The name thyroid-stimulating hormone (TSH) has been given to a substance found in the anterior pituitary gland of all species of animal so far tested for its presence. The hormone has also been called thyrotrophin or thyrotrophic hormone. At the present time we do not know by what biochemical mechanism TSH acts on the thyroid, but for bio-assay of the hormone there are a number of properties by which its activity may be estimated, including release of iodine from the thyroid, increase in thyroid weight, increase in mean height of the follicular cells and increase in the thyroidal uptake of ** f. Here we shall restrict discussion to those methods that appear sufficiently sensitive and precise for determining the concentration of TSH in blood. Brown (1959) has reviewed generally the various methods of assaying TSH, and the reader is referred to her paper for further information on the subject.

As long ago as 1851 it was pointed out by Niepce (1851) that there is a connection between the pituitary and the thyroid. . This connection was clarified by Smith and Smith (1922), who showed that saline extracts of fresh bovine pituitary glands could re-activate the atrophied thyroids of hypophysectomised tadpoles. The first attempts to isolate TSH came a decade later, when Janssen and Loeser (1931) used trichloroacetic acid to separate the soluble TSH from insoluble impurities. After their work other investigators applied salt fractionation techniques to the problem, as well as fractionation with organic solvents, such as acetone. Albert (1949) has concluded that the most active preparations of TSH made during this period, from 1931 to 1945, were probably about 100 to 300 times as potent as the starting material. Much of this work has been reviewed by White (1944) and by Albert (1949). Developments up to about 1957 have been discussed by Sonenberg (1958).