My son was diagnosed with congenital hypothyroidism, he has been very lucky that he has not suffered any set backs. The only problems with him is that he has mood swings and a very bad temper (would like to hear from anybody with kids that have mood swings).
My son is now 14years of age and is doing very well at school. Only problem is trying to get him to take his medication.
With your clarification, I will not suggest the testing. I'd maintain the course you are on, be sure she is receiving a stimulating pre-school program, and obtain a consult re: behavior management. The latter should pose little problem to remediate. If you want some practical behavior management suggestions, take a look at Lynn Clark's SOS: Help for Parents. It is a valuable resource.
It is very likely that your daughter is experiencing two separate conditions, if indeed she is displaying any memory problem at all. Infact, it is probably more correct to say that the behavior problem is also a discrete entity. With the three problems of thyroid, memory and behavior, it may be beneficial to seek neuropsychological assessment to detail the various aspects of her functioning. Many insurances will cover such testing, though it usually requires pre-authorization.
I should have clarified my daughter's age: she has just recently turned 3.