Waggamamma, agree that as you've only been ttc-ing for under a year and your young, you're no way near the 'worry' stage yet (most couples (about 95%) will conceive within 3 years of ttc-ing, and as you've already fallen pregnant, its very early doors for you). I would say though, that not having your thyroid working properly does increase the rate of early miscarraiges (which is why it's important to get your thyroid levels checked as soon as you discover your pregnant, if you know you have hypothyroidism or are sublinical)
Read pages 39-40 of the document I sent, you'll see they recommend a TSH level of 0.4 - 2 in pregnancy, and that T4 should be at the top of the range.
I was diagnosed with an underactive thyroid (although technically subclinical) when ttc for No1, I've been through 2 pregnancies (under consultant care) and my TSH was kept to beneath 2 throughout my pregnancies.
My first GP (who was great) told me that in woman ttc, subclinical hypothyroidism is a factor and that studies have shown that thyroxine therapy in subclinical women ttc - does improve the rates of pregnancy.
A lot of GPs are clueless about thyroxine and pregnancy or ttc - I've had to refer GPs to the guidelines and advice my consultant gave me, as they were unaware of it.
However, this all depends on what your T4 reading is, if that's at the top of the range already, that may be why your GP is loathe to go down that path (given that once you're on thyroxine, its a lifelong condition and you're on it for life).