Hi there. I have no thyroid at all as it was removed 2 years ago. I have been on thyroxine ever since which as has already been said just replaces your body's normal hormones. Without it I would suffer from all the symptoms you have described - tiredness, weight gain, dry skin included.
The thyroxine works a treat - although it is totally normal to have to increase the dose during pregnancy (mine has nearly doubled) as the baby needs the stuff too! So it may be that you had borderline low lower levels before you were pregnant and this has just been exacerbated by pregnancy. You'll go back to normal afterwards.
It's great that you got it checked out as it's miserable to put up with the symptoms, and there can be some side effects on your baby's development if it's not spotted and resolved.
Good luck :-)