Sexually dimorphic features in transsexuals – a female 2nd to 4th finger (2D/4D) ratio in male-to-female transsexuals

Transsexualism is characterised by the feeling of belonging to the wrong gender and the strong wish to achieve gender reassignment by hormonal and surgical treatment in combination with normal body appearance. It has been found that an androgen-dependent, sexually dimorphic nucleus in the hypothalamus which is involved in gender-specific behavior, the BSTc, has the size of females in male-to-female transsexuals (MFT). The ratio of digit 2 to digit 4 in the right hand (2D/4D ratio) has been shown to be sexually dimorphic with a smaller ratio in men than in women and to correlate negatively with testosterone and sperm count. Moreover, in patients with CAH, a condition characterized by elevated prenatal androgen levels, the 2D/4D ratio is lower. Since most sexually dimorphic feautures in the body are assumed to be regulated by prenatal testosterone levels, the D2/D4 ratio has been suggested to be a marker for prenatal testosterone exposure.

We have assessed the D2/D4 ratio in 63 MFT, 43 female-to-male transsexuals (FMT), 65 females (F) and 58 males (M). As expected, the D2/D4 ratio in the right hand was larger in F than in M. The 2D/4D ratio was in MFT was neither significantly different to M nor to F, however, when all non-right handed subjects were excluded, MFT had a significantly higher 2D/4D ratio than M (p=0.01). The right–hand 2D/4D ratio in FMT was also not different to F but significantly higher than in M. There were no significant differences in the left-hand 2D/4D ratios.

These results to our knowledge show for the first time that external sexually dimorphic female-like features exist in MFT. The fact that these differences are only significant in the right hand are consistent with previous findings from other groups. As to wether a higher 2D-4D ratio indicates lower prenatal testosterone exposure remains subject of debate. If this was the case, our findings point to lower prenatal androgen levels in males that will become transsexuals than in non-transsexuals. However, our data do not support a role of prenatal androgens in female transsexualism.