SummaryThe analysis of 200 radioiodine diagnostic tests shows that the iodine collection curve gives results far more valuable than a single 24th hour determination that can ignore an early maximum of collection.The curves showing an early maximum and a significant fall at the 24th hour seem characteristic of hyperthyroidism, even if the values reached at the second and sixth hour are not up to 40 % of the ingested dose.The bell-shaped curves reaching a maximum of uptake superior to 40 % at the 24th hour are less characteristic. Beside cases of hyperthyroidism, the same type of curve can be obtained in euthyroid patients with familial goiters, in certain puberty and juvenile goiters and sometimes after antithy oid drugs.The same shape of curve is found in subjects submitted to a relative lack of iodine due to consumption of water from local sources or wells, what is not exceptional in rural areas.The complementary triiodothyronine test permits to depart the numerous cases where the collection test has given doubtfull results. In hyperthyroid patients, triiodothyronine is usually unable to reduce the high iodine uptake. In euthyroid subjects with goiter, on the contrary, the collection level falls at least of 30 % after 10 days.It is exceptional that this test is not able to provide a valuable answer. Some rare diffuse or nodular goiters in euthyroid patients may notwithstanding keep a high collection level inspite of the administration of triiodothyronine during 10 and even 20 days.