How to Read Your Testosterone Lab Results

Most men who get a testosterone test receive a total testosterone result, a reference range, and either "normal" or a vague suggestion to "come back in 6 months." What they don't receive is an explanation of what the test actually measures, what its limitations are, or what other values would make the result interpretable.

This article fills that gap.

The complete panel: what to request

A single total testosterone value is insufficient for clinical interpretation. The complete picture requires:

Timing: Always collect the sample between 07:00–10:00. Testosterone peaks in the early morning and declines throughout the day. An afternoon draw can read 20–30% lower than a morning draw — a difference large enough to place a eugonadal man in the "low" range.

Testing twice: The Endocrine Society guideline [^bhasin2018] requires two morning draws on separate days to confirm hypogonadism before considering treatment. Single-test variability is significant.

Understanding total testosterone

Reference range: Most labs report 300–1000 ng/dL (10.4–34.7 nmol/L) as "normal." The lower bound (300 ng/dL) is where most guidelines place the threshold for clinical hypogonadism.

The critical problem with reference ranges: they are statistical, not physiological. The range encompasses most of the population, including men with symptoms and men without. A man with total testosterone of 310 ng/dL is "normal" by the lab reference; his symptoms are still real.

Age context: Testosterone declines ~1–2%/year from age 30. A 60-year-old with 400 ng/dL is in a different physiological position than a 30-year-old with 400 ng/dL. The guideline thresholds don't fully account for this.

Kelsey et al. (2014) [^kelsey2014] found that low testosterone (below ~230 ng/dL) was associated with increased all-cause mortality in a large male veteran cohort — establishing that low testosterone is not merely a symptom problem but has health implications independent of symptoms.

Free testosterone: the number that often matters more

Only 2–3% of testosterone circulates freely (unbound). The remainder is bound to SHBG (~44–65%) or albumin (~33–54%). Testosterone bound to SHBG is essentially inactive — it cannot enter cells and activate androgen receptors. Testosterone bound to albumin is weakly bound and considered "bioavailable."

Free testosterone = the fraction actually available to tissue.

A man can have total testosterone of 500 ng/dL and free testosterone in the low-normal range if his SHBG is high. He may experience symptoms of low testosterone despite a "normal" total.

How to get free testosterone: Most labs offer two approaches:

1. Direct free testosterone by equilibrium dialysis — the gold standard but expensive and rarely offered
2. Calculated free testosterone using the Vermeulen formula [^vermeulen1999]: requires total testosterone, SHBG, and albumin. The Vermeulen calculator is available free online — this is more reliable than immunoassay-measured free testosterone, which is inaccurate.

Target: Free testosterone > 70–100 pg/mL (14.5–20.7 pmol/L) is generally considered healthy for adult men under 50.

SHBG: interpreting the binding protein

SHBG determines what fraction of your total testosterone is actually active. High SHBG → more binding → less free T. Low SHBG → less binding → more free T.

Causes of high SHBG: Aging (the primary driver — SHBG rises ~1%/year), hyperthyroidism, liver disease, caloric restriction, high estrogen, low androgen states

Causes of low SHBG: Obesity, insulin resistance/diabetes, hypothyroidism, high androgen states, exogenous testosterone

Clinically, a man with high SHBG may have adequate total testosterone but functionally low free testosterone. A man with low SHBG (often seen in obese insulin-resistant men) may have adequate free testosterone despite lower total testosterone.

LH and FSH: diagnosing the location of the problem

If total testosterone is low, LH and FSH tell you whether the problem is primary (testicular failure) or secondary (hypothalamic/pituitary failure to signal adequately).

Primary hypogonadism (testicular failure):

• Low testosterone + HIGH LH and FSH
• The pituitary is trying hard to stimulate testosterone production (high LH); the testes aren't responding
• Causes: Klinefelter syndrome, orchitis, torsion, chemotherapy, radiation

Secondary hypogonadism (central failure):

• Low testosterone + LOW or normal LH and FSH
• The pituitary is not sending adequate signal
• Causes: obesity, opioid use, hyperprolactinemia, Kallmann syndrome, TBI, sleep apnea
• Importantly: this is more common and more often reversible than primary hypogonadism

This distinction is critical because the treatment approach differs. Secondary hypogonadism caused by obesity, sleep apnea, or opioid use may be reversible by addressing the underlying cause — without testosterone replacement.

Prolactin: the frequently missed suppressor

A prolactin-secreting pituitary adenoma (prolactinoma) suppresses LH pulsatility and is a potentially reversible cause of secondary hypogonadism. Elevated prolactin should be in every initial low-T workup.

Normal range: <15–20 ng/mL. Values >25 ng/mL in men warrant endocrinology referral.

What to do with your results

Total T ≥ 400 ng/dL, Free T normal, no symptoms: Your testosterone is unlikely to be contributing to symptoms. Focus on lifestyle optimization.

Total T 300–400 ng/dL with symptoms: This is the clinical grey zone. Get the complete panel (free T, SHBG, LH, FSH, prolactin). Address reversible factors (sleep, body composition, stress, medication review) before discussing treatment.

Total T < 300 ng/dL confirmed on two morning draws: Referral to an endocrinologist or men's health specialist is appropriate. Identify the cause (primary vs secondary) before considering testosterone replacement.

Low T + high LH/FSH: Primary hypogonadism likely. Endocrinology referral.

Low T + low LH/FSH: Secondary hypogonadism. Investigate reversible causes. Prolactin and TSH required.

The testosterone check quiz below contextualizes your symptoms alongside what the numbers might mean — useful as a preparation tool for a clinical consultation.