Hypothyroid diagnosis late in pregnancy

Ok so I’m 36+2 and my dr called yesterday letting me know my blood work showed a significant decrease in my thyroid hormone and that they’re sending over a prescription for syntheroid to my pharmacy. Has this happened to anyone else? Thing is, I was diagnosed with a 5cm tumor in my thyroid and idk if it’s due to the tumor or pregnancy. I have an appointment to get it biopsied after I have the baby. My thyroid levels have been perfect until this most recent bout of blood work. I’m worried and have had so many complications with this pregnancy, I just want my baby to be ok.