I was diagnosed with hypothyroidism when my dd was 4 months old and was put on 100 mg a day of thyroxine. You really need to take the thyroxine because your body can not get it any other way.
Basically it's the same as being diabetic in that the thyroxine simply replaces what your body doesn't make, as insulin would for a diabetic.
The main advantage of having this condition is that it's classed as a 'chronic' condition so you get free prescriptions for life!
Ask your gp for the form if he hasn't mentioned it already.
I found out I had it because I went to the gp thinking I had PND and he ordered blood tests to rule anything else out. The difference the thyroxine made was unbelievable. I went from being unable to get through a day without crying and not wanting to leave the house to feeling completely normal within less than a week.
According to my gp, 1 in 20 women develop it after having a baby and of those, only 25% need treatment for more than a year.
Please don't be worried by it or feel reluctant to take the thyroxine. There is no other way (dietary or herbal) to treat hypothyroidism and it has no effect on breastfeeding.
What will have an effect on your baby is how you will feel if you don't take it. You haven't said what your levels are but my TSH was 79 and it should be below 5, ideally around 2. I can't explain how awful and miserable I felt - at one point I seriously considered letting go of the steering wheel while driving at 80mph along the M5.
I would go back to your Gp with a list of questions of what's bothering you and then take the thyroxine asap.
Sorry to go on, if there's any questions you think of I'm around on here fairly regularly and I know there are lots of other mners who have it too.