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Most people put on weight because they eat and drink more calories than they burn through everyday movement and body functions.

But, in some cases your weight gain may be due to an underlying health condition. This page covers 9 medical issues that can cause weight gain.

You can also read more about other hidden causes of weight gain.

Underactive thyroid

An underactive thyroid (hypothyroidism) means your thyroid gland is not producing enough thyroid hormones. This slows the body’s metabolism and can lead to weight gain. Although an underactive thyroid can occur at any age and in either sex, it is most common in older women.

The condition is usually treated with daily hormone replacement tablets, called levothyroxine.

What are the symptoms of an underactive thyroid?

Diabetes treatment

Weight gain is a common side effect for people who take insulin to manage their diabetes. Insulin helps to control your blood sugar level. Some people with longstanding diabetes may eat more than they need to prevent low blood sugar, also known as a "hypo" or hypoglycaemia.

Snacking regularly can mean you take in more calories than you need. This can lead to overall weight gain.

Attending a diabetes education course can help you manage your diabetes in a healthy way.

These courses are:

  • DESMOND for people with type 2 diabetes
  • DAFNE for type 1 diabetes

Find out more about Living with diabetes.

Ageing

People begin to lose some muscle as they get older, largely because they become less active. Muscles are an efficient calorie burner, so a loss of muscle mass can mean you burn fewer calories.

If you're eating and drinking the same amount as you always have and are less physically active, this can lead to weight gain. To reduce muscle loss, you should stay active and try to do regular muscle-strengthening exercises.

Find out more about exercising as you get older.

Steroid treatment

Steroids, also known as corticosteroids, are used to treat a variety of conditions, including asthma and arthritis. Long-term use of corticosteroid tablets can increase appetite in some people, leading to weight gain.

Being extra careful about what you eat while you are using steroids will help prevent you from eating more than usual. If you're worried about weight gain, talk to a GP about help to control your weight.

You should not reduce or stop taking steroid treatment without talking to a GP first.

Cushing's syndrome

Cushing's syndrome is very rare and is caused by high levels of the hormone cortisol. It can develop as a side effect of long-term steroid treatment or as a result of a tumour.

Weight gain is a common symptom, particularly on the chest, face and stomach. Depending on the cause, treatment typically involves either reducing or withdrawing the use of steroids, or surgery to remove the tumour.

Stress and low mood

People respond differently to stress, anxiety and depressed mood.

Some people may lose weight, while others may gain weight. Some people turn to food as a coping mechanism and it can lead to a vicious circle. Weight gain from depression can make you more depressed, which can lead to further weight gain.

If you know you're an emotional eater, finding other forms of distraction can help, such as:

  • exercise
  • a hobby
  • calling a friend
  • going for a walk
  • having a soothing bath

You may want to try these 5 steps to improve your mental wellbeing.

Tiredness

Some studies have shown that people who sleep less are more likely to be overweight than those who sleep more. It's not clear why, but one theory is that sleep-deprived people have reduced levels of leptin, the chemical that makes you feel full, and higher levels of ghrelin, the hunger-stimulating hormone.

Read more about how to get to sleep.

Fluid retention

Fluid retention can cause you to gain weight as parts of the body become swollen with fluid. Some types of fluid retention are common, for example:

The swelling can occur in one particular part of the body, such as the ankles, or it can be more general.

More severe fluid retention can also cause breathlessness. Call 999 if you are short of breath or struggling to breathe.

Read more about swollen ankles, feet and legs.

Polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a common condition that affects how the ovaries work. Symptoms can include irregular periods, trouble getting pregnant, growing excess hair and weight gain. The exact cause of PCOS is unknown, but it's thought to be hormone related, including too much insulin and testosterone.

People with PCOS typically put on weight around their waist.

Page last reviewed: 25 February 2020
Next review due: 25 February 2023