Thyroid and Weight Gain

Yes, an underactive thyroid can cause weight gain, and the amount is usually modest, with much of it fluid. The British Thyroid Foundation, reproducing American Thyroid Association guidance, says most of the extra weight is salt and water, that 5 to 10 pounds (roughly 2 to 4.5kg) may be due to the thyroid depending on how underactive it is, and that very large weight gain is rarely caused by it.

The same guidance gives a useful rule of thumb: if weight gain is your only symptom, the thyroid is less likely to be the whole explanation. An overactive thyroid tends to cause weight loss instead, and that weight usually returns once it is treated.

Common symptoms of an underactive thyroid include tiredness, feeling cold and putting on weight, and they usually come on slowly. The NHS lists these:

• feeling extremely tired
• feeling cold more than usual
• putting on weight
• constipation
• difficulty concentrating or thinking clearly
• low mood or depression
• dry skin, dry hair or hair loss
• a croaky voice
• irregular or heavy periods

An underactive thyroid is more likely with:

• being a woman, since thyroid disorders are ten times more common in women than in men
• getting older
• a family history of thyroid or autoimmune conditions
• past surgery or radiotherapy around the neck or thyroid
• past treatment for an overactive thyroid
• having recently had a baby

In the UK the main cause is Hashimoto’s disease, an autoimmune condition that usually starts between the ages of 30 and 50. Many of these symptoms overlap with other conditions, including the menopause.

A thyroid blood test, also called a thyroid function test, measures thyroid stimulating hormone (TSH) and usually free T4. TSH comes from the pituitary gland in the brain and tells the thyroid how hard to work, and T4 is the main hormone the thyroid releases. When thyroid hormone runs low, TSH rises to push the gland harder.

The British Thyroid Foundation explains that TSH is usually measured first, because a normal TSH means the thyroid is very likely working normally, except where a pituitary problem is suspected. Many laboratories then add free T4 automatically if the TSH is outside the range. The test is a single blood sample from a vein in your arm, and it is the same test the NHS uses to diagnose an underactive thyroid.

A high TSH with a low free T4 points to an underactive thyroid, and a low TSH with a high free T4 points to an overactive one. The British Thyroid Foundation gives typical adult reference ranges of 0.4 to 4.0 mU/l for TSH and 9 to 25 pmol/l for free T4, though each laboratory sets its own.

A slightly raised TSH with a normal free T4 is called subclinical hypothyroidism, or mild thyroid failure. It may develop into an underactive thyroid over years, a thyroid antibody test helps show that risk, and the foundation advises regular tests. Illness, some medicines and supplements containing biotin or iodine can all affect results, so tell whoever takes your blood what you take.

A private thyroid blood test costs about £49 to £99 at London clinics and labs. On 30 September 2026, OneDayTests listed a thyroid test at £49 and London Doctors Clinic listed its thyroid profile at £99. Some labs charge separately for taking the blood, and a panel that adds free T3 or thyroid antibodies costs more.

Check which markers a price includes, since a TSH-only test and a panel with free T4 answer different questions. If you have symptoms, your GP can arrange the test on the NHS, and results from the last few months can come with you to a private assessment so the test is not repeated.

Once your thyroid levels are back in the normal range, you lose weight the same way anyone else does. The British Thyroid Foundation, drawing on American Thyroid Association guidance, says that after treatment the ability to gain or lose weight is the same as in people without thyroid problems, and that there is no one-size-fits-all thyroid diet or exercise plan. These are the parts of the plan that matter:

• a steady energy deficit, at about 0.5 to 1kg a week
• strength exercises on at least 2 days a week, as the NHS advises for all adults
• enough protein at each meal
• a regular sleep routine, since tiredness makes every other step harder

If you still feel tired or unwell with normal results, tell your GP, who can review your treatment. Treatment for an underactive thyroid restores normal levels, and the weight plan works on top of it.

Expect a small weight loss once an underactive thyroid is treated, and sometimes none at all. Because much of the extra weight is salt and water, the guidance the British Thyroid Foundation publishes says treatment usually brings only a small loss. It also notes that because an underactive thyroid develops slowly, it is fairly common to see no significant weight loss after successful treatment.

Symptoms can take several months to improve after treatment starts, so give it time before judging. If your levels are normal and the weight has stayed, the rest of it has other causes, and those respond to the same plan as anyone else’s. A medical weight loss programme starts from that point.

A normal TSH makes the thyroid an unlikely cause of your weight gain, so the next step is to check the other causes. The common ones a clinician looks at are:

• raised blood sugar or insulin resistance, found with an HbA1c test
• perimenopause or menopause
• some prescription medicines
• poor sleep or obstructive sleep apnoea
• changes in eating, drinking or activity that built up over months

Rarer conditions, such as Cushing’s syndrome, are tested for by a GP when the pattern fits. The unexplained weight gain page covers the full list.

See your GP if you think you might have an underactive thyroid, since the symptoms are common and can have other causes. The British Thyroid Foundation also suggests asking for a test if you notice swelling or thickening in your neck, an irregular or fast heartbeat, or you have felt unwell since having a baby. If you have a thyroid condition and are pregnant or planning a pregnancy, tell your GP, and if you have a diagnosed thyroid condition, have a blood test at least once a year. If eating or your weight is causing you distress, speak to your GP or contact Beat on 0808 801 0677.

A first assessment arranged by Medical Weight Loss London at UNTIL Soho takes a full history of your symptoms, measures your weight, waist and blood pressure, and arranges thyroid function alongside HbA1c, a lipid profile, liver and kidney function and a full blood count, so a normal thyroid result leads straight to the next question. If your GP has tested your thyroid recently, bring the result. First assessments at London clinics cost about £150 to £310, with tests on top (what a programme costs).

Ask what to get checked and UNTIL Health Concierge arranges the appointment with a registered clinician. If your thyroid needs treatment, that stays with your GP.