Estradiol is a hormone made naturally in the human body by the ovaries. It is incredibly important in the regulation of the menstrual cycle, cardiovascular system, neurologic system, skeletal system, vascular system, and many more.[1] Estradiol is the most potent and most abundant estrogen (E2) during a woman's reproductive years. There are four different kinds of estrogen: estrone (E1), estradiol (E2), estriol (E3), and estetrol (E4).[2]

When women enter menopause, estrogen synthesis significantly decreases due to lower-functioning ovaries. The decrease in estrogen is the reason most women have postmenopausal symptoms. Postmenopausal symptoms include, but are not limited to, hot flashes, vaginal dryness, vaginal itchiness, dysuria, and dyspareunia. These symptoms can be very discomforting to patients and can affect the quality of life, sleep, mood, interpersonal relationships, daily activities, and sexual function. When the ovaries no longer synthesize estradiol, it can then get synthesized by several extragonadal sites.[3] These sites include but are not limited to, adipose tissue, bones, brain, and smooth muscle cells in vascular endothelium. 

Before women enter menopause, estradiol protects women from cardiovascular disease due to increased regulation of cholesterol and triglyceride metabolism, thus decreasing the risk for atherosclerotic heart disease.[4] To combat these symptoms, estradiol can be taken supplementally for the management and treatment of postmenopausal symptoms and for women who have had hysterectomies, salpingo-oophorectomy, or unilateral salpingo-oophorectomy. Estrogen is also useful in female patients with hypoestrogenism due to castration, hypogonadism, or primary ovarian failure such as Turner syndrome. 

The majority of studies find that postmenopausal symptoms experience significant improvement with the use of estradiol hormone replacement therapy (HRT). Also, research has shown that estradiol can decrease stress by reducing the release of cortisol in response to a physical stressor. Estradiol increases the amount of corticosteroid-binding globulin (CBG), thus reducing the free cortisol levels circulating in the body, responding to stress. Less cortisol can act on the body, including areas in the brain that are integral in the stress response.[5]

Estrogen formulations have also served as an off label treatment option for male-to-female transgender patients. However, the levels of blood estrogen levels require close monitoring to avoid complications of the treatment. 

Estrogen has a significant role in bone health. Women in postmenopausal age develop osteoporosis due to decreased levels of estrogen. Estrogen derived formulations such as raloxifene have received approval for osteoporosis prevention and treatment in a selected patient population. 

Estrone is converted to estradiol in the granulosa cells of the ovary by the enzyme 17-beta-hydroxysteroid dehydrogenase (17-beta-HSD).[2] The aromatization of testosterone synthesizes estradiol. Estradiol is a steroid hormone and, therefore, can easily cross the cell membrane. Estrone binds to its specific intracellular receptor and regulates DNA transcription for protein formation.[2] Estrone exerts its effects on the menstrual cycle, breasts, ovaries, brain, musculoskeletal system, cardiovascular system, and more. Estrone appears to affect the gene transcription of other genes that do not have estrogen-responsive elements.

Hormone replacement therapy (HRT) is normally used for a short period in post-menopausal women. The routes of admission can be transdermal (cream and patches), intramuscular, or oral. Estradiol therapy usually comes in two forms: as a vaginal cream or as a capsule. Studies have found that the use of estradiol vaginal cream twice a week has significantly reduced vaginal dryness and dyspareunia compared to placebo pills. However, there was no significant improvement between estradiol vaginal cream and placebo therapy in vaginal irritation, itchiness, and dysuria.[6]

There is still research underway to determine the efficacy of vaginal estradiol capsules. Capsules currently undergoing clinical trials hope to relieve post-menopausal symptoms with very little systemic estradiol exposure. The capsules should be less messy and more patient-friendly than creams and tablets.[7]

In one clinical trial, capsules improved vaginal dryness and dyspareunia. Also, capsules increased the percent of superficial and intermediate cells in the vagina, thus improving vaginal physiology.

Sustained-release estradiol vaginal rings are becoming more popular in research. Rings can be effective for up to 90 days and can be easily inserted and removed by patients.[8] The ring may be beneficial to women who choose not to apply vaginal estrogen creams. 

Adverse effects are generally uncommon; however, there have been reports of the following: 

Cardiovascular: edema, hypertension, thrombophlebitis, retinal thrombosis. 

Central Nervous System: headache, depression, pain, dizziness anxiety, migraine, nipple pain

Respiratory: nasopharyngitis, flu-like symptoms, sinusitis, upper respiratory tract infection, headache, bronchitis, sinus congestion, pharyngitis, asthma exacerbation, cough 

Dermatologic: skin rash, pruritus, erythema multiforme, erythema nodosum, urticaria

Skeletal: arthralgia, weakness, back and neck pain, limb pain, myalgia, leg cramps

Endocrine: weight gain or loss, hot flash, libido changes, hirsutism, menstrual changes, porphyria exacerbation, fluid retention, hypocalcemia, elevated triglycerides, galactorrhea

Gastrointestinal: Abdominal pain, constipation, heartburn, flatulence, bloating, nausea, vomiting, diarrhea, pancreatitis, gastroenteritis, carbohydrate intolerance 

Hypersensitivity: anaphylaxis, angioedema, hypersensitivity reactions

Hepatic: Hepatic hemangioma exacerbation, jaundice

Ophthalmic: conjunctivitis, steepening of the cornea, contact lens intolerance 

Infections: fungal and other infections

Otic: Otitis media 

The United States Food and Drug Administration (FDA) Boxed Warnings: 

• Women who take estrogen plus progestin therapy are at increased risk for breast cancer.
• Women with increased exposure to estrogen are at risk for endometrial cancer. Estrogen stimulates endometrial growth, which results in endometrial hyperplasia, which could result in endometrial cancer.
• Other risk factors to increased exposure to estrogen HRT are cerebrovascular events, coronary artery disease, and venous thromboembolism.[9] There have also been reports of ovarian cancer with estrogen use. 

Women who are at increased risk of breast cancer or endometrial cancer should not begin Estradiol therapy.

Overweight women with exposure to increased levels of estradiol in their lifetime should not add supplemental estradiol to their post-menopause regimen.[10] Adipose tissue carries an increased level of estrogen. Therefore overweight women are at risk of increased exposure compared to average-weight and underweight women. 

Women who have angioedema or anaphylactic reaction to estradiol or its components, abnormal genital bleeding, blood clotting disorders such as deep venous thrombosis or pulmonary thromboembolism, cardiovascular disease (a stroke or myocardial infarction), protein C or S  or antithrombin deficiency as well as thrombophilic disorders, or pregnancy are contraindications to estradiol treatment. 

Estradiol HRT can increase a patient's risk of cardiovascular disease, DVT, and stroke and, therefore, is not a viable option in at-risk patients.  

Women who are at risk can consider other alternatives such as laser therapy, lubricants, dilators, and even physical therapy to strengthen pelvic floor muscles if patients are complaining of dyspareunia.[8]