My daughter has thyroid enlargement. sym were neck bulge & cough, trouble swallowing, tired.ultrasound said thyroid enlarged no mass, what causes this?

Several causes.: Enlarger thryoid is called a goiter, and in many countries is caused by lact of iodine in the diet, but that is rare in the US. More likely, since no masses or nodules are seen on ultrasound, it is caused by Graves disease or hyperthyroidism or inflammation of the gland often called Hashimoto's. Blood tests for T3, T4 and antibodies are helpful. See http://www.mayoclinic.org/diseases-conditions/goiter/symptoms-causes/dxc-20264595