Place chicken thighs at the bottom of the slow cooker.
Add chicken broth, coconut milk, shiitake mushrooms, tomatoes, lemongrass, galangal, and kaffir lime leaves.
Cover and cook on low for 6 hours.
Remove the chicken, shred it, and return to the pot.
Stir in fish sauce, lime juice, and sugar.
Adjust seasoning with more fish sauce or lime juice if needed.
Serve hot garnished with fresh cilantro and red chilies.