Sources/Excerpts: Oral Progesterone Achieves Very Low Levels of Progesterone and Has Only Weak Progestogenic Effects

Aly

Sources/Excerpts: Oral Progesterone Achieves Very Low Levels of Progesterone and Has Only Weak Progestogenic Effects

By Aly | First published August 4, 2018 | Last modified September 25, 2022

Preface

This is a literature sources/excerpts supplement to the main article which can be found here.

Excerpts From Literature Reviews

de Ziegler et al. (1998)

de Ziegler, D., Fanchin, R., de Moustier, B., & Bulletti, C. (1998). The hormonal control of endometrial receptivity: estrogen (E2) and progesterone. Journal of Reproductive Immunology, 39(1–2), 149–166. [DOI:10.1016/S0165-0378(98)00019-9]:

4.2. The oral route paradox and the methodological flaws in progesterone measurement
High levels of progesterone have been reported after oral administration of micronized progesterone (Simon et al., 1993). Yet, despite the high levels reported, the same authors describe incomplete endometrial effects. Today, we believe that the apparent paradox between high progesterone levels and minimal endometrial effects characteristic of oral progesterone has been elucidated by Nahoul et al. (1987, 1993) revealing methodological flaw in the measurement of plasma progesterone after oral administration. Indeed, while direct RIA for progesterone is valid for the measurement of plasma levels in the luteal phase of the menstrual cycle, this is not the case after oral administration. In an elegant paper, Nahoul et al. (1987) showed that because of the high levels of progesterone metabolites encountered after oral administration direct RIA measured values 3–5 times higher than those determined when measurements are conducted after extraction and separation on celite columns using an appropriate assay. Nahoul et al. (1993) showed that plasma levels of progesterone were only minimally elevated after oral ingestion of 100 mg of progesterone (barely reaching 1 ng/ml) and, particularly, were markedly lower than after vaginal administration of the same amounts of progesterone.

de Ziegler & Fanchin (2000)

de Ziegler, D., & Fanchin, R. (2000). Progesterone and progestins: applications in gynecology. Steroids, 65(10–11), 671–679. [DOI:10.1016/S0039-128X(00)00123-9]:

3.1. Oral progesterone
Progesterone is nearly entirely absorbed after oral ingestion when prepared in micronized form. Yet, because of intense inactivation by metabolism during first liver pass, the bioavailability of oral progesterone is notoriously poor at <10% [25]. This poor bioavailability explains that despite the large amounts used (up to 600 mg/day), oral progesterone fails to trigger the full array of endometrial changes seen in the late luteal phase of the menstrual cycle. Particularly, the last step of these changes (the predecidual transformation of stroma cells) fails to be induced by oral progesterone [3].
Some confusion has existed over these findings. Early reports claimed high plasma progesterone levels after oral administration of progesterone. However, fairly low plasma levels of progesterone have been reported when the hormone is given orally, and proper assays are used. Circulating levels of progesterone and its metabolites, determined by sufficiently specific assays, after oral and vaginal administration of 100 mg of progesterone are illustrated in Fig. 4. As can be seen, when taken orally, progesterone accounts for less than 10%, whereas most of ingested progesterone is transformed to 5α-reduced metabolites. The metabolites of progesterone that bind to the GABA_A receptor complex are responsible for drowsiness and other neurologic side effects. Interestingly, however, erroneous readings of circulating progesterone have been obtained after oral intake of progesterone when plasma levels are measured by direct radio-immunoassays. Direct assays have only been validated for measurements of progesterone in the luteal phase of the menstrual cycle. After oral ingestion, the unusually high levels of progesterone metabolites alter the validity of progesterone measurements by direct assays because antibody specificity is insufficient in these unusual circumstances. Conversely, when progesterone is measured after separation on celite columns, plasma progesterone levels are low after oral ingestion; these low levels are concordant with the incomplete secretory transformation of the endometrium observed [25], [26].

Similarly, repeated daily administration of nasal progesterone failed to induce predecidual changes of endometrial stroma, a finding in accordance with the low levels of plasma progesterone achieved (from 2 to 5 ng/ml).

