Cards in hand cannot be items. They are only cards.
The Income Tax rules text:
Discard one item of your choice. Every other player must now discard an item, or items, totaling at least as much value as the item you discarded. If they don't have enough to pay the full tax, they must discard all their items and lose a level.
From the Munchkin FAQ
Q. If you have to lose Items, do you lose the ones from your hand, or just those on the table?
A. Always, always, only those on the table. While cards are in your hand, they're not carried Items yet. If you are meant to lose cards from your hand, this will be specified.
Also from the official rules:
Your Hand: Cards in your hand are not in play. They don't help you, but they can't be taken away except by cards that specifically affect "your hand." At the end of your turn, you may have no more than five cards in your hand.
An item card in your hand does not count until you play it; at that point it is "carried."