Do hairs get thinner in the telogen phase?

This needs an in-person evaluation. 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

Yes it is normal phenomena in telogen efflivium . Because hair get in to resting and shedding phase  in TE that means they get thinner and duller than normal growing anagen phase hair .

Hairs do not get thinning in Telogen Effluvium but rather they reduce their numbers as the hairs are put into their resting stage and then, over a new hair cycle, these hairs regrow