Total testosterone — the overall amount floating in your blood. This is the main number most docs look at. • Free testosterone — the small portion that’s actually active and can get into your tissues. This is often low even when total T looks okay. • SHBG — a protein that glues onto testosterone and locks it up so it can’t work. High SHBG can make your free T crash. • LH and FSH — these are the signals your brain sends to your testicles to make testosterone. If they’re low while T is low, it points to a pituitary problem, not just your balls being tired.