Effects of sex steroids on the neurotransmitter-specific aromatic amino acids phenylalanine, tyrosine, and tryptophan in transsexual subjects.

BACKGROUND/AIMS: Phenylalanine, tyrosine and tryptophan are essential aromatic amino acids and precursors of dopamine, norepinephrine, epinephrine and serotonin. The aim of this study was to assess whether sex steroids affect plasma levels of these aromatic amino acids.

METHODS

15 male-to-female (M-F) transsexuals were treated with 100 microg/day ethinyl estradiol and 100 mg/day cyproterone acetate, and 14 female-to-male (F-M) transsexuals were treated with testosterone esters 250 mg i.m. per 2 weeks. Plasma levels of hormones and amino acids were measured at baseline and after 4 and 12 months of cross-sex hormone administration, and analyzed by general linear models for repeated measures.

RESULTS

Plasma phenylalanine decreased by 7.5% (SD 3.0; p = 0.01); tyrosine by 18.3% (SD 4.6; p < 0.001), and tryptophan by 7.8% (SD 4.7; p = 0.03) after 12 months of estrogen + anti-androgen administration to M-F transsexuals. Administration of testosterone in F-M transsexuals did not induce significant changes in plasma levels of phenylalanine and tyrosine, but increased plasma tryptophan by 18.2% (SD 20.6; p = 0.001).

CONCLUSION

Estrogens and anti-androgens reduce circulating levels of phenylalanine, tyrosine, and tryptophan in men, whereas testosterone administration increases plasma tryptophan levels in women. Sex steroids may influence the availability of neurotransmitter precursors.