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Effect of Long-Term Administration of Cross-Sex Hormone Therapy on Serum and Urinary Uric Acid in Transsexual Persons

Clinical Laboratory (R.Y.), Departments of Endocrinology (I.E., M.C.A., F.S., I.G.) and Psychiatry (J.M.), Endocrinology Research Laboratory (J.J.H.-M., S.M., G.R.-M., J.M.G.-Z.), and family physician (V.H.), Andalusian Gender Team, Carlos Haya University Hospital, Block C, 29010 Málaga, Spain

Address all correspondence and requests for reprints to: Raquel Yahyaoui, Avenida Moliere 22, 2E, 29004 Malaga, Spain. E-mail: yayiobo{at}hotmail.com.

Background: Transsexual persons afford a very suitable model to study the effect of sex steroids on uric acid metabolism.

Design: This was a prospective study to evaluate the uric acid levels and fractional excretion of uric acid (FEUA) in a cohort of 69 healthy transsexual persons, 22 male-to-female transsexuals (MFTs) and 47 female-to-male transsexuals (FMTs). The subjects were studied at baseline and 1 and 2 yr after starting cross-sex hormone treatment.

Results: The baseline levels of uric acid were higher in the MFT group. Compared with baseline, uric acid levels had fallen significantly after 1 yr of hormone therapy in the MFT group and had risen significantly in the FMT group. The baseline FEUA was greater in the FMT group. After 2 yr of cross-sex hormone therapy, the FEUA had increased in MFTs (P = 0.001) and fallen in FMTs (P = 0.004). In MFTs, the levels of uric acid at 2 yr were lower in those who had received higher doses of estrogens (P = 0.03), and the FEUA was higher (P = 0.04). The FEUA at 2 yr was associated with both the estrogen dose (P = 0.02) and the serum levels of estradiol-17β (P =0.03). In MFTs, a correlation was found after 2 yr of therapy between the homeostasis model assessment of insulin resistance and the serum uric acid (r = 0.59; P = 0.01).

Conclusions: Serum levels of uric acid and the FEUA are altered in transsexuals as a result of cross-sex hormone therapy. The results concerning the MFT group support the hypothesis that the lower levels of uric acid in women are due to estrogen-induced increases in FEUA.