Testosterone has often been mistakenly labelled as "the aggressive hormone", but limited studies have supported this notion. Anger itself comes from an interplay between different hormones, of which testosterone plays just one part. In reality, testosterone plays a number of crucial roles in the body which provide benefits that outweigh the minimal risk of anger.¹
Testosterone is a hormone produced in the testicles that is crucial for male health, regulating muscle mass, bone density, erections, libido, mood, cognition, energy levels, cardiovascular health, and weight management.²
Maintaining healthy testosterone levels can contribute to a happier, healthier, and longer life.²
Testosterone is crucial for men's health and plays a central role in many aspects of wellbeing.³ Click on each one to understand more.
Low-T, testosterone deficiency, male ageing syndrome, hypogonadism, andropause, male menopause (or "manopause"), and midlife crisis are all terms you may have come across before.
Whatever you call it, low testosterone is the hidden men's health issue that’s being overlooked by the medical community. With millions of men in the UK affected by this silent problem, we're here to drive change, be the voice of male sufferers, and provide comprehensive diagnosis and treatment support for anyone suffering the symptoms.
Low testosterone is a medical condition where the body doesn't produce enough testosterone to maintain various essential bodily functions.² When testosterone levels are low, it can lead to symptoms such as fatigue, decreased libido, erectile dysfunction, mood changes, reduced muscle mass, and increased body fat.²
Origin: Also known as primary testicular failure, primary hypogonadism arises directly from problems within the testicles.
Common causes: The causes of primary hypogonadism can be diverse, ranging from genetic disorders like Klinefelter's Syndrome to physical issues such as undescended testicles, testicular injury, or the effects of cancer treatment. Ageing can also play a significant role in its development.
In these cases, the testicles themselves are impaired and fail to produce sufficient levels of testosterone.
Origin: Secondary hypogonadism arises from issues in the brain regions that control the testicles—the hypothalamus or the pituitary gland.
Common causes: Causes of secondary hypogonadism can include genetic conditions like Kallmann syndrome, inflammatory diseases, HIV/AIDS, certain medications, or even obesity.
In these cases, the testicles might be healthy but are not receiving enough hormonal signals to function properly.
Even before birth, your testes begin producing small amounts of testosterone. These levels gradually increase during childhood and surge during adolescence, eventually peaking in your early 20s.⁵ For adult men, a typical total testosterone level ranges from 12 to 29 nmol/L. However, these reference values don’t take into account age-related decline or your individual symptoms – which is why clinical context matters.
At Numan, we follow the guidance from the British Society for Sexual Medicine (BSSM):⁵
>12 nmol/L – Normal
Testosterone deficiency is unlikely. Treatment is generally not recommended unless symptoms are severe and another underlying cause is being considered.
8–12 nmol/L – Grey zone
Testosterone deficiency is possible, especially if symptoms are present. Further investigation is advised, including testing free testosterone or SHBG. Clinical judgement is essential.
<8 nmol/L – Low
Testosterone deficiency is likely. If symptoms are present, treatment such as testosterone replacement therapy (TRT) may be appropriate.
We also consider free testosterone levels when assessing deficiency. Levels below 0.225 nmol/L may indicate testosterone deficiency, particularly when symptoms are present.
If these symptoms are holding you back, Numan can help.⁶ Take a blood test to check your testosterone levels.
Erectile dysfunction
Decreased libido
Fatigue
Poor sleep
Mood changes
Difficulty concentrating
Loss of muscle mass
Increased body fat
Reduced facial and body hair growth
There are many symptoms of low testosterone, with some men only exhibiting one or two, whereas numerous of different complications may burden others.
These symptoms may be related to your age and the underlying cause of low testosterone, but they can also be attributed to just how low your levels are. Below is an overview of the symptoms associated with different testosterone thresholds measured in the blood:⁵,⁷,⁸
Borderline low testosterone deficiency (12-15 nmol/L)
Reduced sex drive and an increased occurrence of erectile dysfunction.
A decrease in muscle mass and strength, and a generally lower threshold of tolerance to exercise.
Mild testosterone deficiency (10-12 nmol/L)
Further reduction in libido and more frequent erectile dysfunction.
Increased body fat and more prone to obesity.
Moderate testosterone deficiency (8-10 nmol/L)
Mood fluctuations with an increased risk of depression and anxiety.
