Put it under your tongue? Typical hormone levels for trans women on HRT, and differing effects of oral and parenteral estradiol

Last year, I covered the results of a study of trans men and transmasculine people who had been on masculinizing HRT for at least a year, which were used to establish reference intervals empirically and determine which range of testosterone levels encompasses the middle 95% of trans men on HRT. These reference ranges for laboratory blood tests determine whether a given measurement for a patient is “normal”, or within two standard deviations of the population average.

In the previous study, the reference interval for testosterone found among trans men on HRT spanned a much wider range than the guideline of 400-700 ng/dL that the Endocrine Society most recently recommended as a target range for testosterone treatment. Instead, only about the middle 50% of trans men had testosterone levels measured within that range. The authors noted that this may not be an issue requiring dose adjustment, as the wide variations in timing between the blood draw and the injection of intramuscular T – which can be given every week, every two weeks, or even every three months – can significantly influence the measured levels even when these men may still have levels in the guideline target range at most times.

Now, these researchers have published another study focusing on sex hormone levels in trans women and transfeminine people on various feminizing HRT regimens. In Greene et al. (2020), 93 adult trans women in the United States who had been taking estrogen for at least a year had measurements taken of reproductive hormone levels, with the distribution of these levels being used to establish reference intervals for this population. For comparison, the Endocrine Society clinical guidelines for feminizing HRT recommend target ranges of 100-200 pg/mL estradiol, and less than 50 ng/dL total testosterone. Similar to the findings of trans men’s typical ranges of testosterone, many trans women also had sex hormone levels falling outside of these recommended ranges. The central 95% of trans women in the study were found to have estradiol levels ranging from 16.5-681 pg/mL, and total testosterone from 4-629 ng/dL. The middle 50% of values more closely aligned with clinical target ranges, spanning 88-254 pg/mL estradiol and 4-50 ng/dL testosterone. The authors suggest that, for clinical purposes in evaluating and adjusting trans women’s HRT regimens, these reference ranges should be applied more conservatively than the traditional “middle 95%” calculation:

Due to variations in hormone dose, interval, and mode of hormone administration, and general inter-individual biological variability, the calculated intervals are fairly wide (Table 1). Therefore, the confidence intervals should be considered when choosing the most clinically appropriate reference interval to implement. For example, the exact 2.5/97.5 percentiles for total testosterone would derive a reference interval of 4.0-629 ng/dL. The 95% confidence interval for the upper limit is 227- 1,031 ng/dL. Using a more stringent upper reference limit for total testosterone (such as 225 ng/dL) would likely be more clinically effective – alerting providers to evaluate free testosterone or to adjust hormone dose, as indicated by the specific patient’s presentation.

Trans women’s sex hormone levels were also analyzed between groups of those who were or were not taking the testosterone blocker spironolactone, as well as those who were or were not taking progesterone. There were no significant differences in estradiol or testosterone levels between spironolactone users and nonusers, and progesterone use or nonuse was also not associated with a difference in estradiol or testosterone levels. Those taking spironolactone were found to have lower levels of LH and FSH than nonusers, while those taking progesterone were only found to have higher levels of progesterone. As this was a US-based study, inclusion of trans women using the antiandrogen cyproterone acetate was not possible. Additionally, trans women taking estrogen orally, via injection, or topically were grouped together, and the authors note that as in the study of trans men, timing between doses and blood draws could have a substantial influence here:

First, samples were collected at random intervals, meaning that the wide range of hormone concentrations observed is a function of dose concentration, mode of administration, and the timing since the most previous dose.

However, a followup study of this cohort of trans women examined variations in the sex hormone levels achieved with different routes of administration of estradiol. Cirrincione et al. (2021) found that trans women using transdermal patches achieved the lowest median levels of estradiol (80.1 pg/mL), followed by sublingual tablets (154.4 pg/mL) and injections (192.6 pg/mL). Additionally, among trans women not taking spironolactone, those using injections were found to have significantly lower testosterone levels than those taking tablets. Notably, trans women using tablets were seen to have much higher levels of estrone, a weaker estrogen and a metabolite of estradiol. Those on estradiol tablets had a median of 693 pg/mL estrone, compared to 67.5 pg/mL in those using injections and 58.6 pg/mL in those using transdermal patches – a tenfold difference. It is not known how higher or lower estrone levels may translate to clinical outcomes, and the authors call for further research:

Future studies should investigate clinical implications of elevated estrone, including safety parameters such as thrombogenicity and cardiovascular outcomes, particularly for understudied administration routes such as sublingual and injectable 17b-estradiol. Longitudinal studies are needed to determine whether estrone or the estrone/estradiol ratio enhances feminization in transgender and gender nonbinary adults, and if so, whether this outcome is commensurate with the safety profile of respective 17b-estradiol preparations (e.g., tablets versus transdermal or injectable preparations).

One question this raises is that of perfect use versus typical use of estradiol tablets taken sublingually, that is, absorbed slowly under the tongue. While this has been discussed by some clinicians and the trans community as a way of absorbing estradiol into the bloodstream directly, as opposed to swallowing and digesting tablets where they will be largely metabolized into the weaker estrone, the high estrone levels observed among the “sublingual” group do not appear to reflect this. The authors highlight this discrepancy, noting that among numerous studies of estradiol treatment in postmenopausal cis women, sublingual administration largely resulted in low estrone levels like those seen among trans women in this study using injections or patches, while oral estradiol consistently resulted in high estrone levels (table 2). Conversely, in a previous study of trans women taking various forms of estradiol, those taking it sublingually showed high estrone levels similar to those taking it orally.

A potential explanation: Dissolving estradiol tablets under your tongue is difficult and time-consuming, as many trans women have experienced. Some oral formulations, uncoated and dry, might lurk under your tongue remaining largely intact even after an hour. Is it absorbed? They might break into something like a chalky paste that films the inside of your mouth. Can you swallow it now? Others come with a sweet outer coating that, allegedly, needs to be slurped off to get to the estradiol itself before it can be absorbed sublingually. Is it working? And holding all the twitchy and salivation-prone parts of your mouth perfectly still for an hour – twice a day, every day – can be a real challenge. Is it making a difference?

The authors acknowledge that this difficulty may account for the observed discrepancy:

Sublingual versus oral administration was not distinguished in earlier publications of this cohort, but as per clinical protocols at both study sites, clinicians counsel all patients to dissolve micronized 17b-estradiol tablets under the tongue and assess the administration technique during follow-up visits. These protocols are based on successful sublingual 17b-estradiol use in transgender adults and limited data suggesting that the absorption rate and extent are higher following sublingual than oral administration (p < 0.05). In our study, hormone doses were not directly observed, and the extent to which tablets were absorbed sublingually versus swallowed may have varied. Future studies should account for this source of variability.

In other words, outside of the medical setting, many trans women may either find daily sublingual absorption to be more trouble than it’s worth, or swallow much of it despite attempting to absorb it sublingually, or both. At a minimum, these results show that consistently succeeding at largely sublingual absorption of oral estradiol tablets is not a straightforward matter, and in practice, attempting sublingual absorption appears likely to lead to largely oral absorption for trans women generally. ■