What could be causing my enlarged thyroid if I have no other serious symptoms?

Autoimmunity. Goiters, or enlarged thyroids, may be associated with high, low, or normal thyroid levels. Often, if the thyroid levels are normal, there is no need to do anything but keep an eye on things. Autoimmunity, where the immune system makes antibodies against someone's own proteins, is the cause for most thyroid problems in the US, including goiters and nodules as well as high & low thyroid levels.
Number of things. U may have a smooth or modular goiter. Need blood tests to see that overall function is normal and ultrasound to see if there are any worrisome nodules.