Thank you for your question and welcome to Just Answer.
I am sorry you are having these troubles.
Can you tell me how often you are having a bowel movement?
Are you on any other medications?
Do you have any other health issues?
When was your last menstrual cycle?
Thank you for your reply.
Do you look swollen ?
Look at your hands and feet to see if they look puffy.
With gaining so much so quick you do need to be seen by your doctor.
Synthroid can cause weight gain but it really shouldn't be so quick.
You might want to go to an endocrinologist to get further testing done.