This is a pun on 'iron deficiency' which is when your body doesn't produce enough iron, but 'irony' is used in it's place because it sounds similar and it means the expression of one's meaning by using language that normally signifies the opposite, typically for humorous or emphatic effect. In this case, the irony is that this joke is a pun
I don ' t find health - related puns funny anymore since I started suffering from an irony deficiency .