One low reading does not settle the question. The Endocrine Society describes diagnosis as symptoms plus repeated blood tests, and the order matters if you are a Midtown reader trying to plan your first appointment.
Step one: the symptom history and exam
The Endocrine Society says your doctor will take a thorough history of your symptoms and then complete a physical exam, including your body hair, breast tissue, and the size and consistency of the testes and scrotum.
Nothing here needs a lab yet. It is the part you can prepare for at home by writing down what you have noticed and when it began.
Step two: the blood tests
The Endocrine Society says your doctor will also use blood tests to see if your total testosterone level is low, and that the normal range depends on the lab that conducts the test.
That detail matters when you compare results. A number from one lab is not directly comparable to a reference range printed by another.
Why one result is not a diagnosis
To get a diagnosis of hypogonadism, the Endocrine Society says, you need at least two early morning (7–10 AM) blood tests that reveal low testosterone, in addition to signs and symptoms typical of low testosterone.
In its statement on testosterone replacement therapy, the Endocrine Society says diagnosis requires men to have symptoms of decreased testosterone, plus consistently low, accurately measured blood total and free testosterone in lab tests.
You can read the full statement on testosterone replacement therapy on the society's site.
If you want the research behind treatment
Diagnosis comes first and treatment decisions follow it. A published narrative review of testosterone replacement therapy in men aged 50 and above covers benefits, safety considerations and clinical recommendations.
What to do first
Book the blood draw for the morning, because the society's window is 7 to 10 in the morning. Expect to repeat it, since a single draw does not meet the two-test requirement. Bring your written symptom list to the first visit.