Is thyroid serious?

If her thyroxin levels are high or low it can easily be corrected with tablets. It is not a serious matter.

the T regulates all things in the body - alongside the Thalmus and Pituitary. it helps fight off infection by giving feedback as to the nature of the disease.
No real need to worry

The thyroid gland helps to regulate how the body burns food for energy, it is very easily treated with tablets, and is NOT serious, so relax