Why am I so skinny (male)? Hair is thinning, is this a side effect of being underweight?

This could be a factor in your hair thinning, but an in-person evaluation is needed. I suggest seeing a hair loss expert now to evaluate and go over your condition and provide you information about your best treatment options. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

In my opinion you need to see a hair loss expert. They will do a good history, physical examination, blood work, Wood's light exam and possibly biopsy. Diet and protein intake are important for maintaining quality hair. A clinical dietitian can be consulted regarding your health. In addition there are medications that may cause hair to thin. Hair loss is common with eating disorders and gastrointestinal diseases.PRPHAIRMD.COM 

Hair does need fat to grow and poor nutrition and thyroid disease can affect your hair growth.  You really need to be sure that you have no endocrine (hormonal) or nutritional imbalances that would need correcting and for this the first step would be with your primary care doctor.  That being said, the prevalence of male genetic hair loss is so high that it is still the most likely condition you have.  If so, seek treatment early.  There are a ton of things you can do to prevent further hair loss, and prevention is always easier than restoration. 

At  your age thinning hair may be related to nutrition or other underlying medical condition. As you stated it seems you are underweight and have a hard time gaining weight but would see an internal medicine physician to be sure there is no underlying medical problem causing you to be thin and leading to thinning hair.  If there is no underlying condition, I would ask family members if there is anyone in the family that had thinning hair at such a young age.  You should then consider a consultation with a board certified facial plastic surgeon experienced in hair metabolism who can suggest nutritional supplements and other remedies that can help in restoring your hair health.

Great question and you'll want to see a physician to carefully review all your details. Your diet, body mass index and general health certainly could be linked to your hair loss. However, it may not be. Only a detailed review can arrive at the proper assessment.

Your need to speak with a doctor, get a good history taken, examined and tested with various blood tests to arrive at a reason for you being skinny.