitoring is built into every treatment plan. We do not prescribe Mounjaro without a clear testing schedule, and we review results with every patient before dose adjustments. Our clinical team tracks trends over time rather than looking at single snapshots, which means we catch subtle shifts early.

If you are considering Mounjaro or are already taking it without regular monitoring, speak to your prescribing doctor about setting up a proper testing schedule. For patients who want doctor-led, fully monitored weight loss treatment, CutKilo provides this as standard. You can also learn more about how we use DEXA body composition scanning alongside blood tests to track your progress safely.