I m on lithium, 900mg as well as other medications. i have been feeling tired and very fatigue. how fast can your thiroid become low?

Lithium & thyroid: Clinical hypothyroidism may occur within weeks of beginning lithium, but may also not show up for months to years if at all. Goiter happens in 40-50% of patients on long-term lithium. Your doctor will be checking your thyroid functions every 6-12 months, as well as other labs. Please know that depression can also cause fatigue, so fatigue alone is not diagnostic of hypothyroidism.

Check your thyroid: Typically this is something that happens over time 6-12 months or longer, but not everyone is the same so getting a simple thyroid test can let you know if you are hypothyroid.