A study showing that Nettles extract inhibits enzymes such as 5 alpha reductase (the enzyme that causes testosterone to convert to DHT). It is the DHT metabolite of testosterone that is known to cause excess hair loss at the top of the head.
R. Hartmann et al.
The extract of nettle root (Urtica dioica L., Urticaceae) partially blocked the action of two enzymes involved in the body's production of dihydrotestosterone and estrogen. The in vitro (laboratory) study showed that nettle root extract was effective in inhibiting these two enzymes (5alpha-reductase and aromatase).
Dr. A Vogel, a Swiss herbalist know throughout the world, advocates Nettle for stimulating hair growth and Dr. Rudolf Fritz Weiss, M.D., the dean of German herbal physicians and author of Herbal Medicine also supports the use of Nettles for those with thinning hair.