My 51 year Nanny has a severe cough for the past 2 years. She takes some medicines for thyroid daily and has done blood test and used all possible cough syrups without any long effect. It's a dry cough that prevent her to sleep properly in the night. Blood testis normal. Please suggest any effective syrup or treatment that can help her get rid of this stubborn dry cough that lasts for sooo long. Thanks.