TV ads tout testosterone treatments for “low T,” but surprising new research shows a different hormone may play a role in less sex drive and more fat as men age. Estrogen — the female hormone — is needed by men, too, and the study gives the first clear evidence that too little of it can cause certain “male menopause” symptoms.

“A lot of things we think are due to testosterone deficiency are actually related to the estrogen deficiency that accompanies it,” said Dr. Joel Finkelstein of Massachusetts General Hospital. He led the U.S.-government funded study, which appears in the New England Journal of Medicine.

Testosterone is the main male sex hormone. Men’s bodies convert some of it into estrogen, and levels of both decline with age. Until now, there was no way to tell which hormone was responsible for complaints of diminished libido, strength and energy.

Millions of men have been prescribed testosterone gel, patches or shots for these problems, but it’s not known how much they need or whether this hormone replacement therapy is good, bad or neutral, Finkelstein said. Doctors don’t usually prescribe estrogen to men; the way to remedy low estrogen is to give them testosterone and let the body convert it.

The study didn’t test hormones as therapy, but explored which ones had which effects.

After 16 weeks, researchers saw that muscle size and strength depended on testosterone, body-fat mass depended on estrogen, and both hormones were needed to maintain normal sex drive and performance.

The results mirror animal research — mice altered so they don’t make estrogen grow fat and have no sex drive. And in male-to-female transsexuals, “there’s actually evidence that when those men are given estrogen, it helps their libido,” Finkelstein said.

The study was too short to see long-term benefits or risks, such as the effect of supplements on the heart or mental sharpness.