What Causes Abnormal Thyroid Levels Despite Taking Thyroxine During Pregnancy?

Hi, I ve been on treatment for hyperthyroidism (Carbimazole) but recently my test showed my thyroid level is below normal. My doctor prescribed thyroxine 100 micrograms. As at today my result is t4 and tsh are normal and my t3 is still low. I m supposed to have a c-section by 9th. I m presently 38yrs and 36 weeks and 5 days preg. Is it safe to do the surgery?