Testosterone lab reports and treatment discussions are packed with abbreviations: SHBG, LH, hematocrit, PSA, E2. This glossary defines the terms you are most likely to meet, grouped by where they show up, so you can read a report and follow a conversation with a clinician. It is educational only. It does not tell you whether you need treatment.
If you want a number of your own to apply these terms to, an at-home blood test kit lets you collect a sample without a trip to a lab, and our guide to reading your at-home lab results walks through a full report.
Measuring Testosterone
Most confusion starts with the lab report itself. These terms explain what each number represents and why two people with the same total testosterone can feel very different.
Total testosterone
All the testosterone in your blood, including the portion attached to proteins. It is the most commonly ordered number and is usually reported in ng/dL (or nmol/L). The American Urological Association uses a total testosterone below 300 ng/dL, confirmed on two early-morning measurements, as a reasonable cutoff for low testosterone.
Free testosterone
The small fraction (roughly 1-3%) not bound to any protein and therefore available to tissues. It can be measured directly by a specialized method or estimated by a calculation using total testosterone, SHBG and albumin. It matters most when SHBG is unusually high or low.
Bioavailable testosterone
Free testosterone plus the part loosely bound to albumin. Because albumin releases testosterone easily, this pool is considered usable by tissues. It is less commonly reported than total or free.
SHBG (sex hormone-binding globulin)
A protein made mostly by the liver that binds testosterone tightly. High SHBG can make total testosterone look normal while free testosterone is low; low SHBG does the reverse. SHBG tends to rise with age and fall with higher body weight and insulin resistance.
Albumin
The most abundant blood protein. It binds testosterone weakly, so it is used in free and bioavailable testosterone calculations.
ng/dL and nmol/L
The two common units for testosterone. To convert, multiply ng/dL by 0.0347 to get nmol/L. A 300 ng/dL result is about 10.4 nmol/L. Always compare your number to the range printed for the same unit.
Reference range
The span of values a given lab considers typical, usually drawn from a healthy reference population. Ranges differ between labs and methods, and a result inside the range is not automatically optimal for you. Our guide to testosterone levels by age shows how typical values shift over time.
Diurnal variation
The daily rhythm in testosterone, which is highest in the morning and lower later in the day. This is why guidelines recommend early-morning sampling, and why a single afternoon result can mislead.
Immunoassay vs. LC-MS/MS
Two ways labs measure hormones. Immunoassays are fast and widely used; liquid chromatography-tandem mass spectrometry (LC-MS/MS) separates molecules more precisely and is considered the reference method, especially at the low concentrations typical for estradiol in men. See our piece on DHT and estradiol testing for more.
Dried blood spot
A small, dried sample of blood collected on a card and mailed to a laboratory. Validated methods exist for several steroid hormones. It is the collection format behind many at-home kits, such as an at-home testosterone test kit.
One low result does not mean low testosterone. Levels vary day to day, so guidelines call for repeat early-morning testing and a symptom review before anything is concluded. A licensed clinician makes any diagnosis and treatment decision after reviewing your results.
The Hormone Signaling System
HPG axis (hypothalamic-pituitary-gonadal axis)
The feedback loop that controls testosterone. The hypothalamus signals the pituitary, the pituitary signals the testes, and the testes produce testosterone, which in turn tells the upper parts of the loop to ease off. Most testosterone problems trace back to a break somewhere in this chain.
GnRH (gonadotropin-releasing hormone)
The pulsing signal released by the hypothalamus that tells the pituitary to release LH and FSH.
LH (luteinizing hormone)
The pituitary hormone that tells Leydig cells in the testes to make testosterone. Checking LH alongside testosterone helps show whether the problem is in the testes or higher up.
FSH (follicle-stimulating hormone)
The pituitary hormone that supports sperm production in the testes. High FSH with low testosterone often points to a testicular cause.
Leydig cells
Cells in the testes that produce testosterone in response to LH.
Hypogonadism
A condition in which the testes produce too little testosterone, and sometimes too few sperm. Diagnosis requires both consistently low lab values and symptoms, and is made by a clinician rather than by a number alone.
Primary hypogonadism
Low testosterone caused by a problem in the testes themselves. LH and FSH are typically high because the brain is signaling harder to compensate.
Secondary hypogonadism
Low testosterone caused by a problem in the pituitary or hypothalamus. LH and FSH are low or inappropriately normal. Obesity, certain medications, sleep apnea and pituitary conditions are among the contributors.
Late-onset hypogonadism
Low testosterone with symptoms that emerges in older adulthood, often alongside other health conditions. The European Male Aging Study proposed a combination of low testosterone and three sexual symptoms as a diagnostic definition.
Eugonadal
Having testosterone levels in the typical range for your age and sex. It is the opposite of hypogonadal.
Prolactin
A pituitary hormone. Elevated prolactin can suppress LH and cause low testosterone, so it is sometimes checked when secondary hypogonadism is suspected.
Cortisol
The body's main stress hormone, which follows a similar morning-high pattern. Chronic stress and poor sleep can lower testosterone indirectly.
| Pattern | Testosterone | LH / FSH | Usual meaning |
|---|---|---|---|
| Primary | Low | High | Problem in the testes |
| Secondary | Low | Low or normal | Problem in the pituitary or hypothalamus |
| Eugonadal | Typical range | Typical range | Signaling intact |
Related Hormones and Conversions
Estradiol (E2)
The main form of estrogen. Men make it too, mostly by converting testosterone, and need some of it for bone, brain and sexual health. Both very low and very high levels cause problems. Read more in our estradiol guide.
