Evidence checked as of October 2026. Written by The Wellness Team.
Can a medical condition cause weight gain?
Yes, although medical causes account for a minority of cases. The established medical causes are an underactive thyroid, Cushing's syndrome, which is rare, polycystic ovary syndrome, perimenopause, fluid retention from heart, kidney or liver problems, and a long list of commonly prescribed medicines. Poor sleep and certain mental health conditions shift weight indirectly by changing appetite and activity.
The practical point is that a short set of tests can exclude or confirm most of these in one round. If they come back normal, you have a clean answer and can work on lifestyle without a nagging doubt. If they do not, you have found something treatable.
How does an underactive thyroid affect weight?
An underactive thyroid slows metabolism, and the typical picture is modest weight gain alongside tiredness, feeling cold, constipation, dry skin, low mood and heavy periods. The weight gain itself is usually a few kilograms rather than dramatic, partly from fluid rather than fat. A simple TSH blood test screens for it, and treatment with levothyroxine, where indicated, is effective and inexpensive.
Testing matters because the symptoms are nonspecific. Most people who feel tired and gain a little weight do not have hypothyroidism, but those who do are easily found.
Which medicines commonly cause weight gain?
The commonest offenders are corticosteroids such as prednisolone, some antidepressants, particularly mirtazapine and some SSRIs, antipsychotics, insulin and some older diabetes tablets, certain epilepsy and migraine preventives, beta blockers and some hormone treatments. The effect varies from a kilogram or two of fluid to substantial fat gain over months.
Do not stop a prescribed medicine because of weight. The right move is a review: sometimes the dose can change, an alternative exists in the same class, or the benefit clearly outweighs the gain. That is a conversation for the prescriber, not a reason to abandon treatment.
What blood tests are useful for unexplained weight gain?
A sensible first round includes TSH for thyroid function, HbA1c for blood sugar, a lipid profile, liver and kidney function, a full blood count and ferritin. Depending on the history, cortisol, reproductive hormones and vitamin D may be added. Testing everything is not the point; testing the things that change the plan is.
Where weight gain comes with specific symptoms, the tests follow the symptoms. New stretch marks, easy bruising and a changing face shape point towards cortisol problems. Irregular periods and acne point towards PCOS. Swollen ankles and breathlessness point towards fluid retention, which is examined rather than guessed at.
When is lifestyle the real answer, and what helps?
Most of the time, weight gain is energy balance: portions, alcohol, ultra-processed food, sitting, sleep debt and stress. That answer is easier to hear once the medical causes are excluded, because then the plan is unambiguous. The measures with the best evidence are unglamorous: protein and fibre at each meal, resistance training two or three times a week, a consistent sleep window, and an honest look at alcohol.
As of October 2026, a telephone consultation with The Online GP is £59 and a targeted blood panel starts from £350, with a doctor reading the results with you. Where a medical driver is found, it is treated or referred; where it is not, medically led weight management is available for those who want structured support.
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Evidence and limits
NICE guideline NG145 covers the testing and management of hypothyroidism: https://www.nice.org.uk/guidance/ng145
This is general information, not a diagnosis or a weight-loss plan. The tests that suit your situation depend on your history and should be agreed in a consultation.