ON THE BIOASSAY OF THYROTROPHIN IN PLASMA

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Abstract
The I131 mouse blood level increase method of thyrotropin (TSH) bioassay was investigated with regard to its practical efficiency and statistical validity. This method did not fulfill the requirements of a valid bioassay because of variance heterogeneity and curvilinearity of the logdose response line. Studies on I131 elimination and thyroxine action time in mice yielded a biological half life of I131 of about 5 days, while the estimated period of action time of 20 [mu]g of l-thyroxine was 3 days. The advantages of the method adopted here are briefly summarized as follows no pretreatment with low-iodine diet, no feeding of thyroid powder; the response metameter of choice, preferable to 3 others, is the log cpm-ratio which gave linearity, homoscedasticity, precision, and ease of calculation; the animals can be used from day 2 to day 6 inclusive; small samples of blood (50 [mu]l) and constriction pipettes are preferable to larger samples and the use of syringes: higher precision, the same accuracy and repeated use of the same animals; cross-over design improved the index of precision from 0.23 to 0.15; evidence on specificity: no response to plasma from hypo-physectomized rats and to some other substances, high values after PTU (propyl-thiouracil) and in some clinical conditions. The method is not sufficiently sensitive (mean threshold dose was 0.017 mU of TSH) to detect TSH levels in normal human or mouse plasma, which thus appear to be lower than 0.05 mU/mL In normal rats, the mean plasma TSH level was 0.4 mU/mL Elevated plasma TSH was observed in patients with myxoedema and malignant exophthalmos (highest value recorded was 0.9 mU per ml), and in rats and mice after prolonged treatment with PTU. The plasma levels in mice after PTU treatment for 5 weeks were as high as 4.9 mU/ml.

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