Abnormal body weight, either over or under is mostly controlled by a balance of what you eat and the metabolic rate or the body's metabolism (Speed) When your metabolism is fast, you burn off the fat, thus stay slim.
If your metabolism is slow, you have a problem burning off fat and become overweight. Your Thyroid has much to do with the metabolism, in your case, your thyroid may be secreting too much hormone causing your body's metabolism to burn off the food you eat.This is called Hyperactive Thyroid, Have you had your Thyroid tested (Blood Work) If not, then request the following blood tests, Free T3 and Free T4, they will more than likely try to refuse these two tests if you are in the UK and do the TSH alone (Thyroid Stimulating Hormone) If you are Hyperactive, or have an overactive thyroid, this test result will be very low, indicating you need medication to supress the thyroid production. If you have already been tested please post the results here and include the reference ranges provided with the value on the results. Good Luck FTB4
Do you have a thyroid condition? If so, please post recent labs so we can see how well it's being managed. Do you have other symptoms as well?