Reduction of plasma tyrosine by oral contraceptives and oestrogens: A possible consequence of tyrosine aminotransferase induction

The fasting plasma tyrosine levels of 23 women were determined before, and at least 3 months after they had started to use an oestrogen-progestogen preparation for contraceptive purposes. The group as a whole showed a highly significant decrease of plasma tyrosine when taking an oral contraceptive, and in only 3 cases was the level unaltered. Two women treated with ethinyl oestradiol alone showed a similar decrease of plasma tyrosine. It is suggested that increased levels of glucocorticoids, due to the action of oestrogen, induce elevated levels of tyrosine aminotransf erase, and that as a consequence there is an enhanced rate of degradation of the amino acid in the liver.