First I have a few things to ask.... Do you have any oral habits like smoking of chewing pan masala...? Any gastric issue like frequent heartburns due to acidity..? Or does you

First systemic cause

Bad breath is usually due to poor oral hygiene. So visit a dentist soon to get your teeth cleaned. Also keep a check that you don’t sleep with open mouth as it might add to dryness in the mouth

generally dry mouth is not becoz of thyroid.can b becoz of less intake of water. yellow deposits in teeth are called calculus which needs to be removed with professional cleaning. check your acidity also,as gastic biles or acids which regurgitate in the mouth causing bad smell,make sure ur bowel movements r also fine as intestines have a direct relation with the gums. overall body health including factors like acidity,food intake,water intake n condition of teeth n gums affect the oral cavity n can lead to bad breadth

Hi Bad breath is because of excessive deposits of tartar (dirt ).Go ahead with general cleaning and polishing . Poor oral hygiene will affect your general health too. Take chlorhexidine mouth rinse daily once.  Apply sensoform gum paint .Take Big amla friut daily to avoid bad breath.

Bad breath can commonly a symptom poor oral hygiene which is not addresses ever after 2-3 times of brushing. Seek a dentist help to rule out the common causes first. Secondly, stop making a call that it might be because of thyroid or some thing else of your own.