This is what one of the newer Endocrinology books gives up (David J. Handelsman, Endocrinology: Adult and Pediatric (Seventh Edition), 2016) -
Nandrolone is a naturally occurring steroid that appears as an intermediate in the conversion of testosterone to estradiol by the aromatase enzyme(601); however, it is not normally present in the human bloodstream. The aromatase enzyme complex undertakes two successive hydroxylations on the angular C19 methyl group of testosterone followed by a cleavage of the C10-C19 bond to release formic acid and aromatize the A ring.(602) Nandrolone represents a penultimate step of the aromatization reaction while it is bound to the enzyme complex, including the C19 methyl group excised but a still nonaromatic A ring. Paradoxically, despite being an intermediate in the aromatization reaction, after parenteral administration nandrolone is virtually not aromatized,(603,604) presumably as a poor substrate with hindered access to the human aromatase enzyme.(605) Nandrolone is susceptible to enzymatic 5α reduction with its 5α reduced metabolites exhibiting moderate androgenic activity.(606) The minimal aromatizability of nandrolone makes it suitable for treatment of osteoporosis in women in whom estrogen therapy is contraindicated because of hormone-sensitive cancers (breast, uterus) or for older women, although virilization limits its acceptability.(607)
