These words look as though they are related, but they have different origins and different meanings. Hypothecate means "to mortgage," "to pledge to a creditor as security": "To complete this transaction you will have to hypothecate your stocks and bonds." Hypothetical means "assumed," "supposed," "conjectural": "This is only a hypothetical example." To hypothesize is to form an hypothesis (a proposition, premise, or assertion).
Dictionary of problem words and expressions. Harry Shaw. 1975.