Why Does Cutting Carbs Lower Free Testosterone?

On Huberman Lab, insulin suppresses sex hormone binding globulin. As insulin comes down, sex hormone binding globulin goes up, and if testosterone has not gone up with it, free testosterone falls. Helping a person get metabolically healthy can lower free testosterone in the short run, all things equal.

A patient can show a total testosterone of 900 nanograms per deciliter, about the 70th percentile on the scale they look at, with free testosterone at 8 nanograms per deciliter, less than 1 percent free. A man should be about 2 percent free, closer to 16 to 18 nanograms per deciliter. Sex hormone binding globulin in that situation is really high, probably in the 80 to 90 range, above an upper range around 55 or 56. Estradiol, insulin, and thyroxine are the three hormones that run sex hormone binding globulin, and estradiol is the most important. Higher estradiol means higher sex hormone binding globulin. Somebody on a very low carbohydrate diet who drops water and weight raises sex hormone binding globulin if insulin goes down, and then has less free testosterone.