There are some supplements you shouldn't add to tea (if the tea has tannins), tannins can block the absorption of some things.
Tannins can bind a number of compounds preventing them being utilized by the body. These include iron, alkaloids, and glycosides. Some high tannin sources include coffee, black tea, oolong tea, green tea, white oak bark and uva ursi. Therefore these should never be taken with herbs, supplements or medications.