The Endocrine Society position statement was discussed in this thread. www.mumsnet.com/Talk/womens_rights/3228447-Transing-children
I went through their reference list to see if there was any evidence for their claims - there wasn't.
The first reference, which they use to support their statement that "Data are strong for a biological underpinning to gender identity" is a very short literature review (Saraswat A, et al. Evidence Supporting the Biologic Nature of Gender Identity. Endocr Pract. 2015 Feb;21(2): 199-204.)
It concludes: 'Because the sample sizes of most studies on this subject were small, the conclusions must be interpreted with caution. Further research is required to assign specific biologic mechanisms for gender identity.'
You cannot conclude from this that "Data are strong for a biological underpinning to gender identity".