Rapid weight gain: causes, red flags and when to see a GP

Body fat cannot accumulate very quickly, so a gain of several kilograms in days is almost always fluid rather than fat. The NHS heart failure page puts sudden weight gain alongside swelling in the feet, ankles, legs or tummy, feeling bloated, breathlessness lying down and night-time coughing as symptoms to take seriously. The NHS chronic kidney disease page lists swollen ankles, feet or hands, breathlessness and tiredness among symptoms at a more advanced stage.

Gradual weight gain, over weeks to years, reflects a sustained excess of energy intake over use, shaped by genes, medical conditions, medicines and lifestyle. NICE CKS describes obesity as resulting from an imbalance between energy intake and energy expenditure, with many factors, including lifestyle, genetics, medical conditions and medication, able to influence it.

The table sets out causes named in NHS and NICE sources, grouped by whether the gain tends to be fast or slow.

The NHS heart failure page says to ask for an urgent GP appointment or call 111 if you feel breathless when lying down or during everyday activity, are coughing up frothy pink phlegm or have suddenly gained weight. It says to call 999 for severe difficulty breathing where you are gasping or cannot speak, pale, blue or grey lips or skin, or someone who has passed out and is not responding normally.

Fluid-related weight gain with other warning features, such as swelling that is increasing, breathlessness or chest pain, needs prompt assessment. A swollen, painful, red leg may be a blood clot, in which case sudden breathlessness or chest pain should be treated as an emergency, as the NHS pulmonary embolism page explains. NICE CKS says emergency admission is arranged if a serious complication of hypothyroidism such as myxoedema coma is suspected.

• Weight gain over days with swollen ankles, legs or tummy.

• Breathlessness lying flat or during ordinary activity.

• Coughing up frothy pink phlegm.

• Blood in the urine, or tiredness with swollen ankles, feet or hands, which the NHS lists among later symptoms of kidney disease.

• Gain with a round red face, easy bruising and muscle weakness, particularly with steroid medicines.

• Gain with extreme tiredness, feeling cold, constipation and dry skin, which the NHS lists as underactive thyroid symptoms.

The GP asks how fast the weight has changed and over what period, what you eat and drink, how active you are, sleep, mood, alcohol, smoking changes, medicines (including steroids, antidepressants, diabetes medicines and contraceptives), menstrual and menopausal history, symptoms of thyroid disease, and whether you have swelling or breathlessness. NICE CKS for hypothyroidism lists asking about symptoms, non-thyroidal illness, medicines and risk factors such as family history or autoimmune disease.

Examination includes weight, height, BMI and waist, blood pressure, pulse, signs of fluid in the ankles or abdomen, the thyroid gland in the neck, and skin and body-fat distribution. The NHS says the BMI calculator cannot tell the difference between muscle and fat, and suggests the waist-to-height ratio for people with a BMI under 35.

Blood tests commonly include thyroid function (TSH and T4), kidney and liver function, salts, blood sugar or HbA1c and cholesterol. The NHS says a GP can arrange cortisol tests in blood, pee or saliva if Cushing's syndrome is suspected, and that PMOS is investigated with hormone and insulin resistance blood tests and a pelvic ultrasound in adults. For possible heart failure, the NHS describes an ECG, blood tests and an echocardiogram, and for kidney disease blood and urine tests.

Treatment is for the cause. An underactive thyroid is treated with levothyroxine, usually for life, with regular blood tests to check the dose, as the NHS says. Cushing's syndrome caused by steroid medicine is managed, as the NHS describes, by gradually reducing the steroid dose under medical supervision. PMOS treatment can include hormonal contraception, medicines to reduce hair growth and lifestyle changes. Fluid retention from heart or kidney disease is treated by the specialist team.

For weight gain not due to an identified medical cause, the NHS recommends steady weight loss of 0.5 to 1kg a week through fewer calories and more activity, 45 to 60 minutes a day of activity, cutting alcohol, and says a GP can refer for weight management programmes, medicines or surgery. NICE CKS says multicomponent programmes with behaviour change are the treatment of choice, and that drug treatment may be considered as an adjunct for those who meet BMI criteria. These are prescription decisions made by a clinician after assessment.

A GP for unexplained or rapid weight gain; urgent GP or NHS 111 for rapid gain with swelling or breathlessness; 999 for severe breathing difficulty or collapse. A pharmacist can review medicines. Endocrinology, cardiology, gynaecology or a weight management service may be involved depending on the cause. The NHS says you can self-refer to some weight management programmes.