A patient suffering with manic-depressive psychosis, depressed type, the diagnosis being based upon the history of repeated cyclothymic depressions with typical symptomatology and complete recoveries with remissions free from symptoms, attempted during a depression to commit suicide so that it would be unrecognized as such. Massive, prolonged thyroid overdosage extending over a period of 1½ years resulted in a clinical picture of hyperthyroidism indistinguishable from typical Grave's disease. A marked increase in the overdosage resulted in the clinical picture of psychosis with hyperthyroidism. Removal of the drug resulted in the remission of symptoms due to thyroid toxicity, the clinical picture of a retarded depression remaining. Recovery from the depression followed a period of several months' hospitalization under psychiatric régime. During the acute toxicity the superimposed psychosis sufficiently masked the depression that it was not detected. This unusual method of attempted suicide provided opportunity to study the physiologic effects of massive, prolonged thyroid overdosage—a case of experimental human hyperthyroidism.