Why Can't I Lose Weight? Medical Conditions That Can Hold You Back

In short: If you are eating well and staying active but your weight will not shift, a health condition or a medicine may be playing a part. Common examples include an underactive thyroid, polycystic ovary syndrome (PCOS), insulin resistance, poor sleep (including sleep apnoea), low mood and some prescribed medicines. Most of these can be checked with a conversation and, where appropriate, simple blood tests.

This guide, written by a Ready Health prescribing pharmacist, explains which conditions can make weight loss harder, the signs to look out for, and when it is worth getting checked.

Why can't I lose weight even though I'm doing everything right?

Body weight is influenced by much more than willpower. Genetics, hormones, sleep, stress, medicines, age and your environment all affect appetite and how your body stores energy. When progress stalls, it is reasonable to ask whether something medical is getting in the way, rather than assuming you are doing it wrong.

It is also worth knowing that weight loss naturally slows over time and often comes in uneven steps, so a plateau of a few weeks does not always mean there is a problem.

Which medical conditions can make weight loss harder?

1. Underactive thyroid (hypothyroidism)

The thyroid gland makes hormones that help regulate how the body uses energy. When it is underactive, people may gain some weight and find it harder to lose.

• Possible signs: tiredness, feeling cold, constipation, dry skin, low mood, heavier periods and weight gain.
• How it is checked: a blood test for thyroid-stimulating hormone (TSH), sometimes with thyroxine (T4).
• Treatment: usually a daily thyroid hormone tablet prescribed and monitored by a clinician.

An underactive thyroid tends to account for a modest amount of weight change, so treatment helps but is rarely the whole answer on its own.

2. Polycystic ovary syndrome (PCOS)

PCOS is a common hormonal condition affecting how the ovaries work. It is closely linked with insulin resistance, which can make weight easier to gain and harder to lose.

• Possible signs: irregular or absent periods, excess facial or body hair, acne, thinning scalp hair and difficulty getting pregnant.
• How it is managed: lifestyle support is the foundation, and a clinician may discuss medicines to help with specific symptoms. Even a small amount of weight loss can improve symptoms for some people.

3. Insulin resistance and raised blood sugar

Insulin resistance means the body's cells respond less well to insulin, so the pancreas has to make more of it to keep blood sugar in range. It often has no obvious symptoms, which is why it can go unnoticed for years. Carrying weight around the waist, a family history of type 2 diabetes, PCOS or a history of diabetes in pregnancy all increase the risk.

A blood test called HbA1c can show whether blood sugar is in the normal range, higher than normal (sometimes called prediabetes or non-diabetic hyperglycaemia), or in the diabetes range. We cover this in more detail in our insulin resistance guide.

4. Poor sleep and obstructive sleep apnoea

Short or disrupted sleep affects the hormones that control hunger and fullness, and tiredness makes it harder to stay active and plan meals. Obstructive sleep apnoea, where the airway repeatedly narrows or closes during sleep, is more common in people living with a higher weight and can create a cycle that is hard to break.

• Possible signs: loud snoring, pauses in breathing noticed by a partner, waking up gasping, morning headaches and daytime sleepiness.
• What to do: speak to your GP, who can refer you for a sleep study if needed. If you feel sleepy while driving, seek advice promptly.

5. Low mood, anxiety and stress

Depression, anxiety and long-term stress can change appetite, sleep and motivation. Some people eat more for comfort; others lose their routine altogether. Looking after your mental health is a genuine part of weight management, not a separate issue.

6. Medicines that can cause weight gain

Some prescribed medicines can increase appetite or cause weight gain as a side effect. Examples include some antidepressants and antipsychotics, steroid tablets, some epilepsy medicines, insulin and certain other diabetes medicines.

Never stop a prescribed medicine on your own. If you think a medicine is affecting your weight, ask your prescriber or pharmacist whether there is a suitable alternative.

Other factors worth considering

• Perimenopause and menopause: hormonal changes can shift where fat is stored and affect sleep and mood.
• Stopping smoking: many people gain some weight after quitting, though the health benefits of stopping far outweigh this.
• Rarer hormonal conditions: such as Cushing's syndrome, which a clinician may consider if there are other specific signs.

How is an underlying cause checked?

A good weight assessment usually includes:

• a conversation about your history, eating patterns, activity, sleep, mood and medicines
• measurements such as weight, BMI and waist-to-height ratio (NICE advises keeping your waist to less than half your height)
• blood pressure
• blood tests where appropriate, for example thyroid function, HbA1c, cholesterol, and kidney and liver function

What can I do next?

If a medical cause is found, treating it can make your efforts more effective. If nothing is found, that is useful too: it means a structured plan around eating, activity, sleep and support is likely to work, and a clinician can discuss whether any additional treatment might be suitable for you. Any medicine is only prescribed if a prescriber decides it is clinically appropriate after a full assessment.

Frequently asked questions

Can an underactive thyroid stop you losing weight?

It can make weight loss harder and cause some weight gain. Once thyroid levels are corrected with treatment, many people find their usual approach works better again.

Does insulin resistance cause symptoms?

Often it causes none, which is why a blood test is useful if you have risk factors such as weight around the waist, PCOS or a family history of type 2 diabetes.

Can medication make you gain weight?

Yes, some can. Talk to your prescriber before making any changes, as stopping a medicine suddenly can be harmful.

Which blood tests are useful if I can't lose weight?

Depending on your symptoms, a clinician may suggest thyroid function, HbA1c, cholesterol, and kidney and liver function tests.

Key takeaways

• Difficulty losing weight is common and is not simply a matter of willpower.
• Thyroid problems, PCOS, insulin resistance, poor sleep, low mood and some medicines can all play a part.
• Many of these can be checked with an assessment and simple blood tests.
• Never stop a prescribed medicine without speaking to your prescriber.

Sources and further reading

• NHS: Underactive thyroid (hypothyroidism)
• NHS: Polycystic ovary syndrome
• NHS: Obstructive sleep apnoea
• NICE NG246: Overweight and obesity management

Last reviewed: October 2026 by Shamir Patel, Independent Prescribing Pharmacist, Ready Health. This article is general information and does not replace advice from your own clinician.

If you live in Standish, Wigan or the surrounding area and would like to talk through why weight loss feels so hard, you can book a weight management consultation at Ready Health. Our clinicians will assess your health, discuss whether any tests are worthwhile and agree a plan that suits you.