Because transdermal administration of progesterone is impractical, the vaginal route has long been regarded as the best, if not the only, alternate option for delivering progesterone non-orally to women. With the advent of IVF and other assisted reproductive procedures, interest has refocused on vaginal progesterone. Ultimately, the data suggest that vaginal progesterone is more than merely a non-oral alternative. Early work undoubtedly showed the high efficacy of vaginal progesterone at duplicating all the endometrial changes of the luteal phase. We now know, however, that this is not solely dependent on plasma levels, which remain lower than seen in the luteal phase of the menstrual cycle [30,31].

Stanczyk (2000)

Stanczyk, F. Z. (2000). Pharmacokinetics of progesterone administered orally and parenterally. In Sitruk-Ware, R., & Mishell, D. R. (Eds.). Progestins and Antiprogestins in Clinical Practice (pp. 393–400). New York/Basel: Marcel Dekker. [Google Scholar] [Google Books]:

Progesterone is measured by immunoassay methods that include radioimmunoassay, chemiluminescent immunoassay, fluoroimmunoassay, and enzyme immunoassay. […] Although “direct” immunoassays (i.e., without a purification step) are used most commonly for quantifying progesterone in serum or plasma, such assays should not be used when measuring progesterone in samples obtained following exogenous administration of progesterone. This is because high concentrations of both unconjugated and conjugated progesterone metabolites are found in serum following oral progesterone dosing, which results in overestimated progesterone values. To obtain absolute progesterone levels, it is essential to subject samples to purification (e.g., organic solvent extraction and chromatography) before quantification by immunoassay.

Kuhl (2005)

Kuhl, H. (2005). Pharmacology of estrogens and progestogens: influence of different routes of administration. Climacteric, 8(Suppl 1), 3–63. [DOI:10.1080/13697130500148875] [PDF]:

The oral application of progesterone is associated with an extensive metabolism in the gastrointestinal tract and the liver, which results in high, but individually variable, concentrations of circulating metabolites. Consequently, the investigation of the pharmacokinetics of progesterone by means of radioimmunoassay (RIA) may be hampered by falsely high progesterone levels due to a relative pronounced cross-reactivity of progesterone metabolites. Therefore, either the gas chromatography/mass spectrometry (GC/MS) method or RIA after chromatographic separation are suitable for the measurement of progesterone. This problem is less pronounced after vaginal administration of progesterone owing to the relative low degree of metabolism²¹³.

Wheeler & Barnard (2010)

Wheeler, M. J., & Barnard, G. (2010). Immunoassay of Steroids. In Makin, H., & Gower, D. (Eds.). Steroid Analysis (pp. 283–327). Dordrecht: Springer. [DOI:10.1023/b135931_4]:

In addition, there is a more critical requirement for improved antibody specificity in non-extraction procedures because of the presence of relatively high concentrations of steroid conjugates (glucuronides and sulphates) in unextracted serum. This is particularly true in the case of samples from patients undergoing hormone replacement therapy, which may involve the administration of oestradiol, progesterone and/or testosterone in a variety of combinations and by a variety of routes (e.g. intramuscular, oral, nasal, intravaginal or percutaneous). The route of administration profoundly affects the metabolism of the hormone and leads to the formation of particular profiles of steroid conjugates, some of which may cross-react significantly in the immunoassay (Nisbet and Jomain, 1987; Nahoul et al., 1989). In many cases, commercial direct procedures may be totally inappropriate for use in these situations.

Kuhl (2011)

Kuhl, H. (2011). Pharmacology of progestogens. Journal für Reproduktionsmedizin und Endokrinologie [Journal of Reproductive Medicine and Endocrinology], 8(1), 157–177. [URL] [PDF]:

