What this episode covers
Andrew Huberman explains what hormones are and how the steroid hormones testosterone and estrogen shape sexual development from chromosomes to gonads to brain and body. He walks through the surprising biology of how dihydrotestosterone builds the genitalia, how estrogen (aromatized from testosterone) actually masculinizes the brain, and uses rare conditions like 5-alpha reductase deficiency and androgen insensitivity syndrome to illustrate the underlying principles. He covers environmental disruptors of hormones and fertility, including atrazine, cannabis, alcohol, and cell phone exposure. The episode closes with anecdotes on hair loss biology, the spotted hyena's androgenized genitalia, and plant-to-animal hormonal warfare.
Also referenced (named, not recommended)
Behavioral Endocrinology
Randy Nelson and Lance Kriegsfeld
“today I'm guiding a lot of the conversation on work that on behavioral endocrinology. This is a book by Randy Nelson and and Lance Kriegfield, true experts in the field.”— Andrew Huberman
Androstenedione
various supplement brands
“if you want to read up on androstenedione, androstenedione is made into testosterone through this enzyme 17 beta hydroxysteroid dehydrogenase.”— Andrew Huberman
Pine pollen
various supplement brands
“there are plants that make what is essentially the equivalent of testosterone, like pine pollen is looks a lot like testosterone.”— Andrew Huberman
Big reveals from this episode
- Estrogen, not testosterone, is what masculinizes the XY brain, after testosterone is converted by the enzyme aromatase.
- In the Dominican Republic, a 5-alpha reductase mutation causes children raised as girls to grow a penis at puberty, locally called 'guevedoces' (penis at 12).
- Atrazine, a herbicide in waterways, caused testicular abnormalities in 10 to 92 percent of male frogs at affected sites.
- Average human sperm density fell from 113 million per milliliter in 1940 to 66 million per milliliter by 1990 in the US and Western Europe.
- THC and cannabis increase aromatase activity, raising circulating estrogen and linked anecdotally to gynecomastia in male users.
- In androgen insensitivity syndrome, XY individuals with testes and testosterone look and identify as female because their testosterone receptor is mutated.
- Female spotted hyenas are dominant and have a clitoris larger than the male's penis, driven by high androstenedione, and give birth through it.
Worth remembering
- It is dihydrotestosterone (DHT), not testosterone, that builds the penis in the fetus, via the enzyme 5-alpha reductase.
- A hormone must not only be present but also bind its receptor to have any effect on target cells.
- DHT controls both beard growth and male-pattern baldness, and beard density reflects the density of DHT receptors.
- Hair-loss drugs are 5-alpha reductase inhibitors that block testosterone-to-DHT conversion, but carry severe side effects from low DHT.
- Researchers estimate the volume of semen produced by men dropped about 20 percent between 1940 and 1990.
- Alcohol, especially beer and grain alcohols, can increase estrogenic activity, affecting both boys and girls during puberty.
- Androstenedione was the purported performance-enhancing drug at the center of 1990s and 2000s Major League Baseball scandals.
- Some plants evolved to raise estrogen in animals that eat them as 'plant-to-animal warfare' to suppress rodent populations and protect themselves.