Hi. I had the same problem as you...I had a very enlarged thryoid and when I went to the doctors, they took a blood test and an ultrasound of my throat. It came back that I had hypothroidism. Your normal test should be between 0-3. My blood test came back 428!!! They gave me medication that I take once a day and I haven't had any problems since! I hope this ease's your concerns!! :-)
Most of the time an enlarged thyroid is a benign (non-cancerous) over-growth of normal thyroid. Usually we do an ultrasound to determine if there are nodules (lumps) that may need biopsy -- but only 5-10% of nodules are cancer. Also would test thyroid function - blood tests - which your doctor has probably already ordered.