Progesterone is an important intermediate in the ovarian and adrenal steroid synthesis, but larger amounts are produced only in the corpus luteum and the placenta. During the luteal phase, serum concentrations of 25 ng/ml are reached which may increase during pregnancy up to 200 ng/ml. […]
[…] the half-lives [of progesterone in the circulation] are only 6 min (t1/2α) and 42 min (t1/2β). Progesterone is rapidly metabolised, predominantly by reduction of the keto groups and the Δ4- double bond, and the pattern of metabolites depends largely on the route of administration. The oral application of progesterone is associated with an extensive metabolism in the gastrointestinal tract and the liver which results in high, but individually variable concentrations of circulating metabolites. Consequently, the investigation of the pharmacokinetics of progesterone by means of RIA may be hampered by falsely high progesterone levels due to a relatively pronounced cross-reactivity of progesterone metabolites. Therefore, either the GC/MS method or radioimmunoassay (RIA) after chromatographic separation are suitable for the measurement of progesterone. This problem is less pronounced after vaginal administration of progesterone owing to the relatively low degree of metabolism [54].
After oral administration, progesterone can be metabolised to more than 30 metabolites, among which some exert specific physiological activities. The most important pathway is the formation of 5α-pregnanolone and 5β-pregnanolone that exert considerable sedative effects after binding to the GABA_A receptor. Further metabolites were 20-dihydroprogesterone that has 25–50% of the progestogenic potency of progesterone, 11-deoxycorticosterone (DOC) that is a potent mineralocorticoid, 17α-hydroxyprogesterone, and the inactive end-product pregnanediol (Fig. 7).
There are large interindividual differences in the pattern of metabolites circulating after oral administration [55]. The low oral bioavailability could be increased by the use of micronized progesterone suspended in oil and packaged in a gelatine capsule.
Pharmacokinetics A single oral dose of 100 mg progesterone contained in a gelatine capsule led to a rapid rise in serum progesterone as measured by liquid chromatography–mass spectrometry to a peak level of 1.5–2.2 ng/ml after 1–2 h. Thereafter the levels decreased rapidly to baseline levels within 4–6 h [53, 55]. However, determination by means of RIA revealed a mean peak level of 19.4 ng/ml suggesting a high cross reaction of progesterone metabolites [54]. There was a pronounced rise in the serum levels of 5α- and 5β-pregnanolone up to a maximum of 14 ng/ml and 3.6 ng/ml after 2 h. The DOC levels rose from 120 pg/ml to 680 pg/ml after 2 h and decreased rapidly thereafter [56].
The results cast some doubts on the reliability of progesterone determinations by RIA if metabolites are not separated by means of chromatographically in advance.
After oral intake of 200 mg progesterone, the peak levels of progesterone as measured by RIA after 4 h were 12 ng/ml, while 5α- and 5β-pregnanolone reached serum concentrations of 30 ng/ml and 60 ng/ml [55]. Further metabolites were 20-dihydroprogesterone, DOC, 17α-hydroxyprogesterone, and pregnanediol (Fig. 7).
The results of a large prospective study indicate that oral and transdermal treatment with progesterone does not protect from estrogen-induced endometrial cancer in postmenopausal women. Compared with women treated with estrogen-only preparations who showed an elevated relative risk of 2.52 (95%-CI: 1.7–3.57), the risk of endometrial cancer did not differ significantly during therapy with estrogen plus progesterone (relative risk 2.42; 95%-CI: 1.53–3.83). Contrary to this, synthetic progestogens reduced the estrogen-dependent risk significantly [57]. The lack of endometrial protection during oral progesterone therapy may be explained by the low progesterone serum levels measured with reliable methods. The same phenomenon may also explain the results of another cohort study that, in contrast to synthetic progestins, the addition of progesterone to estrogen therapy did not increase the risk of breast cancer [58, 59].
The finding of an elevated risk of endometrial cancer in postmenopausal women during treatment with estrogens and oral progesterone are in contradiction to various trials that did not find any increase in the rate of endometrial hyperplasia in women treated with estrogens and 200 mg sequential progesterone or 100 mg continuous progesterone [60–62]. However, the effect of oral treatment with progesterone on estrogenized postmenopausal endometria is dose-dependent, and during the use of 200 mg no full secretory transformation was observed, whereas the daily dose of 300 mg seems to be appropriate as an alternative to synthetic progestogens for therapy [63].

