Standardized assessment of 1973 patients with iodine-deficient goitre, treated on average with 100 micrograms thyroxin, revealed clinical signs of thyroid hyperfunction in 2.4%. But an increased hormone concentration alone was not sufficient for the diagnosis. Probable cause of hyperfunction was autonomy of the goitre, as indicated by a negative TRH test before treatment had begun. It was calculated that in this case the risk of hyperthyroidism from thyroid hormone administration was 10%, but only 0.7% in patients with a primarily positive TRH test. Hyperthyroid symptoms disappeared and hormone concentration became normal in 60% of patients after thyroid-hormone administration had been stopped. It is concluded that patients with primarily negative TRH test must be carefully monitored while receiving thyroid-hormone treatment.