Low testosterone (low T), also called male hypogonadism, may be caused by many things, such as aging, hormone changes, chemotherapy, and others. Treatment involves taking various forms of testosterone.

Low testosterone (low T) affects 4 to 5 million men in the US.

Testosterone is an important hormone in the human body. But it starts to decrease each year after age 30. In some men, this can be substantial. Between 19 and 39 percent of older men may have low levels of testosterone.

Older men with low T have increasingly sought testosterone replacement therapy (TRT) in recent years. TRT addresses symptoms such as low libido, poor muscle mass, and low energy.

It’s not just older men who are affected by low T. Young men, even babies and children, can also have this problem.

Low levels of testosterone that are atypical of normal aging are due to other primary or secondary causes of hypogonadism. Hypogonadism in males happens when the testicles don’t produce enough testosterone. Hypogonadism can start during fetal development, during puberty, or during adulthood.

Fetal development

If hypogonadism begins during fetal development, the primary result is impaired growth of external sex organs. Depending on when hypogonadism starts and the level of testosterone present during fetal development, a male child can develop:

  • female genitals
  • ambiguous genitals, neither clearly male nor female
  • underdeveloped male genitals


Normal growth can be jeopardized if hypogonadism occurs during puberty. Problems occur with:

  • muscle development
  • deepening of the voice
  • lack of body hair
  • underdeveloped genitals
  • overly long limbs
  • enlarged breasts (gynecomastia)


Later in life, insufficient testosterone can lead to other problems. Symptoms include:

  • low energy levels
  • low muscle mass
  • infertility
  • erectile dysfunction
  • decreased sex drive
  • slow hair growth or hair loss
  • loss of bone mass
  • gynecomastia

Fatigue and mental fogginess are some commonly reported mental and emotional symptoms in men with low T.

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The two basic types of hypogonadism are primary and secondary hypogonadism.

Primary hypogonadism

Underactive testes cause primary hypogonadism. That’s because they don’t manufacture sufficient levels of testosterone for optimal growth and health. This underactivity can be caused by an inherited trait. It can also be acquired by accident or illness.

Inherited conditions include:

  • Undescended testicles: When the testicles fail to descend from the abdomen before birth. Surgery is advised if the testes have not descended at age 12 months.
  • Klinefelter syndrome: A condition in which a man is born with three sex chromosomes: X, X, and Y.
  • Hemochromatosis: Too much iron in the blood causes testicular failure or pituitary damage.

Types of testicle damage that can lead to primary hypogonadism include:

  • Physical injury to the testicles: Injury must occur to both testicles to affect testosterone levels.
  • Mumps orchitis: A mumps infection can injure testicles.
  • Cancer treatment: Chemotherapy or radiation can damage testicles.

Secondary hypogonadism

Secondary hypogonadism is caused by damage to the pituitary gland or hypothalamus. These parts of the brain control hormone production by the testes.

Inherited or disease conditions in this category include:

  • Pituitary disorders caused by drugs, kidney failure, or small tumors
  • Kallmann syndrome, a condition connected to abnormal hypothalamus function
  • Inflammatory diseases, such as tuberculosis, sarcoidosis, and histiocytosis, which can impact the pituitary gland and the hypothalamus
  • HIV/AIDS, which can affect the pituitary gland, hypothalamus, and testes

Acquired circumstances that can lead to secondary hypogonadism include:

  • Normal aging: Aging affects production and response to hormones.
  • Obesity: High body fat can affect hormone production and response.
  • Medications: Opioid pain meds and steroids can affect function of the pituitary gland and hypothalamus.
  • Concurrent illness: Severe emotional stress or physical stress from an illness or surgery can cause the reproductive system to temporarily shut down.

You may be affected by primary, secondary, or mixed hypogonadism. Mixed hypogonadism is more common with increased age. People undergoing glucocorticoid therapy can develop the condition. It also can affect people with sickle-cell disease, thalassemia, or alcoholism.

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If you’re experiencing symptoms of low T, lifestyle changes may help to ease your symptoms.

A good first step is increasing activity levels and maintaining a healthy diet in order to reduce body fat. It can also be helpful to avoid glucocorticoid medications such as prednisone as well as opioid pain medications.

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If lifestyle changes don’t work for you, you may need to begin testosterone replacement therapy (TRT) for treatment of low T. TRT can be very important for helping teenage males with hypogonadism experience typical masculine development. Sufficient testosterone levels help maintain health and well-being in adult males.

However, it’s important to note that TRT should not be used for teenage males with typical testosterone levels to enhance puberty or athletic performance.

Q: How long does it take for testosterone injections to start working?


A: Testosterone, when given intramuscularly, is slowly absorbed. It’s therefore administered at intervals of 2–4 weeks. Levels peak 2–3 days after an injection and slowly decline until the next dose is given.

Everyone is different, so some people will see improvement in symptoms within a few weeks, while others will need more time to notice any changes. This is mostly dependent on factors such as age and underlying medical conditions.

On average, it takes about 3 weeks to see an improvement in sexual desire (libido), energy, and mood. Changes in erections are slower and may take up to 6 months.

Testosterone replacement also affects metabolic parameters, with effects on cholesterol occurring in 4 weeks, peaking at 6–12 months, and blood sugar levels improving after 3–12 months. Other changes take much longer.

You can expect a change in muscle strength and body composition (fat mass/lean body mass) within 12–16 weeks. Effects on bone mineral density are seen after 6 months, with the full effect occurring around 2 years.

Kelly Wood, MDAnswers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.
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TRT has side effects, however, including:

  • acne
  • enlarged prostate
  • sleep apnea
  • testicle shrinkage
  • breast enlargement
  • increased red blood cell count
  • decreased sperm count

A carefully formulated TRT treatment plan should avoid many of these undesirable side effects. Talk with your doctor to evaluate your options.

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