Hey. So these two conditions can be seen to occur together. But you must realise that weight loss and hair loss are two separate entities with a slight overlap. Weight loss will not directly cause hair loss.
There are certain conditions that will result in both weight loss and hair loss. They may coexist simultaneously but both of them do not have a causal relationship. Basically, what it means is that there is something happening that causes both, for you to lose weight and lose hair.
This means that you don't eat adequately. It is important that you realise adequately not only means the amount of food you eat but it also means what kind of food you eat. You need to consume adequate amounts of protein and vitamins and minerals to ensure your hair is healthy. So if your diet is deficit in required nutrients, it will result in you losing hair and losing weight.
Certain drugs like say anti-cancer drugs basically are designed to kill cancer cells. But because cancer cells are by nature our own cells (just growing abnormally), these drugs may affect normal cells. Thus hair loss is a significant effect seen with use of such drugs. Your hair basically has 2 phases. The growing phase and resting phase. When you lose weight suddenly after a surgery say, then your hair will shift into the resting phase. And in this phase a significant amount of hair fall can occur. But this is reversible, and will resolve in a few month and the hair will begin to regrow
This is perhaps the most important factor. When you are stressed, it causes increased hair fall. You may notice the amount of hair fall increases at times of exams or job interview or when there is emotional stress caused due to the separation or demise of a family member. Not only that, but stress will pull your mind off eating. It makes you feel less hungry and will not eat sufficiently. This will thus cause both hair loss and weight loss.
• Psychiatric conditions
Depression contributes significantly to weight loss and hair loss. There are other psychiatric conditions that cause both but this is the most important.
Lastly, you may be predisposed to hair loss and the current weight loss may be an addition to it due to causes already mentioned and you have found it to coexist with your hair loss.
• Genetic: Male pattern and female pattern baldness is influenced by your family. If your family members have it, you may be losing hair because of it.
• Hair products and styling: Certain gels or chemicals you might be using actually damage your hair and cause it to fall. Hairstyles that pull your hair tightly increase the risk of hair loss.
• Injury to your scalp, or when you hurt your hair follicles with hot oil, it will permanently damage your hair follicle and will cause increased loss.
In Summary, no, weight loss doesn't directly cause hair loss, but both may co-exist due to a common causal condition.