Labs you need before starting TRT, and what each one rules out
Before you start testosterone replacement therapy (TRT), you need at least two separate early morning, fasting total testosterone tests that both come back low, LH and FSH to find out why, and baseline hematocrit and PSA so treatment can be monitored safely. Depending on those results, your clinician may add free testosterone and SHBG, prolactin, iron studies, a pituitary MRI, or a semen analysis if you want children soon. This is the workup laid out by the Endocrine Society and the American Urological Association (AUA). A clinic that prescribes on symptoms alone is skipping it.
Symptoms like low libido, fatigue and lost muscle start the conversation. The labs decide whether testosterone is the answer or whether something else is driving the problem.
| Test | What it checks or rules out | Result that changes the plan |
|---|---|---|
| Total testosterone, twice | Confirms deficiency is real and consistent | One normal result means retest, not prescribe |
| Free testosterone and SHBG | Binding protein shifts that distort total T | Low free T can confirm a borderline total T |
| LH and FSH | Testicular vs pituitary cause | Low or normal LH triggers a pituitary workup |
| Prolactin | Prolactinoma | Persistently high: repeat, then MRI |
| Iron studies | Hemochromatosis | Iron overload is treated first |
| Hematocrit (CBC) | Thick blood risk | Above 50 percent: hold TRT |
| PSA | Prostate cancer baseline | Elevated: urology review first |
| Semen analysis | Fertility baseline | Wanting children soon: consider alternatives |
| A1c, lipids, TSH, estradiol, vitamin D | Conditions that mimic or worsen low T | Treated alongside or before TRT |
1. Total testosterone, twice, in the morning and fasting
Total testosterone measures all the testosterone in your blood. Levels peak in the morning and fall through the day, which is why MedlinePlus notes the test is usually drawn early. Testosterone also drops after a meal, so the Endocrine Society guideline calls for fasting morning samples. Most clinics aim for a draw between 7 and 10 a.m.
One low number is not a diagnosis. A bad night of sleep, a cold, a hard training block or heavy drinking can drag a single result down, and opioids and glucocorticoids suppress it too. The Endocrine Society says to diagnose hypogonadism only when symptoms are paired with "unequivocally and consistently low" testosterone confirmed by a repeat test, and to avoid testing during acute illness. The AUA asks for two separate early morning measurements.
What changes the plan: The AUA uses a total testosterone below 300 ng/dL as a reasonable cutoff. The Endocrine Society uses a harmonized lower limit of 264 ng/dL for labs standardized to the CDC method. If one result is low and the other normal, the next step is another test or a closer look at free testosterone.
2. Free testosterone and SHBG, when total T is borderline
Most testosterone travels bound to sex hormone binding globulin (SHBG) and albumin. Only the small unbound fraction, free testosterone, acts directly on your tissues. When SHBG is unusually high or low, total testosterone can mislead.
The Endocrine Society recommends measuring free testosterone when total T is near the lower limit of normal or when a condition alters SHBG:
- SHBG tends to rise with aging, hyperthyroidism, liver disease and some anticonvulsants.
- SHBG tends to fall with obesity, type 2 diabetes, hypothyroidism and glucocorticoid or anabolic steroid use.
The guideline favors equilibrium dialysis or a calculation from total T, SHBG and albumin over direct analog assays.
What changes the plan: Borderline total T with clearly low free T may still be true deficiency. Low total T caused by obesity lowering SHBG may come with normal free T, which points to metabolic health first. Our guide to free versus total testosterone goes deeper.
3. LH and FSH: finding where the problem starts
Luteinizing hormone (LH) and follicle stimulating hormone (FSH) are the pituitary's signals to the testes. Both guidelines recommend measuring them to locate the breakdown.
- Low testosterone with high LH and FSH means primary hypogonadism: the testes are not responding. Causes include Klinefelter syndrome, past chemotherapy or radiation, and testicular injury.
- Low testosterone with low or inappropriately normal LH and FSH means secondary hypogonadism: the pituitary or hypothalamus is sending a weak signal. Causes include obesity, opioids, past anabolic steroid use, sleep apnea, pituitary tumors and iron overload.
What changes the plan: Secondary hypogonadism sometimes has a reversible cause, and it opens options that raise your own testosterone instead of replacing it. It also triggers the next tests.
4. Prolactin, and when MRI or iron studies come next
When LH is low or normal, the AUA recommends checking prolactin. A persistently high level can signal a prolactinoma, a benign pituitary tumor that suppresses testosterone and is usually treated with medication rather than TRT. A high result is normally repeated first, since stress and some medications can raise it.
The Endocrine Society suggests a pituitary MRI in secondary hypogonadism when total testosterone is below 150 ng/dL, prolactin stays elevated, other pituitary hormones are low, or there are signs of a mass such as new headaches or vision changes.
