Hi, I have an underactive thyroid (I have Hashimotos disease) and can share my experience.
It took me 8/9 months to conceive DC1. This was prior to my diagnosis. Had a pretty straightforward pregnancy and delivery. Was diagnosed when DS1 was around 5 months old. At this point, my TSH was over 180. We quickly got it under control and I began to feel much better.
About a year later, I got another BFP. It ended up being a very early MC/CP and when I had my TSH levels tested, it showed that they were pretty high again (although I can't remember the exact number) I asked my endocrinologist if this had caused the MC, and she said she couldn't really say either way, it could have just been really shitty luck.
I then got pregnant again just 2/3 cycles later. As soon as I got my BFP, I had my thyroid checked and again, we had to make some dramatic changes to my dosage. I ended up having a MMC at around 10 weeks.
The endo then cautioned me not to TTC again until my TSH was stable at below 2. This took a few months. But I fell pregnant again quite easily, after about 2 cycles and I'm now 26 weeks with DC2.
This time my thyroid levels are being reallllllly closely monitored and we are making constant adjustments to my dosage. It's only in the last few weeks that I now seem to be stable taking 225mg every day.
Best of luck!