All of us have a predisposition from the time we're born to be either skinny or full-figured, and your family health history can help explain a lot about it. Many medical conditions are hereditary, and those same conditions can and do affect weight loss and weight gain--everything from diabetes to our metabolism to the way our bodies deal with stress.
You may look at your mom or aunt and think, "She sure looks tired and stressed all the time." It may not just be a quirk of that person's, but how her body physically reacts to outside, stressful stimuli. When stressed, our bodies get tense and our moods alter, and our metabolisms can change. They can slow down or speed up, to where we can eat a gallon of ice cream and not gain a pound, or take one look at that ice cream and gain ten. Like your parents or even grandparents before you, you may be the sort to start shedding pounds when worked up and stressed out.
While you aren't 100% going to develop diabetes just because most of your family has, it does mean you're predisposed to it and should take precautions to avoid it becoming a problem. Diabetes comes in more than one form, so weight loss could be either an intention or unintentional side-effect of it. Many overweight people will drop pounds intentionally in order to get their diabetes in check and more manageable, and this is a good thing.
It's not good, though, when you start shedding pounds too quickly or without even trying. Frequent urination can lead to dehydration, which in turn leads to weight loss.
An overactive thyroid (hyperthyroidism) gland can result in weight loss, just as an underactive thyroid is a cause of weight gain. The thyroid gland regulates a body's metabolism. When the thyroid is overactive, it causes the body to burn calories like crazy, more than it should, and thus causing a person's weight to drop. Graves' Disease – which causes hyperthyroidism – is very hereditary. If you know of anyone in your family who has it, contact your doctor to see about having your thyroid checked, too.
Depression can be caused by a number of different things, including chemical imbalances within the body. Like stress mentioned above, you're more predisposed to depression and other mental illnesses if your family history has a running line of it. Depression is another thing that can swing either way with either extreme weight loss or extreme weight gain. Some people indulge in junky comfort food and put on extra pounds, while others may stop eating all together and begin dropping the pounds. Either way, depression is not healthy, and treatment should be sought after to find the root cause of your depression so it can be treated.
Talk to Your Doctor
There's a long list of things that can cause unintentional weight loss, both hereditary and non-hereditary. If you notice it becoming a problem, start asking around your family. What medical conditions do some of them have that they are aware of? Is it something that you could potentially be suffering from, too? Either way, rapid weight drop (or weight drop at all if you aren't trying to lose pounds) can be dangerous to your body, so seek advice from your doctor to find the cause.