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Dr Ruchira Karunadasa
Authored on 25 Sep 2026 by Dr Ruch Karunadasa,Medically reviewed 25 Sep 2026 by Dara O'Malley.Next review due: 25 Sep 2027

Why Am I Struggling to Lose Weight? Biological Barriers and What to Test

If you are eating well and moving more, yet the scale barely shifts, the problem is often biology rather than willpower. Hormones, thyroid function, blood sugar, and iron stores all influence how readily your body loses fat.

PrivateDoc can look at those markers with a home Weight and Metabolism blood test, then assess whether prescription options such as Wegovy, Mounjaro, or a daily tablet such as Foundayo are appropriate after a full consultation.

Weight and metabolism blood profile at-home kit

Key takeaways: why weight loss stalls

  • Plateaus are common: hypothyroidism, insulin resistance, and low ferritin can all slow progress.
  • Test before you guess: a Weight and Metabolism profile checks TSH, Free Thyroxine, HbA1c, lipids, and iron.
  • Clinician review matters: numbers without context can alarm or mislead. A UK-registered clinician reads the report.
  • Treatment is separate: GLP-1 medicines still need BMI checks, photos, and a named prescriber. A blood test does not skip that step.
  • Two doors, one clinic: start with a kit if you want answers, or a weight-loss consultation if you already meet typical BMI criteria.

The biology of weight plateaus

Weight management is heavily influenced by hormones and metabolic rate. An underactive thyroid (hypothyroidism) or insulin resistance can make your body store fat more readily and burn calories more slowly. Low iron causes fatigue that looks like a slow metabolism. Without those markers, you can keep tightening the plan and still see little change.

That is also why some people regain after stopping GLP-1 treatment. Appetite hormones return while resting energy burn stays lower. See why weight returns after weight-loss medication and our maintenance plans.

What a Weight and Metabolism profile checks

  • TSH and Free Thyroxine for thyroid speed.
  • HbA1c for longer-term blood sugar control.
  • Lipid profile for cardiovascular risk alongside weight.
  • Ferritin because low iron causes heavy fatigue.

The full walkthrough lives in our home blood test guide. You collect a finger-prick sample and a clinician reviews the laboratory report in your account.

From results to a weight-loss plan

If markers point to a metabolic barrier, clinicians can recommend lifestyle changes, GP follow-up, or a prescription pathway. Typical UK eligibility for medicines such as Wegovy and Mounjaro is a BMI of 30 kg/m², or 27 kg/m² with a weight-related condition. Photos and ID still apply. Read how to get a weight-loss prescription online in the UK.

A blood test does not guarantee a prescription. Treatment is only authorised after a successful clinical assessment.

Check your eligibility

If you want medically supervised weight loss, start a consultation. If you first want to understand your metabolism, take the blood-test questionnaire.

Disclaimer: This article is for general information only and does not replace personalised medical advice. Treatment is only authorised after a successful clinical assessment.

FAQs about struggling to lose weight

Is struggling to lose weight just a lack of willpower?

Often no. Thyroid function, insulin resistance, and low iron can stall progress even when diet and activity are consistent. A clinician-reviewed blood test can show whether biology is in the way.

Should I have a blood test before starting Wegovy or Mounjaro?

It can help when progress has stalled or tiredness is unexplained. It is not a substitute for the prescribing consultation. Your clinician decides whether a panel adds value.

Which test should I take for a weight plateau?

The Weight and Metabolism profile is the usual starting point. It checks TSH, Free Thyroxine, HbA1c, lipids, and ferritin. See the home blood test article for the full marker list.

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Did you know?

Over 40% of the population has tried to lose weight at some point in the last 5 years – so you're certainly not alone