Why can't I lose weight?

Effort is usually not the missing piece

Most people who ask this question have already done the obvious things, often several times. They have eaten less, moved more, and watched it work for a while and then stop. That pattern is so common that it is better understood as physiology than as a character problem.

Obesity is recognised clinically as a chronic, relapsing condition with biological, hormonal, psychological and environmental drivers. The body actively defends a weight it has settled at, and that defence gets stronger the harder it is pushed. That is not an excuse and it does not mean nothing can change. It does mean that if weight has stopped moving, the useful question is which mechanism is involved, not how much more willpower to apply.

Nine common reasons weight stops moving

1. Thyroid function

An underactive thyroid slows resting energy use and commonly brings fatigue, cold intolerance, constipation and dry skin with it. It is one of the first things worth excluding because it is cheap to test and treatable. A TSH with free T4 is the usual starting point.

2. Insulin resistance

When cells respond poorly to insulin, the body keeps insulin high, and high insulin favours storage over release. It often shows up as weight that settles around the middle, energy crashes after meals, and blood sugar creeping upwards over years before anything is diagnosed.

3. Sleep

Short or broken sleep raises appetite-driving hormones and lowers the ones that signal fullness, which is why a bad month of sleep can feel like a loss of self-control. Untreated sleep apnoea does this night after night, and it is far more common than most people expect.

4. Stress and cortisol

Sustained stress keeps cortisol elevated, and cortisol pushes the body towards storing energy centrally. This is the mechanism behind one of the most common questions we see: whether stress can cause weight gain even when someone is eating well. It can, and it does not require overeating to do it.

5. Medicines you were prescribed for something else

A long list of everyday medicines change appetite, metabolic rate or fluid balance. Some antidepressants, antipsychotics, steroids, certain blood pressure and diabetes medicines, and hormonal contraception can all contribute. None of this means stopping anything. It means the effect is real and should be part of the conversation rather than a private suspicion.

6. PCOS, now also described as PMOS

Polycystic ovary syndrome affects roughly one in ten women of reproductive age and usually involves insulin resistance alongside the hormonal picture. Weight gain with PCOS is driven by that metabolic component, which is why generic advice so often underperforms.

7. Perimenopause and menopause

Falling oestrogen changes where the body stores fat and how it uses energy, and it frequently arrives alongside disrupted sleep and higher stress. Many women describe doing exactly what used to work and watching it stop working, which is an accurate description of what is happening.

8. ADHD and eating patterns

ADHD affects the timing and structure of eating more than the amount: skipped meals, long gaps, and evening eating that follows a day of not eating. The weight pattern is a consequence of the rhythm, not a lack of discipline.

9. Weight that was lost before

After weight loss, resting energy use falls further than body size alone predicts, and appetite signalling stays elevated for a long time. Regain after a successful loss is the expected physiological response, not evidence that the effort was wasted.

How to find out which one applies to you

Most of the list above is distinguishable with a history and a small number of blood tests. We have set out the tests worth asking for, what each one shows and why it matters, on a separate page you can print and take with you.

There is no cost to reading any of it, and nothing here requires an account or an email address.

What this page is not

This is general information, not a diagnosis and not advice about your situation. A list of mechanisms cannot tell you which one you have. That takes a clinician who can look at your history, your medicines and your results together.