Final answer:
Low levels of the hormone estrogen are detrimental to bone health because it stimulates osteoblast activity.
Step-by-step explanation:
The hormone estrogen is detrimental to bone health when present in low levels because it stimulates osteoblast activity. Osteoblasts are responsible for the formation of new bone. Lack of estrogen may decrease the normal deposition of new bone, leading to impaired bone formation.