I'm unmedicated though have a multinodular goitre and currently going through the diagnosis process as possibly I have central hypo.
But when I was your age around mid to late 30's my cycles started to become erratic, short and long cycles and sometimes very heavy bleeds. It was a suprise to find out my family have a history of early menopause which I'd never been told before and in retrospect now I realised that I was perimenopausal.
This lasted many years but I went on to have an early menopause; I'm now early fifties and my periods stopped around 8/9 years ago.
I was quite shocked when I realised what was possibly happening to me; my gp was no help and said "....what do you want me to do about it?....
I'm not saying you are perimenopausal but it's something to be mindful off.
Hopefully someone with experience of taking erfa and levo and having varying cycle lengths will reply.
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