Aromatase
The enzyme that converts testosterone to estradiol. It is found mostly in fat tissue, which is one reason higher body fat is linked with higher estradiol.
DHT (dihydrotestosterone)
A more potent androgen made from testosterone by the enzyme 5-alpha reductase. It plays a role in hair follicles and the prostate.
5-alpha reductase
The enzyme that converts testosterone into DHT.
DHEA-S
A hormone made mostly by the adrenal glands that serves as a precursor to other sex hormones. It declines steadily with age.
Androgens
The family of hormones, including testosterone and DHT, responsible for male characteristics and many body functions.
Anabolic
Describes effects that build tissue, such as muscle and bone. Anabolic steroids are synthetic drugs and are not the same as medically supervised testosterone therapy.
Therapy and Treatment Terms
TRT (testosterone replacement therapy)
Medical treatment that supplies testosterone to bring levels back toward a typical range in men with a confirmed deficiency. It requires a prescription, an evaluation by a licensed clinician, and ongoing lab monitoring. Our overview of how TRT works goes into detail.
Delivery form
The way a clinician prescribes testosterone, such as a topical gel, a patch, or other formats. Each leads to a different blood-level pattern, so the choice is individualized.
Peak and trough
The highest and lowest points of a testosterone level over a dosing cycle. Monitoring labs are often timed relative to these points so the numbers can be interpreted properly.
Half-life
How long it takes for the amount of a substance in the body to fall by half. It explains why different forms of therapy are dosed at different intervals.
Titration
Gradually adjusting a dose based on symptoms and lab results rather than starting at a fixed amount for everyone.
Monitoring labs
Follow-up blood work, typically including testosterone, hematocrit and PSA, checked at intervals set by the prescribing clinician.
Suppression (of the HPG axis)
When outside testosterone signals the brain that levels are sufficient, LH and FSH fall and the testes make less testosterone and fewer sperm. It is expected during therapy and is why fertility is a key conversation before starting. See TRT and fertility.
Testicular atrophy
A reduction in testicular size that can occur when natural production is suppressed.
Spermatogenesis
The process of making sperm, which depends on high testosterone concentrations inside the testes and on FSH.
Contraindication
A condition that makes a treatment unsafe. For testosterone therapy, examples include prostate or breast cancer, and a desire for near-term fertility is an important consideration.
Safety and Monitoring Terms
Hematocrit
The percentage of your blood volume made up of red blood cells. Testosterone can raise it, and a high value thickens the blood, so it is tracked on therapy. Clinical guidance commonly calls for action when it exceeds 54%.
Hemoglobin
The oxygen-carrying protein in red blood cells, usually checked together with hematocrit.
Erythrocytosis (polycythemia)
An abnormally high red blood cell count. It is the most common laboratory change seen with testosterone therapy.
PSA (prostate-specific antigen)
A protein made by the prostate and measured in blood. It is used for prostate monitoring alongside clinician judgment, and is checked before and during therapy.
Lipid panel
Blood tests measuring cholesterol and triglycerides, sometimes included in monitoring.
Cardiovascular event
A heart attack or stroke. The TRAVERSE trial, published in 2023 in the New England Journal of Medicine, found testosterone therapy was noninferior to placebo for major adverse cardiovascular events in men with hypogonadism and elevated cardiovascular risk. See is TRT safe?
Sleep apnea
A condition in which breathing repeatedly pauses during sleep. It can lower testosterone and can worsen with therapy, so it is part of the safety conversation.
Gynecomastia
Enlargement of male breast tissue, which can be associated with a shift in the testosterone-to-estradiol balance.
FAQ
What does TRT stand for?
Testosterone replacement therapy: a medical treatment, prescribed by a licensed clinician, for men with a confirmed testosterone deficiency.
What is the difference between total and free testosterone?
Total testosterone counts everything in the blood. Free testosterone counts only the small portion not bound to proteins such as SHBG. When SHBG is far from typical, the two can tell different stories.
What is a normal testosterone level?
It depends on the lab, the method, and your age. The American Urological Association uses 300 ng/dL as a cutoff for low total testosterone, but symptoms and repeat measurements matter as much as any single value.
Can I test testosterone at home?
Yes, a kit can collect a dried blood sample at home and mail it to a lab. Our at-home testosterone test kit page explains the options, and the at-home lab kit page has details.
Why does hematocrit matter?
Because red blood cell count can rise during testosterone therapy and thicker blood raises clotting concerns, which is why it is monitored.
Sources
- Mulhall JP et al. Evaluation and Management of Testosterone Deficiency: AUA Guideline. J Urol. 2018.
- Bhasin S et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018.
- Lincoff AM et al. Cardiovascular Safety of Testosterone-Replacement Therapy (TRAVERSE). N Engl J Med. 2023.
- Wu FCW et al. Identification of Late-Onset Hypogonadism in Middle-Aged and Elderly Men (EMAS). N Engl J Med. 2010.
This article is for education and is not a substitute for individualized medical advice. Diagnosis and treatment decisions are made by a licensed clinician after reviewing symptoms and lab results.