This has to do with understanding two things, that maternal hypothyroidism is linked to Cretinism/neural development of the fetus, and normal pregnancy physiology. In pregnancy, increased hormones stimulate liver production of various proteins like Sex Hormone Binding Globulin (SHBG),Thyroxine Binding Globulin (TBG), etc. More TBG produced = more T3/T4 is bound. Less T3/T4 causes more TRH release, more TSH release, and more T3/T4 production until free T3/T4 levels are back to normal. In a normal pregnancy, this happens no problem. In someone with pre-existing hypothyroidism though, already has impaired T3/T4 production, and won't be able to keep up with increased demand. Their T3 will rise, but this won't result in increased T3/T4. Typically, women with Hashimoto's desiring to get pregnant will need to increase their levothyroxine dose in anticipation of this pregnancy change.