WEIGHT LOSS
What Blood Tests Are Useful Before Starting Weight Loss Treatment?
Why baseline testing makes prescribing safer, which markers are most relevant, and how progress should be measured beyond weight alone.
Not every patient needs the same investigations before starting weight-loss treatment, but baseline blood tests are often clinically important. They can identify safety concerns that affect prescribing decisions, reveal metabolic conditions that may be contributing to weight gain, establish a reference point for monitoring progress, and sometimes change the treatment approach entirely — for example, if undiagnosed thyroid disease or pre-diabetes is found.
The purpose is not to delay treatment unnecessarily. It is to make prescribing safer and to ensure the treatment plan is grounded in a proper clinical picture. For the main service, see medical weight loss.
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Which blood tests are most commonly relevant
HbA1c and fasting glucose assess blood sugar control and screen for type 2 diabetes or pre-diabetes. Insulin resistance is both a common contributor to weight gain and a factor that affects how well some treatments work. Patients with undiagnosed diabetes may need a different treatment pathway. Liver function tests (ALT, ALP, GGT, bilirubin) are relevant because non-alcoholic fatty liver disease is common in patients with excess weight, and some weight-loss medications require monitoring of liver markers. Kidney function (creatinine, eGFR) is checked because renal impairment can affect drug clearance and dosing.
Lipid profile (total cholesterol, HDL, LDL, triglycerides) provides a cardiovascular risk baseline — one of the most important markers to track during treatment, since improving lipids is often as clinically relevant as the number on the scales. Thyroid function (TSH, free T4) screens for hypothyroidism, which can contribute to weight gain and fatigue and should be treated before or alongside weight management. Depending on the patient, additional markers such as vitamin D, iron studies, testosterone (in men), or inflammatory markers may also be useful.
How baseline results influence the treatment plan
Baseline testing does more than confirm safety. It can change the approach entirely. A patient with undiagnosed hypothyroidism may find that treating the thyroid condition alone improves energy, metabolism and weight. A patient with pre-diabetic HbA1c levels may warrant a different intensity of treatment or closer monitoring. A patient with significantly elevated liver enzymes may need investigation before any medication is started. These are not uncommon findings — they are part of the reason a GP-led consultation is more useful than a transactional prescribing service that skips the assessment.
Monitoring
Measuring progress beyond weight alone
Baseline testing also establishes the reference point for monitoring progress. Repeat blood tests after 3 to 6 months of treatment can show whether HbA1c has improved, whether liver markers are stable, whether cholesterol has responded, and whether any deficiency has been addressed. These changes matter clinically even if the weight loss itself appears modest.
At Basuto, monitoring is part of the weight loss pathway — baseline blood tests are arranged before treatment and repeated at appropriate intervals to track both safety and clinical progress.
Same-day appointments available. Book online or call 020 7736 7557.
GP-led approach
Why assessment before prescribing matters
Many online weight-loss services prescribe GLP-1 medications based on BMI and a brief questionnaire. A GP-led approach at Basuto includes proper clinical assessment, baseline blood tests, review of contributing factors, and a treatment plan that accounts for the wider metabolic picture. That matters because weight gain is rarely explained by one factor alone, and safe prescribing requires understanding the full clinical context.
For broader health review, a health check or health screening appointment may also be relevant alongside weight management.
Frequently asked questions
Do I always need blood tests before starting Mounjaro or Wegovy?
Not universally, but baseline testing is strongly recommended for most patients. It establishes safety, identifies contributing conditions, and provides a reference point for monitoring. Your GP will advise on what is clinically appropriate for your situation.
What if blood tests show a thyroid problem?
Hypothyroidism is a treatable cause of weight gain and fatigue. If found, treating the thyroid condition may improve energy, metabolism and weight independently. Weight-loss treatment can continue alongside thyroid management where appropriate. See thyroid blood tests.
How often are blood tests repeated during treatment?
Typically at 3 and 6 months, depending on the treatment and the baseline results. The aim is to track both safety markers and clinical improvement. Your GP will advise on the appropriate monitoring schedule.
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