Dry tongue in first half. Be the cause?

After I get up in the morning, I feel my tongue snd throat are too dry. I don't feel hydrated even after drinking 2 to 3 glasses of water. I drink around 2 to 3 litre water. This had been happening since almost 6 months. Since I am underweight and have constipation, I am taking a protein rich and fibre rich diet but in moderation. Can this be the cause? I also have thyroid. I am originally hypothyroid from 30 years but 3 months back showed hyperthyroid. I am now on a reduced dose. Can it be that the current dose is still overdose and it may be linked to my thirst in someway. I also get some blood filled phlegm. In mornings, I have to spit out phlegm sometimes. Please advise the cause of this symptoms and whom should I consult.