What are the treatment options for Graves' disease when trying to get pregnant?

Hello doctor,

My sister is 32 years. After her second delivery, she got the problem of diffuse toxic goiter what is called Graves' disease. She is on medication for the last six years. She is on Carbimazole. But, it is not effective as when she stopped taking the medicine she suffers again from the symptoms (heart rate and sweating). The doctor advised her to get treatment by radioactive iodide. But she is worried this can lead to infertility. So, the other choice was to do a surgery to remove the thyroid gland. But, she will be on Thyroxine for the whole life. I wonder if this is the only choice she has. And, if this is the case, what is the best for her as she wants to get pregnant and she is breastfeeding now.

Note:

1. The thyroid gland is diffusely enlarged with increased tracer uptake and homogeneous distribution.
2. No definite focal lesion seen.
3. 20 minute Tc uptake = 5.2 %.

Thank you and looking forward to hearing from you.

Kindly advise.