Usually Shih Tzu is a very active dog breed...As you said it might be an abandoned dog... having lots of hair indicates there might have been a tick or mite infestation...make sure to deworm the dog...use a medicated shampoo like Clinar M for 3-4 weeks..in order to clear up mite infestation...after bath take a spot on solution for imidocarb...and keep drops of it along neck to tail like drops... Make sure dog get better management and nourishment...
Next Steps
Deworm the dog with a deworming tablet.
bath the dog with Clinar M 2times a week for 3_4 weeks
add spoton imidocarb after total drying of hair
Health Tips
It is strictly important to make sure the shampoo doesn't go into eyes nose and mouth...it is a very great shampoo but if it licks it .it will be poisonous
Imidospot also make sure it becomes total dry before it licks it's fur