I have controlled hypothyroidism by taking levothyroxine. however i have chronic allergies producing urticaria and rhinorhea. which antihistamine best?

Any that works: Try several and find what works best for you and doesn't put you to sleep. It varies from patient to patient. Make sure you are taking brand name T4 replacement, as the generics are cheaper, but unreliable.

Benadryl (diphenhydramine) would be an: Effective antihistamine. Ask your allergist if adding an antileukotriene and a steroid would also help. Take care!