The Effects of Sex-Steroid Administration on the Pituitary-Thyroid Axis in Transsexuals

Eur J Endocrinol. 2006 Jul;155(1):11-6. doi: 10.1530/eje.1.02192.

Abstract

Objective: Estrogen and androgen administration modulate the pituitary-thyroid axis through alterations in thyroid hormone-binding globulin (TBG) metabolism, but the effects of sex steroids on extrathyroidal thyroxine (T4) to triiodothyronine (T3) conversion in humans are unknown.

Design and methods: We studied 36 male-to-female and 14 female-to-male euthyroid transsexuals at baseline and after 4 months of hormonal treatment. Male-to-female transsexuals were treated with cyproterone acetate (CA) 100 mg/day alone (n = 10) or in combination with either oral ethinyl estradiol (or-EE) 100 microg/day (n = 14) or transdermal 17beta-estradiol (td-E) 100 microg twice a week (n = 12). Female-to-male transsexuals were treated with i.m. testosterone 250 mg twice a week. A t-test was used to test for differences within groups and ANOVA with post hoc analysis to test for differences between the groups.

Results: Or-EE increased TBG (100 +/- 12%, P < .001) and testosterone decreased TBG (-14 +/- 4%, P = 0.01), but free T4 did not change. Td-E and CA did not affect TBG concentrations. TSH was not different between groups at baseline or after treatment. CA decreased T3/T4 ratios (-9 +/- 3%, P = 0.04), suggesting that T4 to T3 conversion was lower. Testosterone increased T3/T4 ratios (30 +/- 9%, P = 0.02), which probably reflects higher T4 to T3 conversion.

Conclusion: Oral but not transdermal estradiol increases TBG, whereas testosterone lowers TBG. Testosterone increases T3/T4 ratios. Estradiol does not affect T3/T4 ratios, irrespective of the route of administration.

Publication types

  • Randomized Controlled Trial

MeSH terms

  • Adult
  • Androgen Antagonists / pharmacology
  • Androgens / pharmacology
  • Cyproterone / pharmacology
  • Estrogens / pharmacology
  • Female
  • Gonadal Steroid Hormones / pharmacology*
  • Humans
  • Iodide Peroxidase / metabolism
  • Male
  • Ovary / physiology
  • Pituitary Gland / drug effects*
  • Testis / physiology
  • Thyroid Gland / drug effects*
  • Thyroxine / blood
  • Transsexualism / metabolism*
  • Triiodothyronine / blood

Substances

  • Androgen Antagonists
  • Androgens
  • Estrogens
  • Gonadal Steroid Hormones
  • Triiodothyronine
  • Cyproterone
  • Iodide Peroxidase
  • Thyroxine