I'm on synthroid (thyroxine) for 5 years now and my thyroid is starting to swell now where it's affecting my voice. why is this happening now?

Check TSH level: 1 are you on adequate dose/strength of Synthroid (thyroxine) ? this can be checked by blood test :TSH which has to be 0.5 to 2.5., if level is higher than 5.0 you need to increase the dose of synthroid (thyroxine). 2 Some time around menstrual bleeding phase thyroid gets enlarged. as well during pregnancy. 3 It could be Inflammation of Thyroid : Did U have Thyroid Antibodies levels checked in past . Need sonogram.