Changes in female hormones have been blamed for headaches and other pains.© Punchstock

Women have long been known to experience more pain than men. And the idea that sex hormones are to blame has just been bolstered by a study into pain thresholds in a unique study group: people undergoing sex-change operations.

Men taking female hormones often start to experience chronic pain, says Anna Maria Aloisi, a physiologist from the University of Siena in Italy. In a study of 54 men taking oestrogen and anti-androgens as treatment to become women, 30% reported developing pains, primarily chronic headaches, during their treatment.

"We found that oestrogen in high amounts induced pain in these men," says Aloisi, who presented her work at the 11th World Congress on Pain in Sydney, Australia, this week.

In another study of women taking testosterone to become men, says Aloisi, more than half found their aches and pains improved. "They seemed to feel better generally," she adds.

The results back up previous research on sex differences in pain. Although no one knows exactly how sex hormones affect pain tolerance, researchers think testosterone dulls pain by muting the excitatory pain pathways in the central nervous system, while oestrogen heightens pain by blocking the inhibitory mechanisms that damp pain sensing.

The evidence for this is piling up. But the story about oestrogen is more confusing than it first seemed, conference members heard.

Ups and downs

On the one hand, pain typically arises during the menstrual cycle when oestrogen is at its lowest ebb, as during menstruation. On the other, girls report more pain than boys from the onset of puberty when oestrogen starts to surge.

It seems to be sudden changes in hormone levels that are the problem, rather than the levels themselves, says Linda LeResche, an epidemiologist from the University of Washington, Seattle. "It is the withdrawal or fluctuation of oestradiol that is associated with pain," she says.

To check this theory, LeResche is starting to study women who choose to take the contraceptive pill all the way through their cycle, so that they never menstruate. LeResche will investigate whether keeping their oestrogen levels up affects their experience of pain.

Managing pain

Researchers at the conference added that such findings should be taken into account in pain management.

Serge Marchand of the University of Sherbrooke in Quebec, Canada, who has studied testosterone levels in women with chronic pain, suggests that clinicians should measure levels of sex hormones in patients. An imbalance could be exacerbating their problems, he says.

And Aliosi advocates hormone replacement therapy in some chronic-pain patients. She has started one trial that bolsters testosterone in male patients with low levels of the hormone. She thinks this idea could also be applied to women, although she adds that giving testosterone to women is more complicated than giving it to men. 

University of Siena, Italy