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Let’s clarify this issue once and for all…. Serum/plasma testing [commonly called 'blood testing] for progesterone & Estrogen levels does NOT provide an accurate picture of our hormonal status.

Why?

The serum and saliva progesterone testing levels will not give the same results - because less than 20% of progesterone is found in serum/plasma, whereas 80% of the progesterone are attached to the red blood cell membranes.
[The plasma serum is the white watery liquid portion of the blood without the cellular component of red blood cells and white blood cells.]
Much confusion exists among medical professionals and also the general public about progesterone absorption in the body.
This confusion is based mainly on a misunderstanding of the various test used to measure progesterone levels in the body.

Let us try to clarify the issue!

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Absorption of Trans-dermal Progesterone

Progesterone is a fat loving (lipophilic) molecule. It is for that reason that transdermal progesterone cream is well absorbed through skin into the underlying fat layer. In fact, it is the most fat loving hormone among all the steroid hormones.
From the fat layer, the progesterone gradually attaches itself to red blood cell membranes in capillaries running through the fat tissue.

Only the progesterone transported by red blood cell membranes is readily available to reach all target tissues as well as the saliva.
The progesterone transported on red blood cell membrane is completely bio-available and can be readily measured by saliva testing.

Dr John Lee stated: “Only a small fraction of progesterone is carried by the watery serum. Obviously, serum testing is not a good way to measure trans-dermal progesterone absorption.
Yet, many doctors continue to question the skin absorption of progesterone. A recent example is a report in the April 25, 1998 issue of the Lancet that serum levels did not reflect a substantial rise of progesterone after topical application in postmenopausal women.

This report is being used to argue that progesterone is not well absorbed.
This implication is erroneous. Rather, it means that the authors did not understand the significant difference between serum and saliva progesterone levels. Some even imply that saliva testing is relatively unknown and its reliability is unproven. This is an odd admission since researchers have been using saliva testing for years and a number of laboratories offer routine saliva hormone testing.” Dr John Lee

What does a Conventional "Blood Test" Measure?

The "Blood" tests prescribed by most [uninformed] doctors measure the concentration of progesterone in the watery section of the blood which is called plasma or serum. Serum and plasma, being watery, contain water-soluble (hydrophilic) substances such as water-soluble vitamins, carbohydrates, and proteins. We have already clarified that Serum/plasma does NOT contain fat-soluble (lipophilic) substances.. Sex hormones such as progesterone, estrogen and testosterone are fat-soluble steroids similar to cholesterol and will be attracted to the red blood cell membranes, which is also fatty in nature.

To conclude:

  • Progesterone molecules found in the serum levels (“blood test”) are NOT readily bio-available to attach themselves to the receptors of target tissues. They are on their way to be excreted in the bile via the liver.
  • 80% of Progesterone molecules are bound to red blood cell membranes. As red blood cell membranes are fatty in nature, they will attract the fat-soluble progesterone molecules.
  • The level of bio-available progesterone ready to be used by the body can only be accurately detected by saliva testing.
  • The conventional ‘blood test’ (serum test) for progesterone will NOT detect the great majority of the progesterone present on the whole blood cell membrane.
  • Saliva testing is the gold standard to measure Progesterone levels

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