Doan has been diagnosed with the eating disorder anorexia-athletica-nervosa, which she developed during the recording of KITTIE's "Funeral For Yesterday", Doan's first release with the group.
"It breaks my heart to say goodbye to this family that has given me the best two years of my life," Doan said. "I wish my time with KITTIE did not have to end, but this decision was made based on doctors' orders. Unfortunately, this is the only option that will allow for my full recovery."
Doan has been battling the disease for nearly two years.
"Trish is a part of our family and her talents and friendship will be missed by all of us," said Tara McLeod and sisters Mercedes and Morgan Lander. "We are continuing to be respectful and supportive of Trish's needs during her period of healing. We wish her the best on her road to recovery and will remain close to her throughout this difficult time."