An underactive thyroid, or lack of iodine. Your brain sends a hormone called thyroid stimulating hormone to the thyroid, which then makes thyroid hormones, but it needs iodine to do this. One thing the thyroid hormones do is they tell your brain to stop sending thyroid stimulating hormone, so if the hormones cant be produced properly, due to lack of iodine, or just because of an underactive thyroid then there is nothing to switch off thyroid stimulating hormone production, so the thyroid keeps getting stimulated, causing it go grow bigger and bigger.
also, your thyroid gland may be over- or under-active, you will need to see a doctor so they can take blood tests or an ultrasound on your throat. if this is the case you may be put on medication or depending on the severity, have it surgically removed .
A infection can cause this, this can be normal, depending on the size and the length of time it is enlarged. If you have this and are worried, it is best to seek professional help.
I did today, my Dr. says it's enlarged.. I better not gain any weight! lol