A simple test with 40 mg of oral TRH after overnight fasting was developed and can be characterized as follows: This short oral TRH test has been found to be a most valuable thyroid function test for detection of hyper- and hypothyroidism. It is as reliable as the intravenous TRH test. Oral TRH produces much stronger and more prolonged TSH stimulation similar to that of a depot preparation. Therefore, the incidence of euthyroid controls with absent or subnormal TSH response to TRH is definitely lower than after intravenous TRH. In patients with low TSH response after i.v. TRH the oral TRH test is of particular diagnostic value in distinguishing euthyroid function from TSH suppression or pituitary failure. 40 mg or oral TRH produces reliable stimulation of T3 in most euthyroid subjects. In patients with hyper- and hypothyroidism no increase of T3 is obserable. Oral TRH allows simultaneous assessment of pituitary TSH and thyroid reserve in a single test. The practical advantages (side effects in only 2% [for i.v. TRH 60%], only 1 or 2 blood samples, fewer sampling errors and no i.v. injection) outweigh the disadvantages (longer duration of the test, dependence on food intake and gastrointestinal absorption).