Most fruit and vegetables are rich in fibre, and so are legumes such as beans and peas. An over-active thyroid can cause rapid weight loss, despite causing constant feelings of hunger. Hyperthyroidism results in the production of excessive amounts of thyroxine, which can push your body's metabolism past its limits. Sometimes, unusual weight-loss can occur even when you aren’t on a diet. In cases of prolonged weight-loss where more than 10% of your body weight is lost over a period of 6 months, there is often a more serious underlying medical issue that requires attention.