Why do men lose energy, focus, and drive after 40?

At least eight things produce fatigue, weight gain and low drive in men over 40. Testosterone is one. What separates them, and what testing settles it.
What are the signs of low testosterone?

Symptoms alone cannot diagnose low testosterone. It takes compatible symptoms plus consistently low levels on repeated morning testing, and a reason why.
What Are the First Signs of Perimenopause?

Cycle changes, hot flashes, sleep disruption, mood shifts and brain fog can all be early signs. Why a new pattern matters more than any single symptom.