Kuhl & Schneider (2013)

Kuhl, H., & Schneider, H. P. G. (2013). Progesterone – promoter or inhibitor of breast cancer. Climacteric, 16(Suppl 1), 54–68. [DOI:10.3109/13697137.2013.768806]:

The problem of measurement of progesterone levels by RIA
Oral treatment is associated with a rapid metabolism of micronized progesterone in the intestinal mucosa and during the first liver passage. Although the bioavailability of orally administered progesterone is improved by suspending micronized particles of progesterone in oil, the serum concentrations of progesterone are very low when measured with exact methods like liquid chromatography–mass spectrometry (Table 5) ⁹³. Contrary to this, the metabolism of endogenous progesterone in the luteal phase and of vaginally administered progesterone is low and does not compromise measurement of serum progesterone by RIA methods.
After oral administration of progesterone, however, large amounts of certain progesterone metabolites are circulating which have a more or less pronounced binding affinity to the relatively unspecific antiserum against progesterone used in the RIA. Therefore, if not separated from progesterone, these metabolites will cause falsely high progesterone serum concentrations when using a RIA (Figure 5). Therefore, after oral treatment with micronized progesterone, the separation of progesterone from its metabolites by an appropriate chromatographic system is mandatory ^93–95.
After oral administration of 100 mg and 200 mg micronized progesterone to premenopausal women during the follicular phase, average peak levels of 1.5 ± 0.2 ng/ml and 4.70 ± 1.14 ng/ml, respectively, were measured after 2–3 h using a reliable RIA method after adequate chromatographic separation ⁹⁴.
A direct comparison of the progesterone levels measured by either a direct RIA or liquid chromatography–mass spectrometry revealed that, after vaginal administration of 90 mg of progesterone gel, both methods resulted in similar mean peak levels of 8 ng/ml. Contrary to this, after oral administration of 100 mg progesterone per capsule, the mean peak level of progesterone was measured as 15 ng/ml using the RIA method compared to 2 ng/ml using the gold-standard method of liquid chromatography–mass spectrometry (Figure 5, Table 5) ⁹³.
Micronized progesterone and risk of endometrial cancer
The EPIC cohort study, carried out with 115 000 postmenopausal women, revealed that estrogen-only therapy yielded a 2.52-fold increase in the relative risk of endometrial cancer which was prevented by the addition of progesterone derivatives or nortestosterone derivatives. Contrary to this, progesterone was not able to protect from the development of estrogen-induced endometrial cancer, increasing the relative risk up to 2.42 ⁹⁶.
Randomized trials revealed that, after treatment of postmenopausal women with oral or percutaneous estrogens and cyclic addition of oral micronized progesterone, the rate of endometrial hyperplasia was not elevated and was similar to that in the placebo group ^102,103.
The lack of endometrial hyperplasia was interpreted as evidence for a sufficient protective effect on the endometrium by oral use of micronized progesterone. However, the results of these studies refer to surrogate parameters which cannot be transferred to the clinical endpoint of endometrial cancer. The oral use of daily 200 mg micronized progesterone causes peak levels of progesterone of less than 5 ng/ml, provided that adequate methods are used for measurement of the serum levels ⁹⁴. In face of the large interindividual variations in the pharmacokinetics of progesterone, there will be women with serum progesterone levels too low for a long-term protection of the endometrium ^93–95.
On the other hand, the mitosis rate in invasive breast carcinomas has been observed to reach a maximum during the luteal phase. Very likely, endogenous progesterone in synergism with estradiol is responsible for an increase in mammary epithelial proliferation, vascularity, breast tenderness and mammographic density.
In the 2005 report of the French E3N cohort study, no effect of percutaneous estradiol, with or without the addition of oral micronized progesterone, was found on postmenopausal breast cancer risk, in contrast to an unfavorable effect of combined synthetic progestins. The publication in 2008 revealed that oral micronized progesterone even prevented an estrogen-induced rise in breast cancer risk. In 2009, a new report did not present risk data based on the total case numbers of the three treatment groups. Instead, the cases were divided into eight subgroups according to the gap time between menopause and initiation of HRT and the duration of HRT. Importantly, long-term treatment with estrogen and micronized progesterone was found to increase significantly the risk of breast cancer.
It is hard to believe that progesterone does not promote the estrogen-related risk of breast cancer or even has a favorable effect. […] The oral dose of 100 mg or 200 mg progesterone is too low for endometrial protection in all women, as demonstrated by an increased risk of endometrial cancer. The rapid inactivation of progesterone in the intestinal mucosa and the liver causes very low progesterone serum levels which have been measured using reliable methods (e.g. liquid chromatography–mass spectrometry) instead of a RIA which is compromised by cross-reactions of certain progesterone metabolites. Therefore, a weak proliferative effect of the low progesterone levels may need a longer time interval of oral treatment with micronized progesterone, until an increase in breast cancer can be demonstrated.
The available knowledge suggests that, similar to synthetic progestins, HRT with micronized progesterone may promote the growth of small breast carcinomas which may have developed during the fertile phase of the women. Whether or not there are differences between the various progestogens remains to be elucidated.

