How to Diagnose Low Testosterone
Tests, levels, and what the numbers actually mean before starting TRT.
9 min read
A low testosterone diagnosis isn't made from a single number on a lab report. It requires a specific combination of blood work, timing, and symptoms — and the criteria are more precise than most people expect. Understanding what your clinic is actually testing for (and why) helps you make sense of your results and know whether TRT is a reasonable next step.
This guide walks through the tests used to diagnose low testosterone, the reference ranges and thresholds clinicians rely on, and the official diagnostic criteria from the American Urological Association (AUA).
Symptoms That Typically Prompt Testing
Testosterone testing on its own isn't diagnostic — labs are only meaningful alongside symptoms. Clinicians look for a cluster of the following before ordering blood work:
Reduced energy or persistent fatigue
Diminished sex drive or libido
Erectile dysfunction or fewer morning erections
Depressed mood or reduced motivation
Poor concentration or memory
Reduced muscle mass or strength
Increased body fat, especially abdominal
Irritability
Infertility
Loss of body or facial hair
The Core Diagnostic Test: Total Testosterone
According to American Urological Association (AUA) guidelines, the diagnosis of low testosterone rests on total testosterone — not free testosterone, and not a single blood draw. The official criteria:
Threshold
A total testosterone level below 300 ng/dL is the accepted cut-off supporting a diagnosis.
Timing
Blood must be drawn in the early morning, roughly 7–11 AM, when testosterone is at its daily peak. An afternoon draw can show a falsely low result.
Repetition
Two separate morning measurements are required, typically 1–3 weeks apart, ideally from the same lab using the same method.
Symptoms required
A low number alone is not a diagnosis. The AUA is explicit that testosterone deficiency is only diagnosed when low levels occur together with relevant symptoms or signs.
Source: AUA Guideline on Evaluation and Management of Testosterone Deficiency. This is general education, not a substitute for evaluation by a licensed clinician.
Supporting Tests: Finding the Cause
Once low total testosterone is confirmed, clinics typically order additional labs — not to diagnose the deficiency itself, but to figure out what's causing it and whether TRT is appropriate:
| Test | What it tells your provider |
|---|---|
| Free Testosterone | Not used as the primary diagnostic test, but helpful when total T is borderline (roughly 230–350 ng/dL), since SHBG can affect how much testosterone is actually bioavailable. |
| SHBG (Sex Hormone-Binding Globulin) | Determines how much testosterone is bound vs. freely available. High SHBG can mask a functional deficiency even with a "normal" total T. |
| LH & FSH | Distinguishes primary hypogonadism (testicular issue) from secondary hypogonadism (pituitary/hypothalamic issue) — changes the treatment approach. |
| Prolactin | Elevated levels can indicate a pituitary issue suppressing testosterone production and should be ruled out before starting TRT. |
| CBC (Hematocrit/Hemoglobin) | Baseline before treatment — TRT can raise red blood cell counts, so this gets monitored throughout therapy. |
| PSA (Prostate-Specific Antigen) | Standard baseline safety check in men being considered for TRT, particularly over 40. |
| Estradiol | Often checked to understand aromatization (testosterone converting to estrogen), more commonly used to monitor therapy than to diagnose deficiency. |
What Your Total Testosterone Number Means
| Total Testosterone | General interpretation |
|---|---|
| Below 300 ng/dL (on 2 morning tests) | Meets the AUA threshold for low testosterone — treatment is typically considered if symptoms are present. |
| 300–350 ng/dL | Borderline zone. Some clinicians order free testosterone or SHBG here to clarify the picture, especially with symptoms present. |
| Above 350 ng/dL | Generally considered within a normal range, though "normal" is population-based and doesn't rule out symptoms worth discussing with a provider. |
Reference ranges vary slightly by lab and assay method. These figures reflect commonly cited clinical thresholds, not a substitute for your provider's interpretation of your specific results.
What Happens After a Low T Diagnosis?
If your labs and symptoms meet the diagnostic criteria, your provider will typically walk through treatment options, expected timelines, and monitoring plans before starting therapy. A few things worth knowing going in:
- →Treatment isn't automatic just because a number is low — providers weigh symptoms, goals, fertility plans, and other health factors
- →Baseline labs (CBC, PSA, lipids) are usually drawn before starting TRT to establish a comparison point
- →Follow-up labs are typically done 4–6 weeks after starting treatment to check response and adjust dosing
- →If LH/FSH point to a pituitary cause, further imaging or endocrinology referral may be recommended before TRT
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