Can Excessive Synthroid Cause Memory Loss And Confusion?

My wife has been takin synthroid medication for 15 years after early throid cancer and removal of her throid gland. She was perscribe 1.50 mg for the first 10 years and then her Physician changed and her new Physician perscribe 3.00mg of Synthroid...she has been on this amount until about a year ago when she started having memory loss and confusion...now her doctor has been reducing the amount of synthroid every 6 weeks after checking her blood levels...she is now at 2.00 mg and will be checked again in 6 weeks....my question is... could the excessive amount of synthroid taken for 3 to 4 years be the cause of her memory loss, slowed speech and at times confusion?