First of all, hormone replacement therapy is a term that confuses many of us. “Hormone replacement therapy? Does that mean estrogen?” is a question I still hear. The answer is yes and no. Yes, HRT includes estrogen. No, not estrogen alone, unless your uterus has been removed by a hysterectomy. Even if your uterus has been removed, you and your health care provider may decide that estrogen and other hormones are necessary.

A typical HRT regimen includes:

- a type of estrogen

- a type of progesterone, either natural progesterone or one of the synthetic progestins

- if needed, an androgen such as testosterone

History accounts for some of the confusion between “hormone replacement” and what was once called “estrogen replacement.” In the 1960s and early 1970s, estrogen was prescribed by itself to women who had perimenopausal symptoms. In fact, just about every woman who complained of a hot flash was put on estrogen replacement therapy. Estrogen taken alone, without natural progesterone or a synthetic progestin, is called unopposed estrogen replacement therapy.

Prescribing unopposed estrogen replacement therapy became a routine practice, until an alarming trend started to emerge in the early 1970s: more women taking estrogen were developing uterine abnormalities. Something clearly was wrong.

Estrogen was identified as the culprit, and it was discovered that estrogen alone can stimulate the growth of irregular or even precancerous cells in the uterus. Without progesterone, estrogen causes the uterine lining (endometrium) to build up instead of sloughing regularly. The cells become increasingly crowded and may become misshapen or malformed. Cell changes could result that are potentially dangerous, possibly leading to cancerous conditions.

The discovery of the link between estrogen-only regimens and increased risk of uterine cancer set off widespread alarm. In an abrupt reversal of the promise that estrogen would keep them forever young, women were taken off estrogen en masse. In my view, this was regrettable for three reasons. First, many women really benefited from estrogen. Second, in many cases estrogen therapy was stopped before medical professionals understood that the missing piece of the puzzle was progesterone. Third, anxiety and confusing information about HRT and cancer persist today, even though we now know that a combination of estrogen and progesterone mirrors the body’s natural balance and avoids estrogen-only promotion of irregular uterine cell growth.

Today HRT is different from the estrogen-only approach of the 1960s and early 1970s. For one thing, estrogen dosages, both in HRT regimens and in oral contraceptives, are significantly lower. For another, estrogen is now combined with natural progesterone (or a synthetic progestin) for women who still have their uterus intact. While the estrogen causes the uterine lining to thicken, the progesterone ensures that the lining is regularly sloughed in the form of a menstrual flow.

(A note here for women who have had a hysterectomy and therefore does not have a uterus. Estrogen alone poses no risk of uterine cancer after hysterectomy because there is no uterine lining to be over stimulated. However, some women who have had hysterectomies still choose to take estrogen with natural progesterone because natural progesterone has bone-building properties and can also have a calming effect on mood.)