My Doctor Said I Have A 'very' Enlarged Thyroid And

My doctor said I have a 'very' enlarged thyroid and that it is pressing against the top of my wind pipe. What can cause this? It feels like pressure in my throat, is making me cough a lot, and causes a jittery feeling around the area. Will it have to be removed or will medication make it get smaller? I have gained a lot of weight and lately prone to anxiety. (I have also had severe depression for a long time.) Could these be a result?