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In evaluating data on the concentration of TSH in blood, one must examine critically the bio-assay methods used to obtain them.

The introduction of the United States Pharmacopoeia reference standard in 1952 and the redefinition and equating of the USP and international units of thyroid-stimulating activity have made it possible to compare results published by different investigators since that time.

We should like to re-emphasise the importance of stating results solely in terms of international units of TSH activity and of avoiding the re-introduction of biological units.

For the most part, this discussion will be confined to results obtained since the introduction of the reference standard.

A.

Standard preparations and units of thyroid-stimulating activity

The international unit ( u. ), adopted to make possible the comparison of results from different laboratories ( Mussett and Perry, 1955 ), has been defined as the amount of activity present in 13.5 mg of the International Standard Preparation.

The international unit is equipotent with the USP unit adopted in 1952, which was defined as the amount of activity present in 20 mg of the USP reference substance.