Thyroid increase in second pregnancy

Patient: Dear doctor,i am 8 weeeks pregnant with my second child and ive been told by my gynac to take EUTHYROX 25microgram as i have slight increase in my thyroid,im veryy veryyy depressed and tensed as in my 1st pregnancy i dint have this with this medicine will it help to keep my weight in proper control during my entire pregnancy? and what side effects it can have, and does slightly elevated thyroid effect the fetus or pregnancy period? plzzzz help doc.thankyou,sana