North American Menopause Society (2017)

North American Menopause Society. (2017). The 2017 hormone therapy position statement of the North American Menopause Society. Menopause, 24(7), 728–753. [DOI:10.1097/gme.0000000000000921]:

A higher incidence of breast cancer was seen in the WHI for CEE + MPA compared with placebo, but a reduced incidence with CEE alone (Figure 1).¹⁴ Observational studies have suggested that the risk of breast cancer may be less with the use of micronized progesterone (MP) compared with synthetic progestogens,^15,16 but the bioavailability of oral and transdermal progesterone is poor.
Micronized progesterone needs to be adequately dosed for endometrial protection.^17-19 Improperly formulated or dosed or delivery issues with estrogen plus MP combinations have potentially serious health consequences, including increased risk of endometrial neoplasia.²⁰ In women using EPT, unscheduled bleeding occurring more than 6 months after initiation should be investigated.

In a meta-analysis of trials of women who began HT treatment fewer than 10 years after menopause onset or who were aged younger than 60 years, strong evidence of increased risk of VTE was found in the HT group compared with placebo (RR 1.74; 95% CI, 1.11-2.73).¹⁷⁷ […] Micronized progesterone may be less thrombogenic than other progestins.¹⁹⁰ […]

Some but not all observational data suggest that MP may have less effect on breast cancer risk, whereas more potent progestogens such as MPA may have a more adverse effect,^15,204 but randomized trials are needed.

Davey (2018)

Davey, D. A. (2018). Menopausal hormone therapy: a better and safer future. Climacteric, 21(5), 454–461. [DOI:10.1080/13697137.2018.1439915]:

