There
is said to be variability over the life of each patch & even at different times of day. Some of us try to compensate for the fading effect toward the end of patch life by changing every 3 days instead of 3.5 or 3+4, so similar to what Salad was recommended to do. There are also other factors affecting levels, such as site of application, amount of subcutaneous fat, blood flow etc. The extract below from the following link details some of the variables involved.
https://en.wikipedia.org/wiki/Pharmacokinetics_of_estradiol"
Variability in pharmacokineticsTransdermal estradiol patches are described as delivering a fixed amount of estradiol such as 50 µg/day or 100 µg/day.[10] However,
there is large interindividual variability and intraindividual variability in the pharmacokinetic parameters of transdermal estradiol, and fluctuations in circulating estradiol levels with estradiol patches are almost as great as with oral estradiol.[10][93][12][17] As such, the actual delivery rate of estradiol and mean levels of estradiol achieved with transdermal estradiol patches may be different from what is described and from the mean levels observed in clinical studies, respectively.[10]
A wide range of estradiol levels are measured in women using the same estradiol patch or gel and dosage, with an up to about 10-fold difference in levels.[10][93][17] In a study of estradiol gel and patches, the maximal difference in peak levels between individuals was 11-fold for the gel and 7-fold for the patch, and the maximal difference in area-under-the-curve levels (total exposure) was 6-fold for the gel and 8-fold for the patch.[93] It has likewise been reported that the interindividual variability in bioavailability with Estraderm reservoir patches ranges from 25 to 225%.[17] In as many as 30% of women treated with a 50 µg/day estradiol patch, estradiol levels are low.[10]
There are also significant short-term intraindividual differences in estradiol levels with estradiol patches; estradiol levels can fluctuate considerably from hour to hour.[10][181]
In addition, estradiol levels with estradiol patches are higher in the evening than in the morning, which may be due to circadian variations in skin blood flow that may influence absorption.[10] In terms of area-under-the-curve levels of estradiol, the interindividual variability of transdermal estradiol has been found to be 20 to 44% using different transdermal formulations, and the intraindividual variability with transdermal estradiol has been found to be 20%.[12]"
There are graphs in the full article showing variation over time for anyone who finds them helpful.
It does all seem complex & there's no replicating the way our own bodies fine tune hormonal regulation before ovarian production begins to decline, but there is at least the option of trying different brands & doses to see what suits us best.
Wx