What is Testosterone?

Let's start with the basics. You've heard of testosterone. It's that thing that supposedly makes men want to fight, build muscle, and do other stereotypically "manly" stuff. But what actually is it?

The Simple Version

Testosterone is a messenger. That's it. It's a chemical signal that your body produces and sends through your bloodstream to tell your cells what to do.

Think of it like a text message that your body sends to millions of tiny workers (your cells) saying: "Hey, build some muscle here," or "Make some hair grow there."

The Slightly Less Simple Version

Testosterone is a hormone - a chemical messenger made in specific organs that travels through your blood to affect cells throughout your body. It belongs to a class of hormones called androgens (from the Greek words meaning "man-maker").

Where Does It Come From?

In men, about 95% of testosterone is produced in the testicles (specifically in cells called Leydig cells). The remaining 5% comes from the adrenal glands - those little pyramid-shaped organs sitting on top of your kidneys.

Think of your testicles as a testosterone factory. They're running 24/7, producing testosterone and releasing it into your bloodstream like a steady drip from a coffee maker.

But unlike a coffee maker that you control, this factory is controlled by your brain. More on that in Chapter 4.

What Does Testosterone Actually Do?

Here's a quick hit list of testosterone's greatest hits:

During Development

• Develops male sex organs before birth
• Triggers puberty (voice deepening, facial hair, muscle growth, etc.)
• Causes the growth spurt during teenage years

In Adult Men

• Muscle mass and strength: Helps build and maintain muscle
• Bone density: Keeps your skeleton strong (super important as you age)
• Fat distribution: Influences where your body stores fat
• Red blood cell production: Tells your bone marrow to make more red blood cells
• Sex drive (libido): Major player in your interest in sex
• Sperm production: Essential for making swimmers
• Mood and mental clarity: Affects motivation, focus, and overall well-being
• Body hair: Controls growth patterns (yes, including balding)

A quick note: Not all of these effects come directly from testosterone itself. Your body converts testosterone into other hormones that do some of the heavy lifting:

• DHT (dihydrotestosterone): A more potent androgen responsible for things like facial hair, body hair, and - in genetically susceptible guys - male pattern baldness
• Estradiol: Yes, men need estrogen too. It's crucial for bone health, brain function, and cardiovascular health

We'll cover these in more detail in later chapters. For now, just know that "testosterone's effects" often means "testosterone plus what your body turns it into."

Daily Production: A healthy adult male produces approximately 5-7 mg of testosterone per day

In Your Blood: Normal levels range from about 300-1000 ng/dL (nanograms per deciliter), though this varies by age and time of day

Peak Times: Levels are highest in the morning and lowest in the evening

The Rhythm of Testosterone

Testosterone isn't a steady stream - it has a rhythm. Your levels naturally fluctuate throughout the day, following what's called a circadian rhythm (your body's internal clock).

A Typical Day

Time	What's Happening
6-8 AM	Levels peak (hello, morning wood)
Late morning	Start to decline
Afternoon	Continue dropping
Evening	Hit the lowest point (typically 20-25% lower than morning)
During sleep	Production ramps back up

This is why testosterone blood tests should always be done in the morning - ideally between 7-10 AM. If you test in the afternoon, your levels will naturally be lower, which could lead to an incorrect diagnosis of low testosterone.

More on testing in Chapter 5.

Testosterone's Half-Life

Here's a quick concept that'll become important later when we talk about TRT: half-life.

Half-life is simply how long it takes for half of something to be cleared from your system. Natural testosterone has a very short half-life of only about 10-100 minutes in your blood.

Imagine filling a bucket with a hole in the bottom. The water (testosterone) is constantly draining out. Your body has to keep producing new testosterone just to maintain stable levels - like constantly refilling that bucket.

This is why testosterone production is constant, not just occasional. Your body is always making more to replace what's being broken down.

Quick Recap

• Testosterone is a chemical messenger (hormone) that tells your cells what to do
• It's made mostly in the testicles (95%) with a bit from the adrenal glands (5%)
• It controls tons of stuff: muscle, bones, mood, sex drive, and more
• Levels are highest in the morning and lowest in the evening
• Your body is constantly making and breaking down testosterone

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Next up: How Testosterone Works - We'll dive into how your cells actually "hear" the testosterone signal and what happens when they do.