If this is the reason then this is rather more serious. If the sugar level in your blood is too high, then your body system tries to excrete it. This can make you very thirsty and/or hungry. Your doctor can diagnose diabetes with a simple test.Thyroid disorders. If you suddenly develop an increased appetite but are not gaining any weight, then it could be a metabolic disorder such as a thyroid problem.Your doctor will need to be consulted. These are by no means the only reasons, so if you are at all unsure you will need to visit your doctor for advice.f you suddenly develop an increased appetite but are not gaining any weight, then it could be a metabolic disorder such as a thyroid problem. Your doctor will need to be consulted. These are by no means the only reasons, so if you are at all unsure you will need to visit your doctor for advice.