Thanks for posting your query at DoctorSpring.com
As such, the cut off value for the TSH is 2.5, and hers is just marginally elevated.
There is absolutely no cause for concern.
Yes, ideally it is best to start taking the replacement therapy before planning a pregnancy, but even now, it is best to start them.
It is not too late to avert cognitive and developmental impairments, because the value is just negligibly abnormal.
Also, she should be taking folic acid tablets regularly.
All the best
Please feel free to discuss further.