What Is Testosterone? The hormone, normal ranges, and how injectable dosing works

What testosterone actually is

Testosterone is an androgen, a class of steroid hormone built from cholesterol. In men it is synthesised mainly by the Leydig cells of the testes, with a small amount from the adrenal glands; in women the ovaries and adrenals produce much smaller quantities. Its release is governed by the hypothalamic-pituitary-gonadal axis: the brain sends luteinising hormone (LH) to the testes, the testes make testosterone, and rising testosterone feeds back to dial the signal down again (Nassar & Leslie, StatPearls). Functionally it supports muscle and bone mass, libido, sperm production, mood, and the manufacture of red blood cells.

Most of the testosterone circulating in your blood is not freely active. Roughly 98% is bound to carrier proteins — tightly to sex hormone-binding globulin (SHBG) and loosely to albumin — leaving only a few percent as free testosterone able to enter cells. That is why a lab panel often reports several numbers: total testosterone, free testosterone, and SHBG. This page is about the hormone and its dosing maths; the binding-protein detail is covered in free testosterone explained.

Normal testosterone reference ranges

A blood test usually reports total testosterone — bound plus free — in nanograms per deciliter (ng/dL) in the US or nanomoles per liter (nmol/L) elsewhere. A widely cited harmonised range from four large cohorts put the middle 95% of healthy non-obese men aged 19–39 at 264–916 ng/dL, with a median of 531 ng/dL (Travison 2017). Lab cut-offs vary, so always read your own report against its printed range rather than a number from the internet.

To convert ng/dL to nmol/L, divide by about 28.85 (multiply by 0.0347). The Endocrine Society suggests diagnosing hypogonadism only on a consistently low morning total testosterone confirmed on a repeat sample, alongside symptoms — never on a single value (Bhasin 2018). The numbers above describe a population, not a personal target.

How injectable Testosterone (TRT) dosing works

When testosterone is genuinely low, the most common injectable replacement is an oil-based ester such as testosterone cypionate or enanthate. The ester is a chemical "tail" that slows release so the hormone trickles out over days, producing a peak after injection and a trough before the next one (Nieschlag 1984). That slow release is why Testosterone (TRT) is dosed per week rather than per day, and why injection frequency changes how steady your levels feel.

A prescription gives you a dose in milligrams per week. The vial gives you a concentration in mg/mL — testosterone cypionate is typically supplied at 200 mg/mL (DailyMed, Hikma label), though 100 mg/mL and 250 mg/mL exist. To find the liquid amount to inject, you divide the dose by the concentration. The result is a volume in mL, which you can also read as U-100 syringe units (100 units = 1 mL).

Three numbers are easy to mix up. The dose (mg) is the amount of hormone. The volume (mL) is how much oil carries it. The syringe units are just volume marks printed on the barrel. The same syringe mark can mean a different dose at a different concentration — see testosterone esters explained for how ester choice and strength interact.

How this is calculated

The core arithmetic has three steps. First, split the weekly dose by your injection frequency to get the per-shot dose. Second, divide the per-shot dose (mg) by the vial concentration (mg/mL) to get the volume (mL). Third, multiply mL by 100 to read it as units on a U-100 insulin syringe.

Written as a formula: volume (mL) = dose (mg) ÷ concentration (mg/mL), then units = mL × 100. The only trap is unit matching — mg must pair with mg/mL, and you must convert mg to mcg (×1000) if your figures are mixed. Below are worked numbers for the standard 200 mg/mL cypionate vial unless stated otherwise.

Common mistakes

The most frequent error is copying someone else's syringe-unit number without matching their vial strength. Fifty units from a 200 mg/mL vial is 100 mg; fifty units from a 100 mg/mL vial is only 50 mg — same mark, half the hormone. Always start from the dose, not the mark.

A second is comparing your lab number to the wrong range or unit. A 600 figure is healthy in ng/dL but impossible in nmol/L; check which unit your report uses before reacting. A third is treating the population reference range as a personal goal — diagnosis depends on symptoms plus a repeat morning test, not chasing the 97.5th percentile (Bhasin 2018).

So, what is testosterone?

Testosterone is the main male sex hormone, made mostly in the testes, that drives muscle, bone, libido, mood and red-blood-cell production. A typical male total-testosterone range runs about 264–916 ng/dL (≈9–32 nmol/L); when it is low, injectable testosterone replaces it as a weekly milligram dose you convert into a syringe volume. Convert a weekly dose to units with the testosterone dose calculator.

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