Can I take holy basil with celexa and synthroid (thyroxine)? Will this cause any interactions?

Apparently safe. There are no known interactions between holy basil and these medicines (it does interact with coumadin, (warfarin) but so does almost everything). It might slightly speed elimination of Celexa but this is not likely clinically significant. Holy basil is one of my favorite herbs, with many benefits; it's calming, anti-inflammatory, supports adrenals, lowers blood sugar. See http://www.Holy-basil.Com/6865.Html.
U can check. You can use an interaction checker when you have questions about drug interactions with herbal or natural medications. Here is one example: