Blood test before starting GLP-1

First line

Blood test before starting treatment with GLP-1 or similar medicines, such as Ozempic or Wegovy (semaglutide), Mounjaro (tirzepatide) or Saxenda (liraglutide). It includes glucose, glycated haemoglobin, insulin, cholesterol, kidney, liver, full blood count, TSH and a selection of vitamins and minerals. It provides reference values before the first dose and identifies abnormalities that call for medical context.

No prescription or appointment needed

Results in 1-2 working days

Includes: TSH · Glucose · Glycated haemoglobin · Insulin · HOMA-IR index · Lipid profile and ratios · Creatinine and eGFR · Sodium and potassium. 19 in total. See what each one measures

What it is for

It is used to establish a metabolic, kidney, liver and nutritional starting point before treatment. This makes it possible to compare later checks and to tell a new change apart from an abnormality that was already there.

The blood test does not decide whether the medicine is indicated, nor can it exclude contraindications. Prescribing requires reviewing medical history, medication, digestive symptoms, any history of pancreatitis or gallbladder disease and other questions that a blood sample cannot answer.

When it is recommended

Before the first dose of semaglutide, tirzepatide, liraglutide or another similar treatment
When restarting treatment after a long break, if the doctor considers it necessary to update the values
Shortly after starting, if there is no recent baseline blood test
Before starting, when there is a metabolic, liver, kidney or nutritional history that calls for a reference point

Clinical comment

We have designed this profile for comparison, not to predict that every abnormality will occur. Severe vomiting or diarrhoea can cause dehydration and affect the kidneys and electrolytes. Rapid weight loss is associated with a higher risk of gallstones, but a blood test does not detect stones: ultrasound is the test that shows them.

The micronutrient block provides a reference if food intake falls substantially. It is worth being honest: there is not yet a consensus recommending routine measurement of all these nutrients in everyone treated with GLP-1. They are included on clinical judgement and because comparison is useful if symptoms or substantial weight loss appear.

Thiamine is not included. If persistent vomiting, confusion, difficulty walking or double vision appear, do not wait for a monitoring blood test: urgent medical assessment is required. Calcitonin is not included routinely either, because personal and family history is the basis for assessing the risk of medullary thyroid carcinoma.

Dr. Antonio Mas

What is tested

TSH: The initial test for identifying a thyroid abnormality that could influence weight or cause similar symptoms.
Glucose: Measures fasting blood sugar.
Glycated haemoglobin (HbA1c): Reflects average glucose over the last two or three months.
Insulin: Measures fasting insulin and is interpreted together with glucose.
HOMA-IR index (calculated): Estimates the probability of advanced liver fibrosis from age, transaminases and platelets.
Lipid profile (total cholesterol, HDL, LDL, triglycerides) and ratios
Creatinine and eGFR: Assess kidney function.
Sodium and potassium: Provide a reference for the main electrolytes before any possible digestive losses.
AST, ALT and GGT: Enzymes that give an indication of the state of the liver.
Total bilirubin and alkaline phosphatase: Complete the liver and biliary profile.
Full blood count: Checks red blood cells, white blood cells and platelets.
FIB-4 index (calculated): Estimates the probability of advanced liver fibrosis from age, transaminases and platelets.
Ferritin: Estimates iron stores, although it can also rise with inflammation or fatty liver.
Vitamin D (25-OH): Assesses vitamin D stores.
Vitamin B12: Measures the circulating concentration of cobalamin.
Folic acid (folate)
Uric acid: Provides a baseline value, which is especially relevant with a history of gout.
Calcium: Gives basic information about mineral metabolism.
Magnesium: Measures serum magnesium.

These tests are used to calculate Índice HOMA-IR and Índice FIB-4. They are not separate tests.

How to prepare

Fast for 8 to 12 hours. You can drink water. If you take biotin supplements, stop them 48 hours before, because they can interfere with TSH and other tests. Avoid alcohol and intense exercise the day before.

You can add

Without another needle. It is offered when you order, so you do not have to decide now.

Male androgen profileAssesses androgen function better than total testosterone alone by including SHBG and calculating free testosterone.+€49.00
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Advanced lipid panelExtends standard cholesterol with ApoB, lipoprotein(a) and homocysteine, markers that refine the assessment of cardiovascular risk and are not usually measured routinely.+€59.00

Related blood tests

We have other profiles in Obesity with more or fewer tests:

The most comprehensive is not always the most useful. This is the one we recommend to start with; the others cover specific situations.

What it is based on

American Diabetes Association · Standards of Care in Diabetes 2026 · Diabetes Care 49(Supl 1):S27-S49
Diagnosis and Classification of Diabetes
Recommends screening for prediabetes and diabetes at any age in adults with overweight or obesity and a risk factor, using fasting glucose or glycated haemoglobin. That is the blood sugar block of this baseline blood test.
European Association for the Study of the Liver, EASD y EASO · Diabetologia 67(11):2375-2392
EASL-EASD-EASO Clinical Practice Guidelines on the management of metabolic dysfunction-associated steatotic liver disease (MASLD)
Recommends looking for fatty liver with fibrosis in people with abdominal obesity and another metabolic factor, starting with a blood index such as FIB-4 before an imaging test. That is the reason for looking at the liver before the first dose and not afterwards.
Sibal R, Balamurugan G, Langley J, et al. · Nutrients 17(23):3659
Macronutrient, Micronutrient Supplementation and Monitoring for Patients on GLP-1 Agonists: Can We Learn from Metabolic and Bariatric Surgery?
Reviews the vitamin and mineral deficiencies described with GLP-1 medicines and states that there are currently no consensus guidelines on what to measure or how often, and that the reasonable approach in the meantime is to have a starting point before beginning. The vitamin and mineral block of this profile is based on that reasoning and on clinical practice, not on a guideline recommendation.

Protocol designed by Dr. Antonio Mas, specialist in Endocrinology and Nutrition, Spanish medical licence no. 080846289, and reviewed in September 2026.

This blood test is a standardised protocol and does not replace a medical consultation. If you have symptoms, a known condition or a complex clinical situation, speak to your doctor.