What is a hormone?
A hormone is a chemical messenger made in one part of the body and carried in the blood to tell other parts what to do.
Think of hormones as text messages sent across your body. The pancreas texts "food is here, store it" (insulin), the adrenal glands text "danger, get ready" (adrenaline), and only cells with the right phone receive each message.
Why it matters for aging
Your body uses dozens of hormones. Insulin manages blood sugar, thyroid hormones set your energy pace, cortisol handles stress, estrogen and testosterone shape reproduction and much more, and IGF-1 drives growth.
Many hormone levels shift with age: estrogen drops at menopause, testosterone declines slowly in men, and growth signals fall. Some of these shifts affect energy, sleep, muscle and mood, which is why a hormone conversation with a doctor is often part of figuring out fatigue or brain fog.
A common myth
Feeling tired or foggy after 40 is always caused by hormones.
Hormonal shifts like perimenopause or low thyroid can definitely cause it, and they're worth checking. But sleep, fitness, muscle, mood and medications are just as common causes.
Go deeper
Deep dives
- Why am I always tired after 40? The upstream answer
- Early signs of insulin resistance, and what comes before them
Related terms
Updated Nov 9, 2026 · All terms