We report a patient who underwent, over a mere 3‐year period, three successive cycles of oscillation from hypo to hyper‐thyroidism and back to hypothyroidism. This unusual sequence of events originated In a rare passage of primary hypothyroidism to hyper‐thyroidism. The hy‐perthyroldlsm seemed typical of the autolmune subgroup of toxic multlnodular goitre. Stimulating and blocking TSH receptor antibody activities were measured (by cAMP functional bioassays using cultured human thyrocytes) during the course of the fluctuating phases of hypo and hyper‐thyroidism. Measurement of such antibody activities revealed the coexistence of both stimulatory and blocking types of antibody In several serum samples from the patient. Throughout the whole course of alterations In thyroid function, thyroid stimulating antibodies were present. This was not the case with thyrotrophln receptor antibodies exhibiting TSH antagonist activity which seemed to appear and disappear. Monitoring such activity Indicated that the emergence of blocking antibody seems to herald the onset of hypothyroidism