Hi bizi. I responded to your post about this in another thread. I just thought I'd share here my last TSH result while on Synthroid (levothyroxine), if of interest. My last result was 1.74 in the normal range of 0.40-4.50. On Synthroid, my TSH has been in the normal range for years and I have not had any symptoms of hypothyroidism.

I have read that Tegretol (carbamazpine), which I also take, "can decrease the effects of levothyroxine". See Synthroid and Tegretol Drug Interactions - Drugs.com This is not a big deal, but it is possible that if I didn't take Tegretol that my levothyroxine dose would need to be lowered. I'm not saying Tegretol affects my thyroid (no), just possibly my dose of levothyroxine. I don't know if it would be the case that if I stopped Tegretol (which I won't) that I could develop hyperthyroidism at my levothyroxine dose of 150 mcg. Possibly? That's a reason why it's always important to let doctors manage medication changes. Not to say you would ever tinker with your meds on your own.

I mention the above only because you mentioned an interest in Tegretol in a different thread.

As an aside, both my maternal grandmother and my sister took/take levothyroxine and to my knowledge, neither ever took Lithium. I think there is a genetic element, in some cases.