Progesterone is not only a hormone in and of itself, but can also be used as a building block by the adrenal glands to produce either estrogen or testosterone. While it's not likely that you can meet your estrogen needs solely from progesterone conversion, many women are able to produce enough testosterone when they have an adequate supply of progesterone.
Progesterone, while primarily made by the ovaries, is also manufactured by the adrenal glands. In addition to being used for the production of ovarian hormones, it can also be converted by the adrenals into cortisol, the stress response hormone. Since stress is a survival mechanism, it has a higher priority in the body than fertility or sex. What this means in practical terms is that when we are stressed, our bodies are likely to use our supply of progesterone to create cortisol rather than using it to do progesterone work. Since in surgical menopause our supply of progesterone is limited to adrenal production and any intake rather than feedback-mediated (and hence boostable) ovarian production, this will in turn affect our effective levels of the other hormones. We'll look at this subject in greater detail when we discuss stress.
One word of caution we need to sound on progesterone and its popularizers, most notably Dr. John Lee. With all due respect to Dr. Lee for leading the way in legitimizing this hormone's use, his book must be taken with a grain of salt by women in surgical menopause. While it may or may not be true that women in natural perimenopause can meet their hormone needs by the use of progesterone alone, this is generally not the case in surgical menopause.
While we know of plenty of women who have found his reasoning seductive, as a rule they were not able to stick to progesterone-only HRT. It just doesn't have the desired effect in the absence of estrogen production (remember: in natural menopause, the ovaries continue to produce sub-ovulatory quantities of estrogen). While it could theoretically work in a surgically-menopaused woman with high estrone production by her omentum, we just haven't yet encountered one who has actually pulled it off and felt as though she was fully meeting her needs. While in some instances women who are afraid of estrogen prefer to take only progesterone under the misapprehension that it's "safer because they sell it without a prescription," it cannot generally be considered a full and successful approach for meeting systemic hormone needs in surgical menopause.
What progesterone does
It has a number of metabolic and nutritional effects. It promotes the use of fat for energy, thus opposing the estrogenic tendency to fat storage. It normalizes blood sugar levels, but can cause insulin resistance at high levels by interfering with the action of insulin. It has a thermogenic effect—it makes you warmer by increasing blood flow to the skin. It counters estrogenic binding of zinc and copper, thus normalizing those levels.
Progesterone exerts a diuretic effect, helping to get rid of the fluid bloating that estrogen can cause. At the proper dose level, it is equal in effect to spironolactone, a diuretic used to combat certain types of high blood pressure.
In the brain, progesterone concentrations are up to 20 times higher than in the blood, giving us some idea how important it is there. Progesterone has a soothing effect that is so significant that it is given to treat the (rare) seizures caused by the stimulatory effect of estrogen. Chemically, it can have the same effect as Valium or Xanax or some anesthetic agents. It also exerts a lesser neurovascular effect in decreasing migraines caused by estrogen. It can promote sleep and counteract edginess, anxiety and panic. It contributes to the lessening of the memory problems seen with low hormone levels. It evens moods. In excess, it can cause sedation and depression.
While it has not been demonstrated to have as significant an effect as estrogen on vaginal and urinary tract health, many women report that the addition of progesterone to their HRT does indeed help nourish these tissues. There are progesterone receptors in these areas, so there are grounds to support its action. Part of the effect too may be a result of the "estrogen-sparing" effect whereby progesterone frees up estrogen to circulate elsewhere.
Progesterone is beneficial to thyroid function. It helps keep zinc and potassium in cells, which allows thyroid hormone to enter and be converted into the active form (T3). Given that estrogen inhibits thyroid hormone action, this makes progesterone especially important to women with thyroid dysfunction (and menopausal women are so at risk for this that thyroid testing should be a part of any menopausal workup).
Progesterone in combination with estrogen seems, in some studies, to provide greater cardiovascular benefits than estrogen alone. This is new research and the mechanism is only speculated about, but the benefits do seem to be real. These benefits are not demonstrated by progestins, making the distinction very important in evaluating news articles reporting research results.
Progesterone seems to reduce the severity of allergic reactions and allergies. Women who suddenly seem to develop allergies to everything in sight after a hyst may be demonstrating low levels of progesterone.
The bad news
Given that this is the case, then we need to rethink our use of this hormone. We have come some distance in backing away from the early Lee days, in the late 1990s, when progesterone was touted as totally safe and women were taking humongous doses (100-400 mg transdermally--about four to ten times the current practice). In part, this change is fueled by the less-is-more philosophy, that our primary objective is to mimic natural menopause and expose ourselves to no more risk than the bare minimum to achieve our health goals and meet ongoing body needs.
Because we are finding that lower estrogen doses meet our needs, we are also requiring much less progesterone to defuse the effects of higher estrogen doses. We're reading comments from more surgically-menopaused women finding that the dose of Prometrium, a popular oral progesterone hrt oriented towards women with a uterus who need its protection from endometrial cancer, is enough to put them into symptoms of progesterone excess when taken with today's smaller doses of estrogen. Increasing acceptance and use of testosterone are perhaps also decreasing our need for progesterone, although that may represent switching one risk for another rather than escaping them to any great extent.
Whether or not these lower doses represent any real protection from the risks we're only just beginning to comprehend is anyone's guess. Certainly the conservative position would be to take nothing we don't need, and if we need it, to take only the minimum that is required to meet our needs. This holds with all hormones and so is hardly breaking news, but it's something we need to repeat to ourselves often.
For women with special risks like a history of breast cancer, we're starting to feel that maybe progesterone needs to be weighed seriously on a cost/benefit scale. For women with endo, though, the risks of that disease are much greater with estrogen alone, and so progesterone may well need to remain a cornerstone of control for that disease. Obviously we can all take the message from the present uncertainty that we have an even greater need for increased vigilance with breast exams. And for all those practitioners who recommended rubbing progesterone cream on fibrocystic breasts, we have to suggest that this sounds like an unsupportably dangerous idea. At the very least, keeping progesterone away from breast tissue just as carefully as we do with estrogen seems like a very prudent idea.