What is testosterone?

In one line

Testosterone is a hormone, made mainly in the testicles, that creates male body traits and helps muscle and bone grow in both men and women.

What is testosterone?
The word people call the 'male hormone'
When people at the gym talk about muscle, the word 'testosterone' comes up a lot. It is often called the 'male hormone'. Where is it made, and what does it do in the body?
In one line
The main hormone that shapes a male body
Testosterone is a hormone, a substance that carries signals inside the body, and it is made mainly in the testicles. It creates male body traits such as a deep voice and facial hair. In both men and women, it also helps muscle and bone grow.
What a hormone is
A chemical signal that travels in the blood
A hormone is a substance made in one part of the body that travels around the body in the blood and tells a distant organ what to do. It works more slowly than a nerve signal, but its effect lasts longer. Also, having too much or too little of a hormone causes problems.
Where it is made
Testicles in men, and a little in women too
Men make most of their testosterone in the testicles. Women make a small amount in the ovaries, and both sexes make a little in the 'adrenal glands', small organs that sit on top of the kidneys. So although it is called the male hormone, women have a small amount too.
How the amount is set
The brain sends a signal and cuts it back when there is enough
The brain decides how much testosterone to make. The 'pituitary gland', a pea-sized organ hanging under the brain, sends a signal hormone, and the testicles then make testosterone. When there is enough testosterone in the blood, the brain notices and sends less signal.
What it does at puberty
It creates male body traits
Testosterone is low in childhood and rises sharply at puberty. That is when the voice deepens and hair starts to grow on the face and body. Testosterone drives these changes.
Muscle, bone and blood
It builds muscle and bone, and helps make blood
In both men and women, testosterone helps muscle and bone grow. In men, it also helps make new red blood cells, which carry oxygen. So the signs of too little testosterone include losing muscle and thinning bones.
Changes during the day
Highest in the morning
Testosterone levels rise and fall during the day. They are highest in the morning and drop toward the afternoon. That is why clinics usually take the blood sample between 7 and 10 in the morning.
Changes with age
It slowly drops as you get older
A man's testosterone slowly drops with age. In a study that followed middle-aged men for years, it fell by about 1.6% each year. That pace means about 15% less over 10 years.
Common myth
Does a bigger rise right after training build more muscle?
Testosterone rises right after hard strength training. But in a study of young men, an arm trained so that hormones rose sharply grew no differently from an arm trained so that hormones barely rose. People whose levels rose more right after training did not gain more muscle either.
Summary
The male hormone involved in muscle and bone
Testosterone is a hormone, made mainly in the testicles, that creates male body traits and helps muscle and bone grow. But in studies of young men, a bigger rise right after training did not lead to more muscle growth. Related words: estrogen, growth hormone, TRT (testosterone replacement therapy).