Significance Many therapeutic proteins suffer from short plasma half-lives and, as a consequence, require frequent injections to be therapeutically effective; this in turn can adversely affect patient compliance and quality of life. In contrast, therapeutic antibodies typically have half-lives of weeks in humans. Consequently, there is considerable interest in generating functional antibodies with agonist or antagonist activities. Based on the structure of a natural bovine antibody with an ultralong, well-folded heavy-chain complementarity-determining region, we have developed a strategy for the generation of functional human antibody–hormone chimeras with biological activities comparable to native hormones and significantly enhanced pharmacological properties. This approach likely provides a general, relatively straightforward platform for generating antibody agonists and antagonists for a range of therapeutic applications.