Testosterone exists in several distinct forms inside your body, and when prescribed as therapy, it comes in even more variations. Understanding the differences matters whether you’re interpreting lab results or exploring treatment options, because not all testosterone in your blood does the same thing, and not all replacement formulations work the same way.
Types of Testosterone in Your Blood
The testosterone circulating in your bloodstream isn’t a single, uniform substance. It’s divided into fractions based on what it’s attached to. About 44% is tightly bound to a carrier protein called sex hormone-binding globulin (SHBG), roughly 50% is loosely bound to albumin (another blood protein), around 4% is bound to a third protein, and only about 2% floats freely with no protein attached at all.
These distinctions aren’t just academic. They determine how much testosterone your body can actually use.
Free Testosterone
Free testosterone is the small fraction (2 to 3% of your total) that circulates unbound. Because nothing is attached to it, free testosterone can enter cells directly and activate androgen receptors. It’s considered the most biologically active form.
SHBG-Bound Testosterone
Testosterone bound to SHBG is locked up tightly. While it makes up a large share of your total testosterone, it’s essentially inactive because the bond is too strong for tissues to pull the testosterone away and use it. SHBG levels rise with age, which means a larger proportion of your total testosterone gets trapped in this inactive form over time. This is one reason why total testosterone on a lab report can look reasonable while you still experience symptoms of low testosterone.
Bioavailable Testosterone
Bioavailable testosterone refers to the combination of free testosterone plus albumin-bound testosterone. The albumin bond is weak enough that tissues can strip the testosterone away and use it, giving albumin-bound testosterone real androgenic potential. When doctors suspect that SHBG levels are skewing total testosterone results (common in older men, men with obesity, or those with liver or thyroid conditions), they may order a bioavailable or free testosterone measurement to get a clearer picture.
What Testosterone Converts Into
Testosterone itself is also a precursor to two other important hormones. In many tissues, enzymes called 5-alpha reductases convert testosterone into dihydrotestosterone (DHT), a more potent androgen responsible for traits like body hair growth and prostate tissue stimulation. A separate enzyme called aromatase converts testosterone into estradiol, a form of estrogen. Both conversions happen in peripheral tissues throughout the body, and the balance between testosterone, DHT, and estradiol influences everything from bone density to mood to sexual function.
Normal Testosterone Levels by Age
What counts as “normal” depends on your age and which medical organization you ask. A 2023 study in the Journal of Urology looked at healthy men aged 20 to 44 and found that age-specific normal ranges shift noticeably even within that window. Men aged 20 to 24 had middle-range levels of 409 to 558 ng/dL, while men aged 40 to 44 ranged from 350 to 473 ng/dL.
The cutoff for “low” testosterone is similarly variable. Different medical societies place the threshold for deficiency anywhere from 231 ng/dL to 346 ng/dL, creating a gray zone that affects a significant number of men being evaluated. Insurance providers add another layer of inconsistency, with some requiring levels below 300 ng/dL and others using 250 ng/dL to approve therapy. For free testosterone, most major guidelines converge around a cutoff of roughly 220 to 225 pmol/L, though not all societies agree on whether free testosterone should even be measured routinely.
Injectable Testosterone Esters
When testosterone is prescribed as an injection, it’s chemically modified with an ester (a fatty acid chain) that slows its release into the bloodstream. The length of that chain determines how long each injection lasts.
Testosterone cypionate and testosterone enanthate are the most commonly prescribed injectable forms. They have a half-life of about 173 hours (roughly seven days) and are typically injected every one to two weeks. These are the workhorses of testosterone replacement therapy, widely available and well understood. Some men inject weekly rather than biweekly to keep levels more stable and reduce the peaks and valleys between doses.
Testosterone undecanoate is the long-acting option. Each injection delivers 1,000 mg in a castor oil solution, with a loading schedule of one injection at the start, another at six weeks, and then one every 10 to 14 weeks after that. For about 70% of patients, the optimal interval falls around 12 weeks, meaning roughly four injections per year. The tradeoff is that these injections must be given in a clinical setting because of a small risk of a reaction related to the oil-based carrier, so you can’t self-administer at home.
Topical Testosterone
Testosterone gels are applied daily to the skin, typically on the shoulders or upper arms, and absorb through the skin to deliver a steady, low-level dose. They come in packets or pump dispensers with doses ranging from about 20 mg to 80 mg per application. The main advantage of gels is that they maintain relatively stable blood levels without the peaks and troughs of injections. The main disadvantage is the risk of skin-to-skin transfer: if another person (especially a child or a pregnant partner) touches the application site before the gel has fully dried, they can absorb the testosterone.
Testosterone patches work on the same principle but deliver the hormone through an adhesive patch worn on the body. They’re applied once daily and rotated between sites to reduce skin irritation, which is their most common side effect.
Implantable Testosterone Pellets
Testosterone pellets are small crystalline implants (typically 10 to 14 pellets at 75 mg each) placed under the skin during a brief in-office procedure. They dissolve slowly over three to six months, targeting a peak testosterone level of 500 to 800 ng/dL. Pellets offer the longest gap between treatments of any delivery method and eliminate the need to remember daily applications or frequent injections. The downside is that once they’re implanted, the dose can’t be easily adjusted if levels come back too high or too low. You have to wait for the pellets to dissolve.
Oral Testosterone
Oral testosterone has historically been limited by liver toxicity concerns, since swallowed medications typically pass through the liver before reaching the rest of the body. Newer oral formulations use testosterone undecanoate dissolved in lipids (fatty acids) that get absorbed through the intestinal lymphatic system, largely bypassing the liver. The FDA approved one such oral soft gel capsule formulation in 2022.
These capsules need to be taken with food to ensure proper absorption through the lymphatic pathway. Reported side effects include increased blood pressure, elevated red blood cell counts (which raises clotting risk), fluid retention, breast tenderness, and sleep apnea. These side effects are not unique to oral forms; they’re shared across nearly all testosterone replacement methods and relate to the hormone itself rather than the delivery route.
How Delivery Methods Compare
The choice between formulations usually comes down to lifestyle preferences, comfort with injections, and how your body responds. Here’s a practical comparison:
- Weekly or biweekly injections (cypionate/enanthate): Most affordable option, can be self-administered at home, but produce noticeable peaks and troughs in energy and mood between doses.
- Long-acting injections (undecanoate): Only four times per year, but must be given in a clinic and carry a higher per-visit cost.
- Daily gels: Stable blood levels, no needles, but require careful hygiene to prevent transfer and must be applied consistently every day.
- Patches: Similar stability to gels with less transfer risk, but skin irritation is common.
- Pellets: Longest treatment-free intervals, but require a minor procedure and offer no dose flexibility once implanted.
- Oral capsules: No needles or skin application, but must be taken with meals and are among the newer options with less long-term data.
Blood monitoring is part of all these approaches. Regardless of the formulation, testosterone therapy requires periodic lab work to check hormone levels, red blood cell counts, and other markers to ensure the dose is appropriate and side effects stay manageable.