Testosterone is metabolized through several overlapping pathways, primarily in the liver but also in target tissues throughout the body. The process involves two main phases: enzymatic transformation of the hormone into other active or inactive compounds, followed by chemical tagging that makes those compounds water-soluble enough to be excreted in urine. A healthy young man clears roughly 1,000 to 1,300 liters of blood per day of testosterone, producing about 7 to 9 milligrams daily to keep pace.
Phase I: Liver Enzymes Break Down the Molecule
The liver handles the bulk of testosterone breakdown using a family of enzymes called CYP3A. CYP3A4 is the dominant one in adults, and it’s the same enzyme responsible for metabolizing at least 50% of all prescription drugs. This enzyme adds a hydroxyl group (an oxygen-hydrogen pair) to the testosterone molecule at specific positions, with the most common product being 6β-hydroxytestosterone. A closely related enzyme, CYP3A5, performs similar work, while a third family member, CYP3A7, is the version active in fetal and newborn livers and produces a slightly different mix of metabolites.
These hydroxylated products are less biologically active than testosterone itself. The reaction essentially begins dismantling the hormone so it can be processed further and eventually eliminated.
Conversion to DHT in Target Tissues
Not all testosterone metabolism is about deactivation. In tissues like the prostate, skin, and hair follicles, an enzyme called 5-alpha reductase type 2 converts testosterone into dihydrotestosterone (DHT), a more potent androgen that drives effects like body hair growth, prostate development, and male-pattern baldness. DHT binds to the same androgen receptor as testosterone but with greater strength, which is why it has outsized effects in certain tissues despite circulating at lower concentrations.
People who lack functional 5-alpha reductase produce very little DHT. This causes noticeable differences in sexual development, particularly before puberty, confirming how central this conversion step is to androgen signaling. Some individuals retain a small amount of enzyme activity, which can produce enough DHT to trigger changes during puberty even when levels are well below normal.
Conversion to Estrogen via Aromatase
A portion of circulating testosterone is converted into estradiol, the primary estrogen, by an enzyme called aromatase. This happens in fat tissue, bone, brain, and the gonads. Aromatase strips off a carbon atom from the testosterone molecule and reshapes its core ring structure, transforming it from a 19-carbon androgen into an 18-carbon estrogen.
This conversion has a dual effect: it removes a molecule of androgen and replaces it with estrogen that is, on a molecule-for-molecule basis, 100 to 1,000 times more biologically active. Even small amounts of aromatase activity are physiologically significant. In men, this estrogen production is important for bone density, brain function, and cardiovascular health. In people with higher body fat, increased aromatase activity can shift the balance toward more estrogen and less available testosterone.
Activation and Deactivation Inside Cells
Within cells, a family of enzymes called 17β-hydroxysteroid dehydrogenases (17β-HSDs) control whether testosterone is being created or broken down. The type 3 version, found almost exclusively in the testes, converts the weaker precursor androstenedione into testosterone with high efficiency. The reverse reaction also occurs but is slower. These enzymes sit on the endoplasmic reticulum, the cell’s internal manufacturing network, and use common cellular energy carriers to drive the conversion in either direction.
This means tissues can locally fine-tune their testosterone exposure. A cell doesn’t simply receive whatever concentration the bloodstream delivers. It can amplify or dampen the signal depending on which version of these enzymes it expresses.
Phase II: Tagging for Elimination
After the initial breakdown, testosterone and its metabolites go through a second round of processing called conjugation. Two main reactions handle this: glucuronidation and sulfation. In glucuronidation, enzymes called UDP-glucuronosyltransferases (UGTs) attach a sugar-acid molecule to the steroid, making it far more water-soluble. UGT2B15 and UGT2B17 are the primary ones involved in androgen conjugation. In sulfation, sulfotransferase enzymes attach a sulfate group instead.
Both reactions serve the same purpose: they make the metabolites polar enough to dissolve in urine and bile so the body can excrete them. These conjugated products are biologically inactive, which means conjugation also serves as a final off-switch for any remaining hormonal activity.
What Leaves the Body in Urine
The end products of testosterone metabolism show up in urine as a group of compounds called 17-ketosteroids. The major ones are androsterone, etiocholanolone, and DHEA in its conjugated form. In women, daily excretion of these metabolites totals roughly 10 milligrams, with androsterone accounting for the largest share at 0.6 to 3.2 mg/day, etiocholanolone at 0.2 to 2.9 mg/day, and conjugated DHEA at 0.2 to 1.7 mg/day. Not all urinary 17-ketosteroids come from testosterone; a portion derives from other steroid hormones, including those produced by the adrenal glands.
SHBG Controls How Much Gets Metabolized
Sex hormone-binding globulin (SHBG) is a protein in the blood that binds to testosterone and holds it in an inactive state. Testosterone that’s attached to SHBG can’t enter cells, can’t bind to receptors, and can’t be metabolized. Only the free (unbound) fraction and the portion loosely bound to albumin are available for tissues to use or for the liver to clear.
This means SHBG effectively acts as a metabolic brake. When SHBG levels are high, less testosterone is free, so less reaches enzymes for processing, and less is biologically active. When SHBG is low, more testosterone circulates freely, more gets used by tissues, and more gets cleared. Conditions that raise SHBG (aging, hyperthyroidism, certain medications) slow testosterone turnover, while conditions that lower it (obesity, insulin resistance) speed it up.
How Liver Disease and Obesity Change the Process
Because the liver is central to both testosterone breakdown and SHBG production, liver disease can substantially disrupt hormone metabolism. Fatty liver disease is an increasingly recognized factor: men without other metabolic problems who have fatty liver have a nearly four-fold higher risk of testosterone levels dropping below 300 ng/dL, and a nearly five-fold higher risk of clinically defined testosterone deficiency. The mechanism involves disrupted SHBG production, insulin resistance, chronic inflammation, and oxidative stress, all of which can suppress the hypothalamic-pituitary-gonadal axis that controls testosterone production in the first place.
Obesity compounds the problem through multiple channels. More body fat means more aromatase activity, which converts more testosterone to estrogen. Obesity also lowers SHBG, increasing the fraction of testosterone available for clearance. And the metabolic inflammation that accompanies excess weight further impairs the liver’s ability to regulate hormone metabolism normally. The result is a cycle where excess weight accelerates testosterone loss, and lower testosterone makes it harder to lose weight.
Metabolic Clearance Slows With Age
In young men, the metabolic clearance rate of testosterone averages roughly 1,070 to 1,270 liters per day. By middle age, that drops to around 810 liters per day, a decline of about 35%. Production rates fall in parallel, from roughly 7 to 9 mg/day in young men down to about 4 mg/day in middle-aged men. Both clearance and production follow a daily rhythm, peaking around midday and dropping by evening, which is why testosterone blood tests are typically drawn in the morning when levels are highest.