Bruce Jenner attends the 13th annual Michael Jordan Celebrity Invitational gala in Las Vegas in April. (Ethan Miller/Getty Images for Michael Jordan Celebrity Invitational)

In this excellent post, my colleague Steven Petrow advised that if you're wondering whether someone you know is transitioning from one gender to the other, it's best to "check your curiosity and hold your tongue." But we all have questions about a subject that is increasingly mainstream. Aside from the recent coverage of Bruce Jenner's apparent transition, Laverne Cox last year became the first trans person on the cover of Time magazine, and Amazon debuted “Transparent,” a show about a father who is transitioning.

[UPDATE: Bruce Jenner: ‘Call me Caitlyn’]

Here are answers to some of the questions surrounding this issue.

How many people are transgender?

The Williams Institute, a think tank at the UCLA School of Law dedicated to research on sexual orientation law and public policy, estimates that 700,000 Americans are transgender. But a good piece last year on fivethirtyeight.com noted that there are no national surveys. And if there were, there is no agreement on what "trangender" means.

How many people have sex reassignment surgery?

It's very difficult to know. The Encyclopedia of Surgery says that "the number of gender reassignment procedures conducted in the United States each year is estimated at between 100 and 500. The number worldwide is estimated to be two to five times larger." At least one other researcher says there are many more. Marci Bowers, a transgender obstetrician and gynecologist in Burlingame, Calif., who performs the surgeries, said in an interview that she does about 200 per year herself, about three quarters of them male to female.

Fred Ettner, a physician in Evanston, Ill., who works with people going through transition, estimated that only about 25 percent to 30 percent of transgender people have any kind of surgery.

How does one begin the process of changing one's sex?

Under the standards of care adopted by the World Professional Association for Transgender Health (WPATH), the first step usually is meeting with a mental health professional for a diagnosis and psychotherapy. A diagnosis of gender identity disorder or gender dysphoria and a letter of recommendation from the therapist allows a person to begin hormone therapy with a doctor. That is usually followed by a period of living publicly as a member of the opposite sex and, finally, surgery to alter the genitalia and other body parts.

What do hormones do?

Androgens are given to women to help them develop secondary male sex characteristics such as a beard and body hair. Estrogen and anti-androgens are given to men to help change their musculature, skin and fat distribution, all of which will make them appear more feminine. Body hair also diminishes.

But Ettner said the hormones' most important contribution is to reduce the dysphoria transgender people have been struggling with all their lives. After a month or two, their bodies and brains begin aligning.

"It's very obvious to the individuals," Ettner said, adding, "Their brain finally is getting this hormone. They feel differently. They behave differently.

"The first effect is the brain effect," he said.

What is the Real-Life Experience (also known as the Real-Life Test)

Surgeons who follow the WPATH standards of care (which some patients and experts consider too strict) usually require candidates for surgery to live for as long as a year in their preferred gender role. They must work or go to school; do volunteer work; change their first names and prove to the surgeon that people other than their therapists know they are successfully living this way.

What happens during surgery?

Male to female genital surgery is easier, less expensive and generally more successful than female to male surgery. That's one reason why fewer women choose to have surgery on their genitals, Bowers said. (Another is cost.)

In male to female surgery, the testicles and most of the penis are removed and the urethra is cut shorter. Some of the skin is used to fashion a largely functional vagina. A "neoclitoris" that allows sensation can be created from parts of the penis. Men retain their prostates.

In female to male surgery, the breasts, uterus and ovaries are removed (in two separate procedures). A "neophallus" can be constructed using tissue from the forearm or other parts of the body that allows sexual sensation, an expensive procedure. Extending the urethra to allow standing urination has proved to be perhaps the most difficult part of the process, Bowers said.

What other kinds of surgery are performed?

Jeffrey H. Spiegel discusses the tracheal shave cosmetic surgery procedure to reduce the size of an Adam's apple. (Jeffrey H. Spiegel)

Women wishing to live as men often have mastectomies. Men transitioning to women can have plastic surgery to "feminize" their appearance, including work on their eyes, noses, brows, chins and hairlines. They also can have their Adam's apple shaved down so it is less prominent.

Sounds expensive. Is it covered by insurance?

Bowers and Ettner said a woman who chooses the full range of surgical procedures available would spend $75,000 or more to transition to a male. Switching from male to female might cost in the $40,000 to $50,000 range. Some people have the work done in Thailand, which is well known for the surgery and where the cost is much lower.

Insurance is just beginning to cover some of the costs. Last year, Medicare lifted a 33-year ban on coverage for gender reassignment surgery, and the surgery was performed on 74-year-old Army veteran.

Do many people regret having the surgery?

Anecdotal accounts of regret abound, but overall it appears to be rare. Bowers said that only two of the 1,300 people she has operated on have wanted to go back to their old bodies.

But that doesn't mean that surgery is a miracle cure for the difficulties of gender dysphoria. People who make the transition often lose spouses, families, friends and jobs. They may find themselves completely alone if they relocate to start new lives. Johns Hopkins University, which in the 1960s was one of the first medical institutions to perform sex reassignment surgery, halted the practice when officials concluded that it was not helping the transgendered overall.

A 2o11 study of 324 Swedish transsexuals by the Karolinska Institute showed that "after sex reassignment, [they] have considerably higher risks for mortality, suicidal behaviour, and psychiatric morbidity than the general population. Our findings suggest that sex reassignment, although alleviating gender dysphoria, may not suffice as treatment for transsexualism, and should inspire improved psychiatric and somatic care after sex reassignment for this patient group."

Read more:

The forgotten history of Bruce Jenner

Transgender etiquette 101

Transgender at 7: Tyler gets a legal name change