Influence of sex hormones on the capacity of transport of thyroxine-I-131 by the serum in rabbits

Abstract

Various authors (5, 7, 8) have shown how, in humans, the estrogens increase the capacity of the interalphaglobulin fraction in the serie transport of thyroxin, while the androgens show a contrary effect, though a less evident one.

In the present work an experimental study has been carried out on rabbits, by intramuscular administration to 2 animals of 25 mg of testosterone every second day for 21 days, while 2 other animals have been given injections of 1 mg of stradiol benzoato, at the same rate and for the same period.

The capacity of transport of thyroxin- I131 has been measured in blood samples taken before the beginning of the experiment and after the ist, 2nd and 3rd weeks of the treatment.

The serums charged in vibro with thyroxin-I131 (Radiochemical Center, Amersham), have been separated by electrophoresis on paper with a tampon of tris-maleate pH 8.6, and the radioactivity of the strips has been measured with a Philips unit. On the radioactivity curves there only appears one thyroxin-transporting fraction, which is situated between the albumin and the alpha-globulin. The planimetric measurement of the surface of this fraction has been used to analyse the variations in transport capacity, after making the necessary corrections (table I).
Table II shows how in rabbits, and in the doses employed, the sex-hormones elevate the fixing capacity of the serum for the thyroxin. This behaviour is in accord with that of humans as far as estrogens are concemed, but differs from it with regard to androgens. It may be possible to explain this difference by the doses injected, in our case 12 mg of testosterone per kilo of body weight, on alternate days, whereas the dose indicated by other authors (5, 7, 8) is 0,7 mg of testosterone per kilo of body weight daily.