Dog has Low thyroid – serious answers only please?

My father’s dog has a mild Low Thyroid condition. He has not put her on medication, as per the vets request. What is Low Thyroid and how dangerous is it?
My father is not giving her the meds because he says that she won’t take them – she spits them out and he don’t want to force her. He’s tried hiding them in food, but that don’t work either. If it were my dog, one way or the other, I’d get ’em down!
She is a Shepherd/Husky X and is 12 years old.
SORRY TO ALL! My mistake…I should have been more clear…the vet DID recommend meds, but my dad isn’t giving them to her.