Oral and vaginal micronized progesterone
Oral micronized progesterone
In postmenopausal women with an intact uterus receiving estrogen, the addition of progesterone or progestins is regarded as essential to prevent endometrial hyperplasia and carcinoma. The combination of a progestin with estrogen increases the risk of breast carcinoma and VTE and the risk varies with the type of progestin. Oral micronized progesterone (MP) has been claimed not to increase the risk of breast cancer. When administered orally, MP is rapidly metabolized in the intestinal mucosa and the liver, and the plasma levels of progesterone are very low when measured by specific liquid chromatography and mass spectrometry (LC-MS)³¹.
Oral administration of MP 100 mg daily results in peak levels of less than 2.2 ± 3.06 ng/ml measured by LC-MS. With the doses of MP currently used clinically, the concentrations of MP in the plasma may be insufficient to prevent endometrial hyperplasia and carcinoma when estrogens are given in the short term but may increase the risk when given in the long term (more than 5 years)³². At the same time, the low plasma concentrations of MP following oral administration have may have a weak effect on breast tissue and may not increase the risk of breast carcinoma in the short term but may increase the risk in the long term.
The low plasma concentrations of MP may also have less effect on blood coagulation and on the risk of VTE. The claim that MP does not cause endometrial hyperplasia and carcinoma and is not associated with an increased risk of breast cancer and VTE, however, has been disputed and is discussed in the following sections.
Oral micronized progesterone, oral progestins, endometrial hyperplasia and carcinoma
After a review of 40 studies, an expert committee concluded that oral MP, if applied sequentially for 12–14 days/month at 200 mg/day, provides endometrial protection for up to 5 years³³. In the European EPIC study of 115 474 postmenopausal women in Europe, the risk of endometrial carcinoma was increased both in current estrogen-only users (RR 2.52, 95% CI 1.77–3.57) and in current estrogen–progestin users (RR 1.41, 95% CI 1.08–1.83)³⁴. In estrogen–progestin users, the risk of endometrial carcinoma depended on the type of progestin, the regimen – sequential or continuous – and duration of use. The risk of endometrial carcinoma was not increased in synthetic progestin users but was significantly increased in MP users (RR 2.42, 95% CI 1.53–3.8).
In an analysis of the 65 360 women in the French cohort of the EPIC study, the risk of endometrial cancer was increased in estrogen plus MP users (RR 1.80, 95% CI 1.38–2.34) compared with never users and increased with increased duration of use: <5 years, RR = 1.3 (95% CI 0.99–1.97), and >5 years, RR = 2.66 (95% CI 1.87–3.77). The risk of endometrial cancer with the use of estrogens and progestins other than MP was not increased35.
In a systematic review of 28 studies, continuous combined estrogen–progestin therapy had a lower risk of endometrial cancer than sequential estrogen–progestin therapy. The risk of endometrial cancer was increased with MP given either continuously or sequentially³⁶. The claim that oral MP can prevent the increased incidence of endometrial hyperplasia and carcinoma in postmenopausal women treated with estrogens has not been substantiated.
Oral micronized progesterone, oral progestins and breast cancer
Breast carcinoma is the most common carcinoma in women and an increase in the risk of breast cancer is the most serious risk associated with MHT. The WHI trial of CEE + MPA was terminated prematurely because of the increased risk of breast cancer (RR 1.26, 95% CI 1.00–1.59)². In the CEE-only arm of the WHI trial, in contrast, the risk of breast cancer was decreased (RR 0.77, 95% CI 0.59–1.01)³.
The Million Women Study reported that the risk of breast cancer was increased both in estrogen–progestin users (RR 2.00, 95% CI 1.88–2.12) and in estrogen-only users (RR 1.30, 95% CI 1.21–1.45) and that the increase in risk for combined estrogen–progestin users was significantly greater than in estrogen-only users³⁷.
The UK Generations Study of 58 148 menopausal women followed for 6 years (median 5.4 years) found that the risk of breast cancer was increased in current estrogen plus progestogen users (RR 2.74, 95% CI 2.05–3.6) but was not increased in estrogen-only users (RR 1.00, 95% CI 0.66–1.54)³⁸.
