I’m not familiar with how it works for hyper (which is what you would be looking at), but I wouldn’t worry given it’s only slight (TSH a little below range, it’s not by much, and T4 within range, so they may even say it’s not enough to really do anything about but they’ll keep testing to monitor in case it continues to decrease).
With hypo, the baby doesn’t start producing it’s own thyroxine hormone until early in the 2nd tri so it’s important to make sure the mum has enough hormone as the baby is dependent on the mother’s stores for that time period - for the general development of the baby, thyroxine is a ‘multipurpose’ hormone that helps various functions in the body.
With hyper, it’s that you’re actually overproducing thyroxine - but in your current scenario, only very slightly and they would like already have intervened if it was a concerning level. Thyroid disorders are very easily managed (medically speaking) so it is rare to reach a serious level of hypo or hyper - you would likely have to leave it a while before it reached the point of posing a risk to you or your baby, and again, very rare, and as far as I know hyper doesn’t affect development as such, more it can lead to things like pre-eclampsia. But again, we’re talking if it was to be left completely unmanaged for a while and the levels were significant enough to cause that.
It’s my very clumsy way of saying you are very likely fine and nothing to worry about, but an endo consult is very useful in ensuring how best to manage your levels throughout pregnancy - and routine if you were already hypo or hyper pre pregnancy. I’m pg with DC3 and have had the same routine each time - bloods as soon as I know I’m pregnant (through GP), adjust meds as needed, see the endo, they say, “You’re fine, or, you’re a wee bit under but not much, we’ll keep testing but we don’t need to see you in a clinic again”.
I hope that ramble is somewhat reassuring!