Equal amounts of autonomous thyroid tissue were found with the same frequency before and after 40 years of age in eumetabolic patients in endemic goitre areas. A tentative diagnosis of thyroid autonomy can be made already in euthroid patients with an 80% accuracy by combined estimation of an equivalent of free thyroxine and iodine uptake. The equivalent is taken from determination of thyroid pertechnetate uptake by quantitative scan evaluation. Proof of autonomy is established by the suppression test which in a prolonged fractionated form also permits estimation of the amount of functioning autonomous tissue. There is a linear correlation of technetium thyroid uptake after suppression and the volume of autonomous thyroid tissue. As free thyroxine increases after iodine intake of sufficient amount and duration in proportion with the volume of autonomous tissue, prognosis appears possible: patients with autonomous thyroid tissue above a "critical" volume can be expected to develop hyperthyroidism in case of sufficient iodine uptake. Iodine salt prophylaxis however does not induce hyperthyroidism in patients below 50 years of age provided that the daily additional iodine intake does not exceed 100 micrograms.