When most people hear the word testosterone, they think of aggressive behavior.
There is a link between the twoat least in competitive situations, such as with a peer or for a sexual partner.
However, there appears to be a subtler interplay between testosterone and behavior in other types of situationsin both men and women.
Here are a few facts about the "male hormone."
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Women in love have more
Women in love have higher testosterone for the few months after a relationship starts than women who are single or in long-term relationships, a small Italian study suggests.
The opposite is true for men; those newly in love have lower testosterone than men flying solo or with a long-term partner.
As with early passion, though, the changes don't last. When the researchers tested the study participants again one to two years later, the differences had disappeared.
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It can shrink your belly
Men whose levels of testosterone are below normal may lose their spare tire when treated with testosterone.
"Most of the studies show there's a reduction of abdominal obesity in men who are given testosterone," says Adrian Dobs, MD, a professor of medicine and oncology at Johns Hopkins University School of Medicine, in Baltimore.
Because the long-term effects of testosterone therapy have not been well studied, however, it is generally only recommended in men with below-normal testosterone levels and symptoms such as fatigue, muscle or bone-mass loss, or sexual dysfunction.
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Making money affects it
Young men who are futures traders get a testosterone spike on days when they make an above-average profit, British researchers found.
And on the mornings when men's testosterone levels were higher than average, their average afternoon profits were higher than on their low-testosterone days, suggesting a possible cause-and-effect relationship.
More experienced traders showed an even stronger tie between testosterone and profits.
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Too much can shrivel testicles
In men, taking steroid hormones such as testosterone as performance boosters can cause testicles to shrink and breasts to grow. For women, it can cause a deeper voice, an enlarged clitoris, hair loss from the head, and hair growth on the body and face.
In both genders, steroid abuse can cause acne, mood swings, aggression, and other problems.
Men working with an experienced doctor to treat low testosterone or women taking small amounts of testosterone under medical supervision are unlikely to have testosterone-overdose symptoms.
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Sports fans get a winner's boost
In the run-up to a competition, whether it's wrestling or chess, a man's testosterone levels rise, studies have shown.
After the game, the winner's testosterone will increase even more. And fans' hormone levels seem to mirror those of their athletic idols. In a group of 21 men watching a Brazil vs. Italy World Cup match, the Brazil fans' testosterone levels increased after their team won, but the Italy fans' testosterone fell.
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Fat can lower testosterone
Obese men tend to have lower testosterone than thinner men, Dr. Dobs says. It's not clear why, she adds, although one possible reason is that obesity promotes a state of widespread inflammation in the body.
"When there's fat cells, there's a lot of inflammatory factors," she says. "These inflammatory factors have been associated with suppression of testosterone synthesis."
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Hands reveal hormone secrets
In men and boys, the right pointer finger is shorter in relation to their right ring finger than it is in girls.
This has even been found in other five-fingered creatures, such as rats. Scientists have found that the difference is a clear marker for fetal exposure to testosterone. The higher your testosterone level before birth, the lower your pointer-finger-to-ring-finger ratio.
Men with the lowest ratios made the most money and stayed in business for the longest time, according to the U.K. study of traders and testosterone.
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It's hard to measure accurately
Men are often diagnosed with low testosterone after a single test. This is a big problem, says Neil Goodman, MD, an endocrinologist and professor of medicine at the University of Miami School of Medicine.
"If I take blood on a guy and I send it to three labs, I'm going to get three different levels," he says.
Efforts are underway to standardize blood tests. In the meantime, testosterone should be checked more than once, Dr. Goodman says, and done in the morning when testosterone is highest.
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It's not the fountain of youth
It would be great if an aging man's vigor, muscle power, and sex drive could be restored with testosterone.
But it is not clear whether therapy will do anything for the 75% to 80% of men over 65 who have normal levels of testosterone.
Men with below-normal levels, however, may get a boost in libido, sexual function, and bone mass from supplemental testosterone. And it may help diabetic men with low testosterone build lean muscle mass.
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Taking it doesn't cause prostate cancer
It has long been thought that taking testosterone increases the risk of prostate cancer. Testosterone treatment can boost levels of prostate-specific antigen, or PSA, a nonspecific marker for prostate cancer, which may lead to more prostate biopsies and more prostate-cancer diagnoses, Dr. Goodman says.
There are now, however, major questions about whether it's worthwhile to treator even diagnoseprostate cancers in older men, given that they're common and often slow-growing.
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Low levels are linked to sleep apnea
Men with sleep apnea are more likely to have low testosterone, and treating sleep apnea can help return it to normal.
But if a man with sleep apnea is diagnosed with low testosterone alone, taking the supplemental hormone can worsen sleep apnea. That's why it's crucial for men with low testosterone to get a thorough workup by an endocrinologist so underlying conditions that can cause low testosterone, such as sleep apnea or pituitary-gland tumors, don't go undiagnosed, Dr. Goodman says.
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It may hurt men's hearts
In 2010, researchers halted a study of testosterone therapy in older men because of a higher rate of cardiovascular problems such as heart attack in the group taking testosterone instead of placebo.
The reason isn't clear, but caution should be used in prescribing testosterone to older men in poor health, Dr. Goodman says. Declining testosterone in men is associated with health problems, but this doesn't mean giving older men testosterone will extend lifespans, he says.
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Too much may kill brain cells
It's only known to happen in a petri dish, but Yale researchers showed that nerve cells exposed to high levels of testosterone were more likely to self-destruct. The hormone boosted a "cell suicide" mechanism known as apoptosis, which, under normal circumstances, is supposed to help the body wipe out cancerous or otherwise abnormal cells.
And the higher the testosterone level in the dish, the shorter lived the cells were. Exposure to low levels of testosterone, however, had no effect on the cells.
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