As estrogen levels wax and wane over the course of the month, a woman's cholesterol readings may do the same. This could make cholesterol tests less accurate at certain times of the month, according to the researchers, a fact which might need to be taken into account in the future.

The research team studied this phenomenon in 259 women who ranged in age from 18 to 44. They measured the women's estrogen, cholesterol, and triglyceride (blood fat) levels multiple times over the course of two menstrual cycles. The participants used at-home fertility tests to keep track of where they were in their cycles, and the likely time of ovulation (estrogen peaks just before ovulation takes place).

The women's total cholesterol levels, as well as their LDL (or "bad") cholesterol alone, were initially higher during the first half of the menstrual cycle, known as the follicular phase. In fact, more women were classified as having borderline high cholesterol (above 200 mg/dl) during this early part of the cycle, even though the rise in LDL and total cholesterol was mild (5-8%).

However, as estrogen levels began to peak as ovulation drew near, LDL and total cholesterol levels actually fell. This indicates that estrogen may have a favorable effect on the kind of cholesterol we want to keep down. More evidence for this idea comes from the fact that women's HDL ("good") cholesterol levels also fluctuated throughout the cycle. But here, the "good" cholesterol rose right along with estrogen levels and peaked right around the time of ovulation.

Sunni L. Mumford and colleagues at the University of Buffalo conclude that "[e]strogen...appears to have beneficial effects on the lipid profile" of women across their monthly cycles. But whether this will change how doctors do business will remain to be seen. Doctors may simply need to keep in mind that readings should be taken at a particular point during a woman's cycle, particularly if a woman requires repeated cholesterol testing.

The study was published in the Journal of Clinical Endocrinology and Metabolism's June 9, 2010 online edition.