Unexplained weight gain: possible causes and when to see a GP

Weight naturally varies from day to day with fluid, food and hormones, and gradually over years with age, activity and life events. Weight gain is considered unexplained when it is steady, rapid or out of proportion to your eating and activity, or when it comes with other changes such as tiredness, swelling, mood change or menstrual change.

It helps to separate two types. True gain of body fat usually builds over weeks to months. Fluid retention can add kilograms in days and is often noticed in the ankles, legs or abdomen. They have very different causes and urgency, so it is worth noting the speed of change as well as the amount.

An underactive thyroid (hypothyroidism) is one of the commonest. The NHS lists tiredness, feeling cold, putting on weight, constipation, difficulty concentrating, low mood, dry skin, dry hair or hair loss, a hoarse voice and irregular or heavy periods, and says it usually develops slowly. A GP can diagnose it with a blood test.

Polyendocrine metabolic ovarian syndrome (PMOS), which the NHS says used to be called polycystic ovary syndrome (PCOS), can cause weight gain or difficulty losing weight alongside irregular periods, excessive hair growth or hair loss, difficulty getting pregnant, mental health problems, oily skin and acne, and dark patches of skin on the neck or armpits, according to the NHS.

Cushing's syndrome is rare but important. The NHS says it is caused by too much cortisol in the body, most commonly from taking high doses of steroid medicine over a long time. Symptoms include more body fat on the neck, upper back, chest and tummy while the arms and legs look thin, a red round face, easy bruising, stretch marks, muscle weakness, low sex drive, mood change, irregular periods and excess hair growth.

Menopause and perimenopause often bring weight gain around the stomach and upper body, according to the NHS, alongside other changes in mood, sleep and periods.

Many medicines can increase weight. Steroid tablets are the best known. Others, including some antidepressants, antipsychotics, antiepileptics, insulin and some diabetes and blood pressure medicines, can affect appetite or metabolism. Do not stop a prescribed medicine without advice, because stopping can be harmful. Instead, ask your GP or pharmacist whether it could be contributing and whether an alternative is suitable.

Other contributors include poor sleep, sleep apnoea, low mood or depression, stopping smoking, alcohol, reduced activity after illness or injury, and eating patterns that have changed without being noticed. The NHS includes sleep apnoea and low mood among the day-to-day problems linked with excess weight, so the relationship can run both ways.

Weight gain over days, with swollen ankles or legs, breathlessness or a bloated tummy, may be fluid. The NHS says that symptoms of heart failure include feeling out of breath when doing everyday activities or lying down, feeling weak or very tired and being more out of breath than expected. It advises asking for an urgent GP appointment or contacting NHS 111 if you feel breathless lying down or from everyday activity, cough up frothy pink phlegm or have suddenly gained weight, and calling 999 for severe difficulty breathing or pale, blue or grey lips or skin.

Kidney and liver disease can also cause fluid build-up, and an underactive thyroid can contribute. Do not treat swelling with over-the-counter water tablets without medical advice.

A GP will ask how quickly weight has changed, about diet, alcohol, activity, sleep, mood, periods or menopause symptoms, and all medicines including those bought over the counter. They will usually measure weight, height, waist and blood pressure. The NHS says that the BMI calculator, with a BMI of 25 to 29.9 classed as overweight and 30 or more as obese for most adults, with lower thresholds for some ethnic backgrounds, and waist size less than half your height are the usual yardsticks.

Tests may include thyroid function, blood glucose or HbA1c, kidney and liver tests, a full blood count and sometimes hormone tests. If Cushing's syndrome is suspected, the NHS says a GP can arrange for the cortisol level in a blood, urine or saliva sample to be checked, and a hormone specialist may do further tests.

Keeping a record of your weight once a week, measured in the same way, makes patterns visible. Note associated symptoms in a diary. For gain linked to lifestyle and menopause, the NHS Better Health service offers free tools and support to lose weight and be active, and the NHS recommends losing weight if you are overweight for people with PMOS.

If a medical cause is found, treating it, for example with thyroid hormone replacement, often helps but does not always return weight to what it was, so combined lifestyle support is usually needed.

Ask for an urgent GP appointment or contact NHS 111 if weight gain is sudden with breathlessness lying down, swelling of the legs or tummy, or frothy pink phlegm, as the NHS advises for possible heart failure. Call 999 for severe difficulty breathing, pale, blue or grey lips or skin, chest pain or collapse.

See a GP in the ordinary way if weight gain is persistent and associated with fatigue, feeling cold, a hoarse voice, stretch marks and easy bruising, irregular or absent periods, or low mood.

On the NHS, the GP can arrange the first blood tests and refer to endocrinology, gynaecology, cardiology or weight management services depending on what is found. NHS weight management and Better Health resources are free.

Private health checks and blood panels are available, but a single set of results needs interpretation alongside symptoms. If a private test finds an abnormality, share it with your GP so that it can be repeated and acted on within one pathway.