ttc for baby

hi I have alredy 2 kids and my fiancee have 4 kids after my second daughter I was diagnosed with underactive thyroid.I heard that thyroid can make problems to conceive or keep baby? what can help u conceive when u have underactive thyroid?I am on levothyroxine 75mg atm.I really would like to give my other half baby.plus I got other health problems like diabetes type 2, high BP and migraine.please help.I am really worried I won't be able to give him baby