I was born with congenital hypothyroidism and have been taking Synthroid (thyroxine) since. I have missed about 2-3 weeks of taking my dosage. Side effects?

Oh no! These are not side effects, but rather effects of your disease. Lethargy, weight gain, headaches, anxiety, depression, hair loss, cold intolerance for starters. You should be feeling quite awful by now!
Slow decline. This leads to subtle changes in metabolism and physical symptoms that may change so slowly you are not aware of them. Temperature sensitivity, changing energy levels, weight gain, constipation, etc.A blood test would likely show your brain sending out messages to turn on your non-working thyroid.

Related Questions

If I had low thyroxine free when I was Preggy and then my baby was born with congenital hypothyroidism should I of been treated during my pregnancy?

No. I understand how you feel but Thyroxine does not pass very well through the placenta and one cannot treat hypothyroidism in the fetus before delivery. As long as the infant was diagnosed early and is receiving Rx then all sound be OK. Read more...