Iron studies (transferrin saturation and ferritin) screen for hemochromatosis, an inherited iron overload that can lower testosterone by depositing iron in the pituitary. Untreated, it also damages the liver, heart and pancreas, so catching it matters well beyond hormones.
What changes the plan: A prolactinoma or hemochromatosis gets treated first. Starting TRT on top of either would hide the real cause.
5. Safety baselines: hematocrit, PSA and blood pressure
These tests do not diagnose low testosterone. They confirm treatment is safe to start and become the reference point for every follow-up lab.
Hematocrit. Testosterone stimulates red blood cell production, and hematocrit that climbs too high thickens the blood. The AUA advises withholding testosterone if baseline hematocrit is above 50 percent until the cause is evaluated.
PSA. Known prostate cancer is a reason not to start TRT, so a baseline is essential. The Endocrine Society recommends assessing prostate cancer risk in men aged 55 to 69, and in men 40 to 69 at higher risk, such as Black men and those with a family history. The AUA recommends a PSA for men over 40 before starting. The Endocrine Society advises a urology evaluation before TRT if PSA is above 4 ng/mL, or above 3 ng/mL in higher risk men.
Blood pressure. In 2025 the U.S. FDA required class-wide labeling changes for testosterone products, adding results of the TRAVERSE cardiovascular safety trial and a warning about blood pressure increases. A baseline reading shows whether treatment moves yours.
Untreated severe sleep apnea, a recent heart attack or stroke, and uncontrolled heart failure are also reasons the guidelines give to pause. After you start, these values are rechecked on a schedule; see understanding your bloodwork on TRT.
6. Fertility check: semen analysis if children are in your plans
Testosterone from outside the body tells the pituitary to stop sending LH and FSH, and sperm production can drop sharply, sometimes to zero. The AUA says men should be told this and that testosterone should not be prescribed to men currently trying to conceive. Recovery after stopping is common but not guaranteed.
Neither guideline requires a semen analysis for everyone. If you want children in the next year or two, though, a baseline is a sensible step, and it may change the treatment choice. At BioCure, men focused on fertility can be evaluated for bioRestore, which uses medications that raise the body's own testosterone without direct hormone replacement.
7. Optional but useful labs, and where to begin
These do not diagnose low testosterone, but they catch conditions that mimic or worsen it.
- Estradiol: The AUA recommends it before treatment in men with breast tenderness or enlargement.
- A1c: Low testosterone is common with obesity and type 2 diabetes.
- Lipids: A cardiovascular baseline.
- TSH: Thyroid disease causes fatigue and low libido on its own and shifts SHBG.
- Vitamin D: Evidence that it raises testosterone is weak, but deficiency is common and worth correcting.
BioCure's Comprehensive Male Panel, part of our diagnostics, covers hormones, thyroid, lipids, and metabolic and organ markers across 90+ biomarkers. An in-home phlebotomist can draw it, or you can go to a lab, with results in 7 to 14 days and a specialist reviewing every result with you. If your first results point to something that needs a closer look, ask your physician whether second-line tests such as prolactin or iron studies make sense for you.
If you are starting from zero: complete the 5 minute online intake, book a $199 physician video consultation (credited back with a membership), get your labs drawn, and review the results 1:1 before any prescription. From there, our testosterone therapy tiers range from foundational support to full replacement. BioCure is licensed in 47 states and does not bill insurance directly; many patients use HSA or FSA funds.
Frequently asked questions
Do I need to fast before a testosterone blood test?
Yes. Testosterone drops after eating, so the Endocrine Society guideline calls for fasting morning samples. Most clinics draw blood between 7 and 10 a.m., when levels are highest.
Can I start TRT after just one low testosterone test?
Guidelines say no. The Endocrine Society and the AUA both require a repeat test, because poor sleep, illness and other factors can lower a single result.
What testosterone level qualifies you for TRT?
The AUA uses a total testosterone below 300 ng/dL on two morning tests as a reasonable cutoff, and the Endocrine Society uses a harmonized lower limit of 264 ng/dL. A number alone does not qualify you; your symptoms have to match.
Will testosterone therapy affect my fertility?
It can. Outside testosterone suppresses the hormones that drive sperm production, sometimes to zero. If you want children soon, get a semen analysis and ask about options that raise your own testosterone instead.
Can I get pre-TRT blood work done at home?
Yes. At BioCure, an in-home phlebotomist can draw your Comprehensive Male Panel, or you can visit a lab. Results arrive in 7 to 14 days, and a specialist reviews them with you.
A testosterone plan is only as good as the workup behind it. The right labs tell you whether replacement makes sense, what else needs attention first, and what numbers to watch for years to come. That is the difference between treatment by default and treatment by design.
Let BioCure Health lead you in the right direction. Call or text us to schedule an introductory call at 754-206-0838. Your future self will thank you.