Carbimazole is very effective at stopping your thyroid from making any thyroid hormone. When you start taking it, your manufacture of thyroid hormone drops almost immediately.
However, anyone who is seriously hyperthyroid will have lots of stored thyroid hormone - in their thyroid, in their blood (largely bound to proteins), etc. Once your thyroid is not making more thyroid hormone, what is already there starts to get used up.
Trouble is, once your thyroid hormone levels drop towards a more healthy level, there is nothing to stop you going flying straight past the good point, and heading right down into being hypothyroid.
We see this time after time. Typically two to four weeks after starting carbimazole but it can vary. At this point you (or your doctor) can choose two different routes. First, reduce your carbimazole and try to balance the dose so that you make just enough thyroid hormone. This can take some time - and you would typically end up on something like 5, 10 or 15 milligrams a day. Or keep your carbimazole high and take levothyroxine as your source of thyroid hormone.
What should never happen is that you are left to find this out for yourself on a patient forum. It should have been properly explained to you. And when it happens you should have an immediate appointment to review and decide what to do. It is almost as predictable as night following day - just the precise timing isn't so accurate!