12 week old chick very lethargic
I'm sorry for your loss. Please update your thread when you receive necropsy report. Hopefully, it wasn't anything contagious, such as Mareks or another virus. Necropsies are very helpful to others here to learn about what certain diseases look like. Was your pullet vaccinated for anything? Some viruses such as Mareks and lymphoid leukosis can cause poor immunity to common diseases, weight loss, and just a lack of vigor. Did she have any problems with lameness or wings drooping?