18 years old and losing hair on scalp and eyebrows

There can be many factors for hair loss, some you have already mentioned. I suggest seeing a hair loss expert 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

Based on your description it sounds like you are suffering from a more severe form of Alopecia Areata(Totalis)where the body reacts against its own hair follicles.I would recommend a consultation with a Dermatologist to make the diagnosis and render treatment.

It's hard to say without seeing your scalp, eyebrows and chest hair. Genetic hair loss does not affect the chest and eyebrows.
You either have two conditions or one single condition that explains everything. What could this one single condition be?
Well three main possibilities.
1. Seborrehic dermatitis
2. Alopecia areata
3. Telogen effluvium

Most males with your story however have early genetic hair loss with seborrheic dermatitis (two conditions). It's way more likely than alopecia areata, especially since you have seen one or more dermatologists. But I would advise you to see a dermatologist specializing in hair disorders for confirmation if you are still worried Any of these diagnoses are theoretically possible.