Short answer

Yes, an underactive thyroid can cause some weight gain, but usually only a few kilos, and much of it is retained salt and water rather than fat. When the thyroid is treated, people often lose a little of this, but treatment rarely leads to large weight loss. If you have gained a lot of weight, other causes such as diet, sleep, medicines, PCOS or insulin resistance are usually more important and worth discussing.

“Doctor, I think my weight gain is because of my thyroid.” It is one of the most common things people say in an endocrine clinic. Sometimes it is partly true. Often the thyroid is only a small part of the story.

How does the thyroid affect weight?

Thyroid hormone sets your metabolic speed. When the thyroid is underactive, the body burns slightly fewer calories, and it holds on to more salt and water. That is why people with an untreated underactive thyroid often look puffy, especially around the eyes. Large population studies show that even small differences in thyroid function are linked with small differences in body weight.

How much weight can it explain?

Usually only a few kilos. In overt hypothyroidism, where TSH is high and T4 is low, much of the extra weight is fluid. A mildly raised TSH with a normal T4 usually explains very little weight gain. Read about overt hypothyroidism.

Will thyroid treatment help me lose weight?

Treatment usually brings a modest change, mostly as the retained fluid goes. It does not usually lead to large weight loss. Thyroid tablets should never be used as a weight-loss medicine. Too much thyroid hormone strains the heart and thins the bones.

What else could be causing my weight gain?

If your thyroid is normal or already treated, look at other causes with your doctor:

eating patterns, refined carbohydrates, late meals and less activity
poor sleep or sleep apnoea
medicines such as steroids, some antidepressants, insulin and some epilepsy medicines
PCOS in women
insulin resistance and prediabetes, very common in Indians
rarely, cortisol excess (Cushing’s syndrome)

If your thyroid tests are normal but you still feel unwell, read normal thyroid tests but still symptoms. To understand whether your weight puts your health at risk, check the BMI calculator for Indians and read what obesity really is.

When to see an endocrinologist

Rapid weight gain with swelling of the face, legs or belly, or breathlessness
Weight gain with very high blood pressure, easy bruising, purple stretch marks or muscle weakness
Weight gain with irregular periods, excess facial hair or acne

Chest pain, fainting, severe breathlessness or confusion: call 108 or 112, or go to the nearest emergency department.

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This page is general health information and does not replace a consultation with your own doctor. Please read our medical disclaimer.

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