There are different types of thyroid disease; some women get post partum issues that tend to be non autoimmune and this can be characterised by hyper then hypo symptoms, often then settling or going on to remain hypo.
Weight gain with hypothyroidism isn't always a given; I loose weight and was very weak and thin when first diagnosed at 20. When I possibly wasn't on enough thyroxine / had probably missed some doses post partum with my son, again I lost weight.
You can loose muscle mass and weight seems to be dependent on different people's metabolism. I'll gain a little in water retention but over time I start to struggle and either don't loose weight or waste away if I'm not on the correct dose.
You must start to record your blood test results and how much thyroxine you're on as this can be useful if you need to adjust slightly (eg a 25 every other day on top of say 100 or 125.) most people on thyroxine need to be just below 1 or even lower.
Regarding ttc; thyroxine is extremely important for foetal development in the first trimester and tsh must be below 2.5 - though I expect you will need to be less than one to feel well. If you end up on a dose of around 125/150, you're likely to have no functioning thyroid left. So when ttc, you must be aware of your levels before conception and get a blood test between 4-6 weeks (ideally 4/5) and then raise by 25. Bloods should be tested every 4-6 weeks throughout pregnancy and tsh kept below 3.
The above guidelines have been worked out with a gp and endo by the British Thyroid Foundation and should be due to be published soon. If your gp is in any doubt ask them to call either an endo or the obstetricians at the hospital. Annoyingly, you don't see a consultant till 12-14 weeks which is too late and not all gps are up to speed with this. The nice guidelines do actually say to refer to an endocrinologist but in practice that doesn't seem to happen.
I tend to find it's between 10-14 weeks you need the most thyroxine. I briefly went to 200 from 125 pre pregnancy at that point before settling at 175 (I felt like I was on too much after a while)
It's also very important - for everyone- to be getting enough iodine during pregnancy through dairy and white fish but a pregnancy multivitamin with it in is useful. When on thyroxine, iodine isn't as important (it's used by the thyroid to make t4 and t3 along with selenium and iron) but it's still worth making sure you have some from your diet.
When very hypo you can become anaemic or have low iron stores (ferritin) which it's worth checking - 'normal' is over 70 but they tend not to treat. Many feel better if this is over 70 however.
It's also worth keeping on top of vitamin D and B12 (especially post partum and if BF) as both can have similar symptoms to hypothyroidism, which is useful to rule out if you're trying to work out your correct thyroxine dosage.
Apologies that this is a bit of a brain dump, I've been there and got the teeshirt a few times!
Excellent source of info (if you join you get quarterly magazines) and as I say, soon to update the pregnancy and fertility section:
www.btf-thyroid.org
And this book is a really handy resource too:
www.amazon.co.uk/Thyroid-Disorders-Understanding-Family-Doctor/dp/1903474191