I have not had it done, but the thyroid is really important in regulating your metabolism, it can impact/determine your mood/depression etc.
Just relax and stay calm until they do the additional testing- did you ask your doctor about what it could mean and how low it really is when they advised that you needed more testing done?
After our anatomy scan, the doctor noted that the umbilical cord only had one artery instead of 2, but she was not worried about it as everything else was looking good. Then the next week they called to scheudle a follow up ultrasound to look at it further- that completely freaked me out. But it is all fine. This can be a really scary time, but just do your best to relax and take it as it comes.