India, May 29 -- Last week, I saw a 51-year-old woman in the clinic with fatigue, mood changes, body aches, poor sleep and bouts of night sweats. Routine tests were normal. "Doctor, there is something wrong with me, but no one seems able to diagnose it," she said. Her periods had stopped a year earlier. It was clear that she might benefit from estrogen replacement. The real question was whether she should receive it, and whether it would be safe.

That question is worth asking more often, because estrogen is one of medicine's most misunderstood hormones. Most people still think of it simply as a female sex hormone, but that narrow label hides its wider importance. Estrogen shapes puberty, menstrual function, fertility, bone strength, brai...