Role of Steroid Hormones in Fibroid Formation Oestrogen
Until recently, the steroid hormones, estrogen and progesterone were considered the most important regulators of fibroid growth and oestrogen especially promotes fibroid growth.
This would explain why they appear during a woman's middle reproductive years (when oestrogen levels are high) and stop growing after the menopause (when oestrogen levels drop) or during gonadotropin releasing hormone (GnRH) agonist therapy (in the presence of low levels of estrogen).
Also women who weigh over 70kg may be more likely to have fibroids.
This is thought to be due to higher levels of oestrogen in heavier women.
In the past, the contraceptive pill was thought to increase the risk of fibroids, but that was when the pill contained higher levels of oestrogen than it does today.
Some studies suggest that the newer combined pill (oestrogen and progestogen) and the mini pill (progestogen only) may actually help prevent or slow the growth of fibroids.
have higher estrogen concentrations
bind more estrogen
have more estrogen receptors
and convert estradiol (a more active form of estrogen) to estrone (a less active form of estrogen) more slowly than normal myometrium
Progesterone is also thought to play a role in fibroid growth.
Progesterone seems to facilitate the growth of fibroids.
For example, fibroid size increases during treatment with synthetic progesterones.
Fibroids regress with the antiprogesterone agent, RU-486, further supports the role of progesterone as a promoter of fibroid growth.
Fibroids have higher progesterone receptor concentrations than normal myometrium.
Growth hormone (GH) and prolactin (PRL) are also thought to promote fibroid growth, but their role is less well defined.
More recently, growth factors (small proteins that affect cell growth), have been shown to mediate the growth-promoting effects of estrogen and to play an important role in the development of fibroid tumors.
There are other potentially important factors in fibroid growth:
transforming growth factor-beta,
basic fibroblast growth factor,
epidermal growth factor,
insulin-like growth factor,
platelet-derived growth factor.