In the first report of the French cohort of the E3N-EPIC study in 2005, in 54 548 postmenopausal women with a mean duration of use of MHT of 2.8 years, the risk of breast cancer was found not to be significantly increased with MHT with MP (RR 0.9, 95% CI 0.7–1.2) but was increased with MHT containing synthetic progestins (RR 1.4, 95% CI 1.2–1.7)³⁹. In a later report of the E3N study in 2009, the risk of breast cancer was found to be increased with MHT with MP if MHT was initiated in the 3-year period following onset of the menopause and continued for 5 or more years (RR 1.54, 95% CI 1.28–1.86) but was not increased if initiated after 3 years (RR 1.00, 95% CI 0.68–1.47)⁴⁰.
A separate French CECILE case–control study of 1555 menopausal women (739 cases and 816 matched controls) found that, compared with never use, the risk of breast cancer was increased with current use of estrogen plus synthetic progestins for 4 or more years (RR 2.07, 95% CI 1.26–3.39) but was not increased with estrogen plus MP for the same period (RR 0.79, 95% CI 0.37–1.71)⁴¹.
A number of factors influence the increase in risk of breast cancer associated with MHT including the interval between the menopause and starting MHT (gap time), duration of MHT, and body weight and body mass index; the interpretation of the effect of MP and progestins on risk of breast cancer may be difficult. It has been suggested that the low plasma concentrations of MP given orally may have a weak effect on breast tissue and may not increase the risk of breast carcinoma over short periods, but may increase the risk with longer periods of 5 years or more³². MHT is usually initiated for the relief of menopausal symptoms within a year or so of the menopause and it is often necessary to continue MHT for more than 5 years. An increased risk of breast cancer with oral MP given for 5 years or more cannot be ruled out, and a possible reduction in risk of breast cancer with MP cannot be regarded as a reason for preferring MP to progestins in MHT in the light of the increased risk of endometrial hyperplasia and carcinoma with MP.
Oral micronized progesterone, progestins and venous thromboembolism
Most studies have shown an increased risk of VTE with MHT with combined estrogen and progestin compared with estrogen only, and the risk appears to vary with different progestins. In the CEE + MPA arm of the WHI trial, the risk of VTE in women age 50–59 was increased (RR 1.27, 95% CI 1.19–4.33), but in the CEE-only arm the risk was not significantly increased (RR 1.22, 95% CI 0.62–2.42)^2,3.
The UK NHS record linkage study reported that the risk of VTE was significantly greater for estrogen–progestin than for oral estrogen-only therapy (RR 2.07, 95% CI 1.86–2.31 vs. RR 1.42, 95% CI 1.21–1.66). The risk of VTE with MPA was greater (RR 2.67, 95% CI 2.25–3.17) than with other progestins (RR 1.91, 95% CI 1.69-2.17), heterogeneity = 0.0007²³.
A Dutch study found that the risk of VTE was increased with oral CEE and MPA (RR 4.0, 95% CI 1.8–8.2) and with oral estradiol plus norethisterone acetate (RR 3.9, 95% CI 1.5–10.7) compared with oral estrogen-only MHT and that there was no significant difference between the progestins²⁴.
In a Swedish study, the risk of VTE in combined estrogen–progestogen users was double that of estrogen-only users (RR 2.18, 95% CI 1.21–3.92, p = 0.009). The risk was increased by both medroxyprogesterone acetate (MPA) (RR 2.94, 95% CI 1.67–5.36) and norethisterone acetate (RR 2.25, 95% CI 1.50–3.40) and there was no significant difference between the progestins²⁵.
It has been claimed that the risk of VTE is less with MHT with MP than with other progestins. In the E3N study of 80 308 postmenopausal women with 549 cases of incident VTE, the risk for VTE was not significantly increased with the use of estrogens combined with MP (RR 0.9, 95% CI 0.6–1.5), nortestosterone derivatives (RR 1.4, 95% CI 0.7–2.4) or pregnane derivatives including MPA (RR 1.3, 95% CI 0.9–2.0), compared with oral estrogens only but was increased with norpregnane derivatives combined with estrogens (RR 1.8, 95% CI 1.2–2.7)²¹.
In the four studies of transdermal estrogens with oral MP or progestins, the risk of VTE was not increased^20,23–25. The use of transdermal rather than oral estrogens in all women receiving MHT would obviate any possible increased risk of VTE with all types of progestins as well as with MP and is another good reason for using transdermal estrogens in preference to oral estrogens in all perimenopausal and postmenopausal women.

