This microhistory analyzes the transformation of French medical practice after World War II. Before the 1940s, coordinated clinical and experimental studies on a patient with thyroid disease were nonexistent. Starting in 1945, thyroid endocrinology was pushed forward by the use of radioiodine, which led pediatricians to rethink the hereditary transmission of thyroid diseases, stimulated by the school of biochemistry headed by Jean Roche, who participated in the elucidation of thyroid-hormone metabolism. Roche knew how to listen to hospital clinicians, who, in return, gave him free access to patients to conduct investigations on thyroid metabolism. Radioiodine proved to be sufficiently flexible to respond to the respective information needs of clinicians and biochemists, and was also responsible for the profound transformation of scientific reasoning: the chemical framework that had dominated thyroid studies since the discovery of thyroid iodine (1896) was replaced by the hormone paradigm.