HI, thanks for using healthcare magic

It is not unusual for adolescents to experience irregular cycles, in fact they can be irregular through adolescence.
In a study done by the world health organisation looking at over 3000 girls, the average length of a cycle is 34 days with 38% of girls having cycle lengths over 40 days.
In that same study 10% of the adolescent girls have cycles longer than 60 days.

If she's had repeated tests that have shown no abnormality then you can consider watching and waiting to see if they regularise or restart the birth control pills.

I hope this helps

Hypothyroidism is a known cause of amennorhea, however, you reported that her levels are well controlled. She may need to have her prolactin levels checked as well, as this hormone can cause the same problem and is often found in women with a history like your daughter's. It could be that your doctor has already checked this. It would also be a good idea to check for pregnancy, as she has stopped the birth control and then didn't have a period yet, this could be a cause as well. If both of those issues are not the cause, there may be other problems. PCOS can sometimes be the cause even with normal labs, or obesity itself can cause the problem even without PCOS. I do not know your daughter's weight, but if obesity is present than that alone could cause it too. On the other hand extremely low body mass can be a cause as well, such as if she in annorexic or severely underweight. There are other even more rare possibilities, and you should talk to your doctor about how to test for them. I hope this was helpful to you. Please let me know if you have any more questions.