Difficulties sleeping, including more frequent awakenings and finding it more difficult to get to and stay asleep.
Brain fog and a lack of ability to concentrate or focus.
Further weight increases and heightened risk of developing type 2 diabetes.
Lower bone mineral density, which increases the risk of osteoporosis and fractures.
Severe testosterone deficiency (<8 nmol/L)
Significant decrease in libido, frequent erectile dysfunction, and risk of infertility.
Hot flushes.
Increased risk of lower urinary tract symptoms, particularly in those with type 2 diabetes.
Increased risk of arterial hypertension (higher blood pressure), especially in those who smoke and/or are obese.
More pronounced mood swings with severe depression and emotional instability.
As well as age, testosterone deficiency (TD) is also more common among men with obesity, type 2 diabetes, or cardiovascular disease. Even without these conditions, it can affect men from a surprisingly young age.⁹
If you have TD, you may be showing symptoms such as fatigue, low libido, loss of muscle mass, and depression. Although these symptoms alone don’t mean you have it, experiencing even one of them could point to something deeper.
Age
Testosterone levels decline as men age, typically by 1% per year after the age of 30, primarily as a result of gradual reduction in testicular function.¹⁰
Obesity
Excess body fat boosts the enzyme aromatase, which turns testosterone into oestrogen. This lowers testosterone levels and disrupts overall hormone balance.¹¹
Sleep irregularity
Testosterone production follows the circadian rhythm. Disrupted sleep patterns can interfere with this natural cycle, leading to decreased testosterone production over time.¹²
Diabetes
Type 2 diabetes is characterised by insulin resistance, which can disrupt hormone signalling pathways, including those involved in testosterone production.⁹,¹¹
Cardiovascular disease
Men with cardiovascular disease may suffer from low testosterone due to impaired blood flow to the testes.¹³
¹ Peterson CK, Harmon-Jones E. Anger and testosterone: Evidence that situationally-induced anger relates to situationally-induced testosterone. Emotion. 2012;12(5):899–902.
² Nassar GN, Leslie SW. Physiology, testosterone. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2025.
³ Morley J. The benefits and risks of testosterone replacement therapy: a review. Ther Clin Risk Manag. 2009;427.
⁴ Sizar O, Leslie SW, Schwartz J. Male hypogonadism. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2025.
⁵ Hackett G, Kirby M, Rees RW, Jones TH, Muneer A, Livingston M, et al. The British society for sexual medicine guidelines on male adult testosterone deficiency, with statements for practice. World J Mens Health. 2023;41(3):508–37.
⁶ Morales A, Bebb RA, Manjoo P, Assimakopoulos P, Axler J, Collier C, et al. Diagnosis and management of testosterone deficiency syndrome in men: clinical practice guideline. CMAJ . 2015;187(18):1369–77.
⁷ Zitzmann, M., Faber, S., & Nieschlag, E. (2006). Association of specific symptoms and metabolic risks with serum testosterone in older men. The Journal of Clinical Endocrinology and Metabolism, 91(11), 4335–4343. doi:10.1210/jc.2006-0401
⁸ Shigehara, K., Izumi, K., Kadono, Y., & Mizokami, A. (2021). Testosterone and bone health in men: A narrative review. Journal of Clinical Medicine, 10(3), 530. doi:10.3390/jcm10030530
⁹ Traish AM, Miner MM, Morgentaler A, Zitzmann M. Testosterone deficiency. Am J Med. 2011;124(7):578–87.
¹⁰ The ‘male menopause’ [Internet]. nhs.uk. [cited 2025 Feb 17]. Available from: https://www.nhs.uk/conditions/male-menopause/
¹¹ Traish AM. Testosterone and weight loss: The evidence. Curr Opin Endocrinol Diabetes Obes. 2014;21(5):313–22.
¹² Auyeung TW, Kwok T, Leung J, Lee JSW, Ohlsson C, Vandenput L, et al. Sleep duration and disturbances were associated with testosterone level, muscle mass, and muscle strength—A cross-sectional study in 1274 older men. J Am Med Dir Assoc. 2015;16(7):630.e1-630.e6.
¹³ Kelly DM, Jones TH. Testosterone and cardiovascular risk in men. In: Frontiers of Hormone Research. S. Karger AG; 2014. p. 1–20.
Experiencing decreased libido, fatigue, or increased body fat? You might have low testosterone. Check if low-T is affecting you today.
