The short answer.

The American Urological Association uses total testosterone below 300 ng/dL as a reasonable diagnostic cutoff, but diagnosis requires symptoms or signs and confirmation with two separate early-morning measurements. A single number, even below 300, is not a diagnosis on its own.

That last part is critical. Low testosterone is not a lab diagnosis alone. It's a clinical diagnosis: low levels plus symptoms plus appropriate evaluation to rule out other causes.

Why one test isn't enough.

Testosterone fluctuates. It's highest in the early morning and drops throughout the day — sometimes by 30% or more by afternoon. It fluctuates with sleep quality, stress, acute illness, alcohol intake, and even how recently you exercised.

A single testosterone level drawn at 3pm after a bad night of sleep, a stressful week, and two drinks the night before can easily come back 150 points lower than a properly timed morning draw.

This is why guidelines specifically recommend confirmation: two separate morning fasting testosterone levels. If someone diagnoses you — or tells you you're fine — based on one test drawn at a random time, that's not a proper evaluation.

Illustrative example: A 42-year-old man's primary care doctor tells him his testosterone is "normal" at 340 ng/dL. The test was drawn at 2:30 in the afternoon. He has classic symptoms: fatigue, reduced strength despite consistent training, low libido, difficulty concentrating, irritability. Two properly timed morning fasting draws, one week apart, come back 285 and 298. With consistent symptoms and two confirmatory levels below 300, the clinical picture becomes clearer. The original result was not wrong that 340 is technically within the reference range, but the test was drawn at the wrong time, and a single number without symptom correlation is not a diagnosis.

Total testosterone vs. free testosterone.

Most standard lab panels report total testosterone. But total testosterone includes testosterone that's bound to proteins in the blood — primarily sex hormone-binding globulin (SHBG) and albumin. Most circulating testosterone is protein-bound. Testosterone tightly bound to SHBG is less readily available to tissues, while albumin-bound testosterone is more loosely bound. Free testosterone represents the unbound fraction.

Free testosterone — the small fraction that's unbound and directly available to receptors — is clinically important because it correlates with the symptoms men actually experience. It typically represents about 2-3% of total testosterone.

Here's why this matters clinically: SHBG rises with age, with certain medications, with liver conditions, and sometimes for reasons that aren't entirely clear. A man can have a total testosterone of 450 ng/dL — clearly "normal" — but if his SHBG is elevated, his free testosterone may be quite low. He'll have all the symptoms of low testosterone despite a normal total level.

This is why I check both total and free testosterone (or calculate it from total testosterone and SHBG) in any man I'm evaluating for hypogonadism. Total testosterone alone misses a significant subset of symptomatic men.

The reference range problem.

The reference range on your lab report — typically something like 264-916 ng/dL or 250-1100 ng/dL, depending on the lab — is not a clinical guideline. It's a statistical range derived from a sample population. It tells you where 95% of tested individuals fell. It does not tell you where you should be, or whether your current level is consistent with your symptoms.

A symptomatic man with a borderline total testosterone deserves a careful evaluation, but a borderline level alone does not automatically mean he has hypogonadism. The diagnosis requires confirmed low levels, consistent symptoms, and exclusion of other causes.

Conversely, a 60-year-old man with a testosterone of 380 ng/dL and no symptoms doesn't need treatment just because someone told him his level is "low for his age." Treatment is for symptomatic men with confirmed low levels, not for numbers on a page.

What else I check — and why.

Testosterone doesn't exist in isolation. Before prescribing TRT, I need to understand why the level is low. The answer changes the treatment plan.

LH and FSH

These are the brain hormones that tell the testes to produce testosterone. If testosterone is low and LH/FSH are high, the brain is trying to stimulate production and the testes aren't responding (primary hypogonadism). If testosterone is low and LH/FSH are also low, the brain isn't sending the signal (secondary hypogonadism). The distinction matters because secondary hypogonadism can sometimes be caused by correctable factors — obesity, sleep apnea, medications, pituitary issues — and may not require lifelong testosterone replacement.

Estradiol

Some testosterone converts to estrogen through an enzyme called aromatase. This is normal and necessary. But in men with higher body fat or elevated aromatase activity, the conversion can be excessive, leading to symptoms (gynecomastia, water retention, mood changes) and masking the testosterone deficiency picture. I check estradiol at baseline and monitor it during treatment.

CBC and hematocrit

Testosterone stimulates red blood cell production. This is a known effect. In some men, TRT can push hematocrit too high, increasing blood viscosity and cardiovascular risk. I check baseline hematocrit and monitor it regularly on treatment. If it rises above a threshold (typically 54%), we adjust the dose or dosing frequency.

PSA

Prostate-specific antigen. The relationship between testosterone and prostate cancer is more nuanced than the old dogma suggested — current evidence has not established that appropriately prescribed TRT increases prostate-cancer incidence, but long-term safety questions remain and appropriate risk assessment and monitoring are important. I check baseline PSA and monitor it, because a significant rise on therapy warrants further evaluation.

Metabolic panel, lipids, thyroid

Low testosterone is frequently associated with metabolic syndrome, insulin resistance, hypothyroidism, and dyslipidemia. Sometimes treating the underlying metabolic issue improves testosterone levels without TRT. Sometimes TRT improves the metabolic picture. Either way, I need the full context before making a treatment decision.

When I prescribe TRT — and when I don't.

I prescribe testosterone replacement when a man has:

I don't prescribe testosterone for:

This is the difference between a physician and a TRT clinic. A clinic's business model depends on putting men on testosterone. My business model depends on getting the diagnosis right.

The bottom line.

A testosterone number is one data point. A proper evaluation looks at the number, the timing of the draw, the confirmation on repeat testing, the free testosterone, the upstream hormones, the metabolic context, and — most importantly — how you feel.

If your doctor checked one total testosterone at a random time, said "normal," and moved on — that wasn't a testosterone evaluation. If a clinic checked one level, said "low," and started you on injections — that also wasn't a proper evaluation.

The right approach is somewhere in between: thorough, honest, and built around your clinical picture.

The goal is not to chase the highest testosterone number. Treatment aims to restore testosterone to an appropriate physiologic range, improve symptoms, and minimize adverse effects.

References.

Author: Seth Lampert, MD, Founding Physician, Wellesley Medical.

Disclaimer: This article is for general educational purposes only and does not constitute medical advice, diagnosis, or treatment. Individual symptoms and circumstances vary. Consult a qualified physician for evaluation and care tailored to your situation.