Glossary

Testosterone replacement therapy is the medical administration of exogenous testosterone to restore physiologic hormone levels in men with hypogonadism. It is distinct from anabolic steroid use in that doses target the normal reference range rather than supraphysiologic levels, though the line is frequently blurred in practice.

Anabolic-androgenic steroids are synthetic derivatives of testosterone engineered to maximize anabolic (muscle-building) effects while modifying androgenic (masculinizing) activity. They include both testosterone esters and structurally modified compounds like nandrolone, trenbolone, oxandrolone, and stanozolol.

Post-cycle therapy is a protocol of pharmacological agents used after an AAS cycle to stimulate the hypothalamic-pituitary-testicular axis (HPTA) and accelerate recovery of endogenous testosterone production. Common PCT agents include clomiphene (Clomid) and tamoxifen (Nolvadex), which act as selective estrogen receptor modulators (SERMs) to increase LH and FSH secretion.

Aromatization is the enzymatic conversion of androgens (testosterone, androstenedione) into estrogens (estradiol, estrone) by the enzyme aromatase (CYP19A1), primarily located in adipose tissue, liver, and the brain. On TRT and AAS, aromatization is the primary mechanism driving estradiol elevation.

The HPTA is the hormonal feedback system that regulates endogenous testosterone production. The hypothalamus releases GnRH, stimulating the pituitary to secrete LH and FSH, which signal the testes to produce testosterone. Exogenous testosterone (TRT or AAS) suppresses this axis via negative feedback, shutting down LH, FSH, and natural testosterone production.

Sex hormone-binding globulin (SHBG) is a transport protein that binds testosterone and estradiol with high affinity, rendering the bound fraction biologically inactive. Only free testosterone and albumin-bound testosterone (loosely bound) can exert physiologic effects. SHBG levels determine how much total testosterone is bioavailable.

Bioavailable testosterone refers to the free testosterone plus the albumin-bound fraction — both of which can dissociate from their binding proteins and enter cells to act on androgen receptors. Unlike tightly SHBG-bound testosterone, albumin-bound testosterone is weakly held and readily available to tissues.

Hematocrit is the percentage of blood volume occupied by red blood cells. As hematocrit rises, blood viscosity (resistance to flow) increases non-linearly — at 50% hematocrit, blood is approximately 2× more viscous than plasma; at 60%, it is 4× more viscous. This relationship makes elevated hematocrit on TRT a genuine thrombotic risk, not merely a lab value concern.

LDL-C (LDL cholesterol) measures the cholesterol content of LDL particles, while ApoB counts the total number of atherogenic lipoprotein particles (each LDL, VLDL, and IDL particle carries exactly one ApoB molecule). When LDL particle size is small and dense — which AAS use promotes — LDL-C underestimates actual atherogenic particle burden while ApoB remains accurate.

Hepatotoxicity from oral anabolic steroids results from the 17-alpha alkylation (17-AA) modification that allows these compounds to survive first-pass hepatic metabolism. This modification impairs normal hepatocyte function, blocks bile secretion (cholestasis), and in severe or prolonged cases can cause peliosis hepatis (blood-filled cysts), hepatic adenomas, or hepatocellular carcinoma.

Erythrocytosis refers to an abnormal increase in red blood cell mass, and is the most common adverse effect of TRT affecting up to 40% of patients. Testosterone directly stimulates erythropoietin (EPO) production in the kidneys, driving red cell synthesis. Secondary erythrocytosis from TRT is distinct from primary polycythemia vera but carries similar thrombotic risk when hematocrit exceeds 52–54%.

Estrogen rebound is a rapid rise in estradiol following abrupt discontinuation of an aromatase inhibitor (AI). During AI use, aromatase is suppressed and estradiol is low. When the AI is stopped, aromatase activity rapidly normalizes while testosterone levels may temporarily be at their highest point, resulting in a sudden surge in aromatization and estradiol that can overshoot baseline.

Androgen receptors (ARs) are intracellular proteins that bind testosterone and DHT to mediate gene expression changes that drive protein synthesis, bone density, and virilization. Contrary to early fitness culture belief, chronic AAS exposure does not persistently upregulate androgen receptor density — receptor density normalizes after cessation, and receptor sensitivity returns to baseline.

Insulin sensitivity describes how effectively cells respond to insulin and take up glucose. Many AAS compounds impair insulin signaling — particularly nandrolone, trenbolone, and high-dose testosterone — by interfering with GLUT4 translocation and post-receptor signaling pathways. This can gradually elevate fasting glucose and HbA1c, especially in athletes with existing metabolic risk factors.

AAS profoundly alter lipid metabolism through two primary mechanisms: suppression of hepatic lipase activity (reducing HDL clearance is reversed, causing HDL suppression) and upregulation of LDL receptor downregulation (increasing circulating LDL). The result is an atherogenic lipid profile with low HDL and high LDL/ApoB that closely resembles a familial hypercholesterolemia pattern.

Long-term AAS use causes pathological cardiac remodeling — distinct from the physiological cardiac adaptation seen in endurance training. AAS-driven remodeling includes left ventricular hypertrophy (LVH), myocardial fibrosis, diastolic dysfunction, and potentially irreversible structural changes. These changes are not captured by standard bloodwork and require echocardiography to detect.

Prolactin elevation is a specific risk of 19-Nortestosterone derivatives (nandrolone, trenbolone) due to their agonism at dopamine D2 receptors in the pituitary and direct stimulation of lactotrophic cells. Elevated prolactin (above 25 ng/mL) suppresses GnRH secretion, further impairing HPTA recovery, and can cause sexual dysfunction, gynecomastia, and galactorrhea.

AAS can suppress thyroid function through several mechanisms: reducing TBG (thyroid binding globulin) which artificially lowers measured total T4/T3, suppressing TRH/TSH secretion at high doses, and potentially causing direct pituitary effects. However, many athletes on AAS have subclinical hypothyroidism that predates AAS use and becomes symptomatic under the metabolic demands of training.

Creatinine is the standard marker of kidney function but is produced in proportion to muscle mass, making it unreliable in high-muscle-mass athletes. An athlete with 100 kg of lean mass will have elevated creatinine from muscle turnover alone, triggering false-positive kidney disease screening. Cystatin C is a kidney filtration marker not influenced by muscle mass, making it the superior renal marker for athletes and AAS users.

C-reactive protein (CRP) and interleukin-6 (IL-6) are markers of systemic inflammation. AAS can both suppress inflammation (via glucocorticoid-sparing effects) and promote it (via cardiovascular and hepatic stress). High-sensitivity CRP (hs-CRP) above 3 mg/L in an AAS user is a cardiovascular risk marker even when standard lipid panels appear acceptable.