Thyrotrophin Assay by Plasma I¹³¹ Measurements

The thyrotrophin assay, based on measurement of the effect of the hormone in causing a rise in the plasma I 131 level of thyroxine-injected guinea pigs, has been studied further. The dose–response relationship from 0.0001 to 0.015 International Units of thyrotrophin is described. It is directly proportional from 0 to 0.0006 I.U., and closely approximates linearity over any threefold range of dosage from 0.0006 to 0.015 I.U. A modified design for the assay is suggested, embodying this knowledge of the dose–response relationship and the use of a Latin square arrangement to eliminate much of the unwanted variation. The proved limit of detection is 0.0001 I.U., the minimum concentration detectable being 0.00005 I.U. per ml., when 2 ml. of test material is injected. Accurate assays can be performed with small numbers of animals.