Both my big toes have a burning sensation under the nail beds. They are cracking and bout to fall off! No thyroid problems and don't think its fungus!

Toenails. Your medical history is complicated. The problem with your toenails has many possible causes have them evaluated by a dermatologist. It could be due to an infectious (bacterial or fungal) , or immunologic, or physical etiology. The burning may or may not be related to the cracking of your nails.