Effect of Progesterone on Testosterone Concentrations and Breast Development in Transwomen

* Purpose. The purpose of this research is study the effect of progesterone therapy on testosterone levels in transwomen who are currently on gender affirming treatment with estradiol.
* Duration. It is expected that participation will last for 6 months (24 weeks).
* Study Procedures. Participants will be asked to sign an informed consent, complete a medical history, have a physical exam conducted, have a fasting blood sample taken and asked to take a study drug once daily.
* Risks. Some of the foreseeable risks or discomforts include temporary discomfort, bleeding, bruising and/or swelling in the arm from blood draw. There are no known direct risks with the use of Progesterone in Transwomen for gender affirming therapy.
* Benefits. Some of the possible benefits that may be expected include increase in fat mass, increase in breast size/fullness, decrease in masculine hair patterns.

The number of individuals with gender incongruence who present to their physician for hormone therapy has increased manifold in the last decade. Testosterone therapy in transgender men (also known as female-to-male transgender or transmen) and estrogen therapy in transgender women (also known as male-to-female transgender or transwomen) respectively is the mainstay of hormone regimen.Transwomen, who are genetically men, receive estradiol replacement with the aim of suppressing serum testosterone and achieving serum estradiol concentrations that mimic the serum concentrations of biological women. This leads to an increase in fat mass, breast growth and decreases in lean mass and masculine pattern hair. However, the results of these changes are often less than satisfactory and additional therapy is required.

Breast development is a major concern for transgender females. There is a great deal of variability among individuals, as evidenced during pubertal development.Transgender women do not achieve the same level of breast development as cisgender women do after puberty. Typically, transgender women plateau at Tanner stage III and half of the transgender women have a AAA cup size or less.Investigators plan to conduct a randomized, placebo controlled double blind study evaluating the effect of adding progesterone for 6 months to transwomen who are being treated with estradiol. The hypothesis is that progesterone will decrease serum testosterone concentrations as compared to placebo and increase breast size. Investigators will also assess its role in mood, sleep, scalp hair and androgenic hair growth.

Eligibility Criteria:
Inclusion Criteria:

* Transwomen, currently on treatment with estradiol therapy for at least 6 months (as their standard of care gender affirming therapy)
* Has achieved serum estradiol \>100 pg/ml at least once, based on clinical labs in past.

Exclusion Criteria:

1. Treatment with progesterone in the last 2 months
2. HIV
3. Planning to go for breast enhancement or gender reassignment surgery in the next 6 months
4. Known history of peanut allergy (because the study drug contains peanut oil)
5. Active deep vein thrombosis, pulmonary embolism or history of these conditions
6. Active arterial thromboembolic disease or history of these conditions
7. Active cardiovascular disorders or history of these conditions (e.g. myocardial infarction, uncontrolled hypertension \>150/90 mmHg)
8. Known, suspected, or history of breast cancer
9. Known liver dysfunction or disease
10. Known or history of gallbladder disease. This does not apply to subjects who have undergone cholecystectomy
11. Known or history of hypertriglyceridemia (\>400 mg/dl)