Results of a purely in-vitro test were compared with previously available methods in 425 patients. For triiodthyronie, a normal range must be differentiated from the so-called euthyroid range (0.8-2.0 ng/ml) for patients with euthyroid goitre. The accuracy for triiodothyronine is then more than 95% and in hyperthyroidism 97.5%. It is, therefore, superior to other in-vitro measurements. But a disadvantage is that it is easily influenced by thyroid and extrahyroid factors. Similar accuracy is obtained for the TRH function test (thyrotopine-releasing hormone). Noteable is a negative TRH test in 20% of patients with euthroid goitre. The significance of triiodoathyrone determination is decreased in autonormous adenoma, in which the TRH test has an accuracy of 96.5%. In rare diseases of the thyroid all diagnositc possibilities must be taken into account. In ordinary practice stepwise course is suggested. RT-3U, T-4, T-3 and thyroid scan with 99mTc pertechneate are useful as basic tests. PBI is still thought to be important. The second step involves the TRH test rather than 131I function test. Triiocothyronine determination is best for therapeutic and follow-up perposes, a situation in which TRH test is of limited value.