What Are Effects Of Taking Hypothyroid Medication During Pregnancy?

I am 20 weeks pregnant and received a call from my OBGYN today. He told me that my TSH level came back at 4.75 (lab test done at 19 weeks) He also mentioned that when I was 12 weeks my TSH level was higher 5.25 and now they have dropped to 4.75. He prescribed me Centroid. I also had quad screening test and ultrasound at 19 weeks, and test came back negative, Ultrasound showed fetus measuring and growing normal with no abnormalities. My concern is being treated for hypothyroidism so late into my pregnancy (20 weeks) Will this have any effect on the growth of my baby or cause any problems? Is 4.75 a high level? and how concerned should I be? He said he will retest me at 7 months...Seems too long to wait to be retested again. Thank you for answering my questions.