Why can not use antihistamines and decongestants in patient with thyroid disease? What is the machanism of action? What medicine can use to relief?

Decongestants may. Decongestants may complicate and exacerbate hyperthyroid states. In general I do not recommend oral decongestants to any age group as there is a higher risk of cardiovascular disease with decongestant use. Antihistamines alone may be used in thyroid disease in general.