Lithium's working dose is very close to a toxic dose level. Even carelessness in not being well hydrated can tip you over into the toxic range, so I urge you to never neglect remaining well hydrated.
I urge you to seek professional advice (from your doctor) for answers to your questions.
I too take Lithium and I have Hypothyroidism. It took a while for my dr to try different dosaging in order for all of my Thyroid levels to be in range. My levels are always checked every 3-4 months. I am on Armour Thyroid, 30mg Mon and Wed and 60mg the other days.
Just to clarify. When I stated trying different dosaging that was on the Thyroid med.