Hi, I Am Trying To Work Out Whether Taking Antihistamines

Hi, I am trying to work out whether taking antihistamines such as clarytn or zyrtec reduce the effectiveness of medication I am taking for under active thyroid - thyroxine or somac
It seems that question has been asked before and perhaps their is no clear answer. ?
The reason I am taking anithistamines is because of a cat allergy. I cannot get rid of the cat as I have undertaken to mind it for over 1 year for a very dear friend.
Like a lot others I am miserable with the allergy, it is really bad or feel deeply depressed when taking them. It would seem self evident, except I am trying to work out if it is real probelm or just me?