The Real Magnitude
An underactive thyroid can cause weight gain — but the American Thyroid Association notes it is usually modest, commonly about 5–10 lb, and much of it is fluid and slowed clearance, not fat. Large, unexplained weight gain is rarely due to the thyroid alone.
The Critical Caveat
Once hypothyroidism is treated to a normal TSH (euthyroid), metabolic rate returns to normal. Any residual weight issue is then driven by the same factors as everyone else — calorie balance, activity, and other conditions like PCOS or menopause.
Use the Hypothyroidism Calculator — it applies a conservative adjustment only if you are currently untreated or not yet euthyroid. See hypothyroidism metabolism and the Hypothyroidism hub.
Educational only. Weight changes despite normal TSH on treatment usually have non-thyroid causes — discuss with your clinician.