SOME OBSERVATIONS ON A SENSITIVE BIOASSAY FOR THYROTROPHIN. APPLICATION TO NORMAL, HYPER- AND HYPOTHYROID HUMAN SERUM

Abstract

A four-point in vitro bioassay for human serum thyrotrophin (TSH) has been described by Kirkham (1962). In this assay, thyroid tissue of PTU-treated guinea pigs is incubated with I131 -iodide. Release of I131 after addition of KCNS is measured. This release is inversely proportional to the logarithm of the TSH activity in the incubation medium. We have found that erratic responses occasionally occurring in this assay are due to "non-specific" stimulation. The non-specific material affecting I131 release is removed from human serum and other preparations by filtration through Sephadex G-25 medium. Rabbit anti-bovine TSH abolishes the bioassay response to bovine TSH and to human serum TSH. The system does not respond to LATS levels which give a very high response in the McKenzie assay. It is suggested that the amount of I131 bound in the incubated tissue is directly related to the TSH concentration and that therefore the remainder of the trapped I131 which is free to diffuse out gives an inverse measure of the TSH concentration.