I've developed low thyroid levels due to lithium, what should i do now?

Treat if low: Need to replace your deficiency as lithium is known to cause hypothyroidism (and simetimes hyperthyroidism). Although lithium-induced hypothyroidism is usually reversible upon cessation of lithium, the development of hypothyroidism is not a contraindication to continuing lithium, and most experts advocate thyroid augmentation.

See below: If the low thyroid is causing you symptoms then replacement is necessary or another medication other than lithium needs to be prescribed that doesn't have this effect.