Kuhl & Wiegratz (2021)

Kuhl, H., & Wiegratz, I. (2021). Pharmakokinetik und Pharmakodynamik der in der Assistierten Reproduktion Verwendeten Gestagene. [Pharmacokinetics and Pharmacodynamics of Progestogens Used in Assisted Reproduction.] Gynäkologische Endokrinologie [Gynecological Endocrinology], 19, 105–117. [Google Scholar] [DOI:10.1007/s10304-020-00372-5] [Translation] [Translated]:

Abstract
[…] Oral administration of progesterone causes extensive metabolism resulting in the formation of more than 30 metabolites, some of which may cause anesthetic/sedative effects. The use of a direct radioimmunoassay (RIA) without prior separation of progesterone from its metabolites may simulate falsely high serum concentrations of progesterone owing to cross-reaction of some metabolites with the specific antiserum of the RIA. This effect is not observed after parenteral treatment with progesterone due to minimal metabolism. […]
Progesterone pharmacokinetics
Problems with the determination of progesterone in serum
Because of the large number of progesterone metabolites that circulate in variable concentrations after oral administration, false-high progesterone levels must be expected when using an immunoassay. The cause is the cross-reactions of some progesterone metabolites, such as 5α- and 5β-DHP and 5α-pregnan-3β-ol-20-one [13]. For this reason, a radioimmunoassay (RIA) can only be used, for example, if the progesterone has previously been isolated by means of a suitable chromatographic separation. The determination of progesterone with exact liquid or gas chromatography-mass spectrometry (LC-MS or GC-MS; [14]; Table 1) is more reliable, albeit more complex.
The problem of cross-reactions does not play an essential role in parenteral progesterone treatment due to the low metabolism of progesterone. The specificity of the antiserum used is of particular importance for the comparability of the results of different RIAs, as it influences the cross-reactivity of the numerous progesterone metabolites [11].
The determination of the progesterone level, which was measured after oral administration of a capsule containing 100 mg of micronized progesterone, resulted in a false high peak value of 19.4 ng/mL in the RIA, which was 8 times as high as that with the exact LC-MS method which determined a true value of 2.4 ng/mL (Table 1). The cause was metabolites, the high concentrations of which simulated a false high concentration via a cross-reaction with the RIA antiserum [11]. In contrast, after intravaginal application of a gel containing 90 mg of progesterone, the metabolism of progesterone and thus the influence of cross-reactions is so low that the progesterone concentration of 10.5 ng/mL measured with the RIA agrees with the result of the exact LC-MS method (Table 1; [11]).
“Enzyme-linked immunosorbent assay”
As with the RIA, the “enzyme-linked immunosorbent assay” (ELISA) is a procedure based on an immune reaction between an antigen and a specific antibody. For this purpose, a specific antibody is produced in a separate standard process for the antigen, for example progesterone, which binds the antigen with high affinity. An enzyme is coupled to this specific antibody that cleaves an added chromogen, producing a dye, the intensity of which is measured in a photometer. The measured intensity of the color correlates with the concentration of the antigen (for example progesterone).
With regard to the cross-reaction of some progesterone metabolites, which leads to false high values of the progesterone concentration, the same restriction applies to the ELISA as to the direct RIA (Table 1).
Summary
If a progesterone determination is carried out in the serum after oral progesterone application in the RIA or in the ELISA without prior chromatographic separation, one measures incorrectly high progesterone values, since progesterone metabolites are also measured! This problem does not exist with parenteral application (intravaginal, intramuscular, or subcutaneous).
Oral application of progesterone
After separation of important progesterone metabolites by means of column chromatography, an increase in progesterone in the serum to a maximum of 4.7 ± 1.15 ng/mL was found in the RIA after oral administration of 200 mg micronized progesterone [14].
Summary
In summary, it can be stated that oral administration of 100 to 200 mg progesterone leads to an increase in serum levels to around 3–5 ng/mL, provided the laboratory diagnostics are carried out correctly, i.e. after chromatographic separation.
Conclusion for practice
After oral progesterone administration, more than 30 metabolites are formed, which in immunoassays can cause false high results of the systemic progesterone concentration. This problem can only be solved by prior chromatographic separation of the progesterone or by direct measurement methods.

Individual Studies and Papers

Comparative Immunoassay versus Immunoassay with Chromatography or Mass Spectrometry Studies

Nahoul, K., Dehennin, L., & Scholler, R. (1987). Radioimmunoassay of plasma progesterone after oral administration of micronized progesterone. Journal of Steroid Biochemistry, 26(2), 241–249. [DOI:10.1016/0022-4731(87)90078-1]
Levine, H., & Watson, N. (2000). Comparison of the pharmacokinetics of Crinone 8% administered vaginally versus Prometrium administered orally in postmenopausal women. Fertility and Sterility, 73(3), 516–521. [DOI:10.1016/S0015-0282(99)00553-1]
Sapin, R., Neamtu, D., Gasser, F., Ohl, J., Grunenberger, F., & Grucker, D. (2000). De la prudence lors de l’utilisation des dosages directs de progestérone. [Caution when using direct progesterone assays.] Immuno-analyse et Biologie Spécialisée, 3(15), 203–204. [DOI:10.1016/S0923-2532(00)80010-1] [Translation]

Non-Comparative Immunoassay with Adequate Chromatography or Mass Spectrometry Studies

Nahoul, K., Dehennin, L., Jondet, M., & Roger, M. (1993). Profiles of plasma estrogens, progesterone and their metabolites after oral or vaginal administration of estradiol or progesterone. Maturitas, 16(3), 185–202. [DOI:10.1016/0378-5122(93)90064-O]
Lobo, R. A., Liu, J., Stanczyk, F. Z., Constantine, G. D., Pickar, J. H., Shadiack, A. M., Bernick, B., & Mirkin, S. (2019). Estradiol and progesterone bioavailability for moderate to severe vasomotor symptom treatment and endometrial protection with the continuous-combined regimen of TX-001HR (oral estradiol and progesterone capsules). Menopause (New York, NY), 26(7), 720–727.[DOI:10.1097/GME.0000000000001306]

Commentaries on Studies

Nahoul, K., & de Ziegler, D. (1994). “Validity” of serum progesterone levels after oral progesterone. Fertility and Sterility, 61(4), 790–791. [DOI:10.1016/S0015-0282(16)56666-7]