I have the same problem. It´s intriging. I´ve done lots of research and nothing, except the male pattern baldness and some extrange hair shaft diseases. The fact is, if it is not male pattern baldness, the hair will come back to his originary status just with time or healthy life, in every case I found, but I´ve had this thin hair on sides and top (and yes, even in sideburns) for five years. Now I started some treatments (see below) and I´m still waiting. I´ve got hope becouse I´ve had a heavy shed (even in sides) since the beggining (now recovering) and I think it´s a possitive sign.
Just for making a profile, for the past years I have weird sleep pattern, late in the night (3 a.m.) and waking about 12 p.m., i had anxiety/stress, moderate (weekends) use of drugs (anphetamine, pot, alcohol), I do sports (basketball) regularly and a healthy diet, I was wearing very long hair (using pony-tail) when noticed thining for first time, and I´ve got a healthy looking hair w/ no recession, just poor density.
Any ideas on us, global thinners?
P.S.: Thyroid disease is discarded on me.