# TRT Glossary: Key Terminology

> Last updated: February 2026

## Testosterone
The primary male sex hormone (androgen) produced mainly in the Leydig cells of the testes, with small amounts from the adrenal glands. Responsible for the development and maintenance of male sexual characteristics, muscle mass, bone density, fat distribution, red blood cell production, libido, and mood regulation. Normal range: 300-1000 ng/dL in adult men.

## Hypogonadism
A clinical condition in which the body produces insufficient testosterone. **Primary hypogonadism** originates from testicular dysfunction. **Secondary hypogonadism** results from problems with the hypothalamus or pituitary gland. Diagnosis requires total testosterone below 300 ng/dL on at least two morning blood draws plus clinical symptoms.

## TRT (Testosterone Replacement Therapy)
A medically supervised treatment that restores testosterone to normal physiological levels in men diagnosed with hypogonadism. Administered via injections, topical gels, oral medications, pellets, or nasal preparations. Requires a prescription from a licensed physician.

## Free Testosterone
The fraction of total testosterone (typically 1-3%) that circulates unbound to proteins in the blood. Free testosterone is biologically active and available for use by tissues. It is considered a more accurate indicator of testosterone status than total testosterone alone, especially when SHBG levels are abnormal.

## Total Testosterone
The combined measurement of all testosterone in the blood — both bound (to SHBG and albumin) and unbound (free). This is the standard initial screening test for hypogonadism. Measured in nanograms per deciliter (ng/dL).

## SHBG (Sex Hormone-Binding Globulin)
A glycoprotein produced by the liver that binds tightly to testosterone, making it biologically unavailable. Elevated SHBG reduces free testosterone even when total testosterone appears normal. SHBG increases with age, liver disease, hyperthyroidism, and certain medications. Obesity and insulin resistance tend to lower SHBG.

## Estradiol (E2)
The most potent form of estrogen, produced in men primarily through the conversion of testosterone by the aromatase enzyme. Some estradiol is necessary for bone health, brain function, and cardiovascular protection. Elevated E2 during TRT can cause water retention, mood changes, and gynecomastia. Typically managed with aromatase inhibitors if symptomatic. Optimal range on TRT: approximately 20-40 pg/mL.

## Aromatase
An enzyme (CYP19A1) that converts testosterone to estradiol. Found in adipose tissue, liver, brain, and other organs. Higher body fat percentage leads to greater aromatase activity and higher estrogen conversion from testosterone.

## Aromatase Inhibitor (AI)
A class of medications (e.g., anastrozole, letrozole) that block the aromatase enzyme, reducing the conversion of testosterone to estradiol. Used in TRT to manage elevated estrogen levels and associated side effects. Prescribed at low doses (typically 0.25-0.5 mg anastrozole, 1-3 times per week).

## HCG (Human Chorionic Gonadotropin)
A hormone that mimics LH and stimulates the Leydig cells in the testes to produce testosterone and maintain testicular size. Often used alongside TRT to prevent testicular atrophy and preserve some degree of fertility. Typical dose: 250-500 IU, 2-3 times per week.

## Clomiphene (Clomid)
A selective estrogen receptor modulator (SERM) that blocks estrogen feedback at the hypothalamus and pituitary, stimulating increased LH and FSH production. This results in increased natural testosterone production while preserving fertility. Used as an alternative to TRT, especially in younger men who want to maintain sperm production. Typical dose: 25-50 mg daily or every other day.

## LH (Luteinizing Hormone)
A gonadotropin hormone produced by the anterior pituitary gland. LH signals the Leydig cells in the testes to produce testosterone. Exogenous TRT suppresses LH through negative feedback on the HPG axis.

## FSH (Follicle-Stimulating Hormone)
A gonadotropin hormone produced by the anterior pituitary gland. FSH stimulates the Sertoli cells in the testes to support spermatogenesis (sperm production). TRT suppresses FSH, which is the primary mechanism by which it impairs fertility.

## HPG Axis (Hypothalamic-Pituitary-Gonadal Axis)
The hormonal feedback system that regulates testosterone production. The hypothalamus releases GnRH → the pituitary releases LH and FSH → the testes produce testosterone → testosterone feeds back to suppress GnRH and LH/FSH. Exogenous TRT disrupts this axis by providing testosterone from an external source.

## DHT (Dihydrotestosterone)
A potent androgen converted from testosterone by the enzyme 5-alpha reductase. DHT is 2-3 times more potent than testosterone at the androgen receptor. It is the primary hormone responsible for male pattern baldness and prostate growth. 5-alpha reductase inhibitors (finasteride, dutasteride) block this conversion.

## PSA (Prostate-Specific Antigen)
A protein produced by the prostate gland, measured via blood test as a screening tool for prostate health. TRT may cause modest PSA increases. A rise of >1.4 ng/mL within 12 months or an absolute value above age-appropriate norms warrants urological evaluation.

## Hematocrit
The percentage of blood volume occupied by red blood cells. TRT stimulates erythropoiesis (red blood cell production), which can raise hematocrit. Levels above 54% increase blood viscosity and the risk of thromboembolic events (blood clots). Managed through dose reduction, therapeutic phlebotomy, or treatment modification.

## Polycythemia
An abnormal increase in red blood cells, often measured by elevated hematocrit or hemoglobin. This is one of the most common side effects of TRT and requires regular monitoring. Secondary polycythemia from TRT is distinct from polycythemia vera (a bone marrow disorder).

## Testosterone Cypionate
The most commonly prescribed injectable form of testosterone in the United States. An esterified form that provides sustained release over approximately 7-10 days. Typically administered via intramuscular or subcutaneous injection at weekly intervals. Standard dosing: 100-200 mg per week.

## Testosterone Enanthate
An injectable form of testosterone similar to cypionate, with a slightly shorter half-life. More commonly used outside the United States. Functionally interchangeable with testosterone cypionate for clinical purposes.

## ng/dL (Nanograms per Deciliter)
The standard unit of measurement for testosterone levels in the United States. One nanogram equals one billionth of a gram. Normal total testosterone range: 300-1000 ng/dL. In other countries, testosterone may be reported in nmol/L (multiply ng/dL by 0.0347 to convert).

## Dosing Interval
The time between testosterone administrations. For injections, this is typically every 7 days (weekly) or every 3.5 days (twice weekly for more stable levels). For gels, daily application is standard. For pellets, every 3-6 months.

## Subcutaneous (SubQ) Injection
An injection method where testosterone is administered into the fatty tissue beneath the skin, typically in the abdomen or thigh. Uses smaller needles (27-30 gauge) than intramuscular injections. Growing in popularity as studies show comparable absorption to intramuscular administration.

## Intramuscular (IM) Injection
The traditional injection method for testosterone, administered into muscle tissue — typically the gluteal, deltoid, or vastus lateralis (outer thigh). Uses 22-25 gauge needles. Provides reliable and well-studied absorption.

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*This glossary is for informational purposes only. Terms are defined in the context of testosterone replacement therapy and male hormone health.*