When you can start testosterone depends on why you need it. For adult men with low testosterone, treatment can begin once blood tests confirm levels below 300 ng/dL on two separate morning draws and symptoms are present. For transgender and gender-diverse individuals, testosterone can be initiated after meeting informed consent criteria and discussing reproductive options with a provider. For adolescents with delayed puberty, the typical starting point is age 14. Here’s what each pathway looks like in practice.
Starting Testosterone for Low Levels in Men
A diagnosis of low testosterone (hypogonadism) requires more than just feeling tired or having a low sex drive. You need two separate blood draws, both taken in the early morning while fasting, showing a total testosterone level below 300 ng/dL. Morning timing matters because testosterone naturally peaks after waking and drops throughout the day. A single low reading isn’t enough for diagnosis.
Beyond the blood work, you also need at least one clinical symptom. The most closely linked symptoms are low libido and erectile difficulty, but less specific signs also count: persistent fatigue, irritability, depressed mood, poor concentration, and sleep problems. Men over 65 face the same criteria, though unexplained anemia can also qualify as a suggestive condition.
Before your first dose, expect a round of baseline blood tests. These typically include hemoglobin, hematocrit (the percentage of your blood made up of red blood cells), liver function, a hormone called luteinizing hormone that helps pinpoint the cause of low levels, a prostate screening marker called PSA, and prolactin. These aren’t just formalities. Hematocrit is especially important: if yours is already above 50%, most guidelines say testosterone shouldn’t be started at all, because the therapy tends to push red blood cell production higher, increasing the risk of blood clots. The threshold for stopping treatment once you’re on it is 54%.
Starting Testosterone as Gender-Affirming Care
For transgender men and gender-diverse individuals assigned female at birth, the current standards of care (version 8, published in collaboration with the World Professional Association for Transgender Health) outline four conditions that should be met before starting testosterone. You need a marked and sustained difference between your sex assigned at birth and your current gender identity. You need the capacity for informed consent. Any physical or mental health conditions that could be affected by hormone therapy should be assessed. And you need to have discussed reproductive implications and options with your provider.
There is no single required age for adults. Once you meet these criteria and a provider confirms readiness, treatment can begin. The process varies by clinic and region, but many informed-consent clinics can initiate therapy within a few appointments.
For adolescents, the timeline is more structured. Hormone suppression (puberty blockers) may be offered after the first signs of puberty appear, at what’s called Tanner stage 2. Testosterone itself comes later, typically after a period of psychological support and evaluation, though specific age cutoffs vary by country and clinical setting.
Fertility Counseling Before You Begin
Regardless of why you’re starting testosterone, fertility is a conversation that should happen before the first dose. Testosterone suppresses the body’s own reproductive hormone signals, which can significantly reduce or halt sperm production in men and affect egg quality in transmasculine individuals. National and international medical organizations, including the American Society for Reproductive Medicine and WPATH, recommend that anyone interested in having biological children be offered fertility preservation beforehand.
This matters because there isn’t yet enough long-term data to guarantee that fertility fully recovers after stopping testosterone. For some people it does, but the timeline and degree of recovery are unpredictable. Sperm banking or egg freezing before starting therapy removes that uncertainty.
Testosterone for Delayed Puberty in Adolescents
Boys who haven’t started puberty by a typical age may be diagnosed with constitutional delay of growth and puberty. Treatment with low-dose testosterone is generally considered starting at age 14, provided specific physical and hormonal criteria are met. These include small testicular volume and low serum testosterone levels, confirmed through blood work. The goal of treatment at this age isn’t long-term replacement. It’s a short course designed to jumpstart puberty, after which the body’s own hormone production usually takes over.
What to Expect in the First Weeks and Months
Once you start testosterone, changes don’t happen overnight, but some arrive faster than you might expect. Sexual interest and morning erections typically improve within the first three weeks. Mood shifts, including reduced anxiety, better concentration, and improved self-confidence, can appear in that same three-week window. Depression symptoms often begin lifting within three to six weeks, with maximum improvement taking up to 30 weeks.
Fatigue and listlessness tend to decrease within one to six weeks. Quality of life improvements show up around weeks three to four, though they continue building for months. Physical changes like improved cholesterol levels start appearing after about four weeks but don’t peak until six to twelve months in. Blood sugar regulation may begin improving within days, but meaningful changes in long-term blood sugar control take three to twelve months.
These timelines come from studies on men with hypogonadism, but they offer a reasonable general picture. For transmasculine individuals on gender-affirming testosterone, physical masculinization (voice deepening, facial hair growth, fat redistribution) follows its own longer timeline, often unfolding over one to two years.
Conditions That May Delay or Prevent Starting
Not everyone is cleared to start right away. A hematocrit above 50% is one of the clearest reasons a provider will hold off, since testosterone raises red blood cell counts further and the risk of clotting becomes too high. Certain prostate conditions, particularly active prostate cancer, are also reasons treatment won’t be initiated. Elevated PSA levels on baseline screening may prompt further evaluation before a provider is comfortable prescribing.
Severe untreated sleep apnea, uncontrolled heart failure, and a desire for near-term fertility are other common reasons to either delay or reconsider. In many of these cases, the barrier isn’t permanent. Once the underlying issue is managed, testosterone can be revisited.