The measurement of thyroid stimulating hormone in body fluids. histometric and radiometric techniques using the xenopus larva

Abstract

The work described in the preceding chapters was undertaken to determine whether the tadpole of Xenopus laevis is a suitable test-animal for measuring TSH in biological fluids. It was intended that the assay should be applicable on a routine basis for use in investigation of thyroid disorders and the findings were assessed accordingly. Studies using a histometric technique demonstrated that this method reaches the required standards of sensitivity and reliability when large test-groups are employed; accordingly, it has been used to estimate the concentration of TSH in serum. The limiting factor, where clinical application of the method is concerned, was found to be the time and labour involved in completing the estimation. An attempt was made to overcome this difficulty by application of a projection technique whereby the time required to make cell-height measurements was reduced and the method made more objective. Although, from the point of view of technical simplicity, the histometric method does not complete with either of the radiometric techniques described, it has a lower limit of sensitivity of 0.1 imu/ml. of TSH and has yielded meaningful results in assays on serum. It may be concluded, therefore, that the histometric method can be employed in this type of work provided that the time required to complete an estimation is not of major importance. The factors influencing thyroidal iodine accumulation were investigated with the aim of developing an assay method based on estimation of -uptake by the glands. A considerable individual variability in iodine uptake was demonstrated in both Xenopus larvae, reared under artificial conditions, and in larvae of the common toad, collected from their natural surroundings. Although neither diet nor temperature was found to influence this variability greatly, pretreatment with a dilute solution of potassium iodide was found to be effective to a united extent. In animals pretreated with potassium iodide it was possible to demonstrate an increase in 131I-uptake in response to administration of TSH at a concentration of 10 imu/ml. Administration of TSH was also shown to have an effect on discharge of as demonstrated by increase in the radioactivity released into the water in which the animals are immersed. This method of measuring thyroid stimulation is indirect and probably less accurate than direct determination of the activity present in the glands. It is further complicated by the presence of comparatively large amounts of iodine retained in the extrathyroidal tissues. Improvement in the sensitivity of the radiometric techniques is entirely dependent on control of individual variability in iodine uptake. In their present form, however, both the 131I-uptake method and the 131I-discharge method are suitable for application where the concentration of is 10 imu/ml. or more, and where very little material is available. Such a situation arises in comparative work on vertebrate endocrinology where small quantities of pituitary material are available in which the expected concentration of TSH would be high as compared with that in serum samples. (Abstract shortened by ProQuest.)