I don't think that any given medication guarantees permanent stability, at least not at any given dose all of the time. Breakthrough episodes can happen. Period. Occasional medication tweaks are just the name of the game for most of us. I'm not saying that Lithium is the best medication for you, at least not at your current dose. However, clearly that dose couldn't withstand the strength of your recent episode. Something does need tweaking and/or some other change needs to be made. The latter could be life situation, improvement in use of coping skill use, etc.

Hypothyroidism is a common side effect from Lithium, especially in people with family dispositions, like mine. I developed it from Lithium and still have it years after going off that med. I take Synthroid (levothyroxine) and it controls it perfectly. There are far worse side effects. Far worse!