Is Low Testosterone Causing Your Fatigue? Signs, Tests, and Solutions

You used to have energy. You used to sleep well, concentrate easily, and wake up feeling like yourself. Now you're dragging through every afternoon, your motivation has evaporated, and you can't remember the last time you felt genuinely sharp. Your GP says your bloods are "normal." But something is clearly off.

For a significant number of men, the answer hiding in plain sight is low testosterone. It's one of the most underdiagnosed conditions in men's health — partly because the symptoms are vague, partly because GPs don't routinely test for it, and partly because many men simply accept declining energy as an inevitable part of ageing.

It doesn't have to be.

The Symptoms Most Men Ignore

Low testosterone — clinically known as hypogonadism — doesn't announce itself with a single dramatic symptom. Instead, it creeps in gradually, producing a constellation of changes that are easy to dismiss:

• Persistent fatigue — not just tiredness, but a deep, unrelenting lack of energy that sleep doesn't fix
• Low mood or irritability — feeling flat, unmotivated, or emotionally blunted without an obvious cause
• Reduced libido — declining interest in sex, often accompanied by weaker erections
• Poor concentration and brain fog — difficulty focusing, forgetting things, feeling mentally sluggish
• Loss of muscle mass — despite maintaining exercise, your strength and muscle definition decrease
• Increased body fat — particularly around the abdomen, even without changes to diet
• Poor sleep quality — difficulty falling asleep, staying asleep, or waking unrefreshed
• Reduced confidence and drive — a subtle but pervasive sense that something fundamental has changed

The British Society for Sexual Medicine (BSSM) estimates that testosterone deficiency affects a substantial proportion of men over 40, with prevalence increasing with age. Yet many affected men never receive a diagnosis because they attribute their symptoms to stress, poor sleep, or simply "getting older."

Is It Really Low T? The Differential Diagnosis

Here's the honest complexity: every symptom above can be caused by conditions other than low testosterone. Before jumping to a diagnosis, consider:

• Thyroid dysfunction — hypothyroidism produces almost identical symptoms: fatigue, weight gain, brain fog, and low mood
• Iron deficiency — low ferritin causes crushing fatigue and exercise intolerance
• Vitamin D deficiency — affects energy, mood, and muscle function; extremely common in the UK
• Depression and anxiety — can cause fatigue, low motivation, and reduced libido independently of testosterone
• Sleep apnoea — disrupted sleep causes daytime fatigue and also suppresses testosterone production
• Type 2 diabetes — strongly associated with low testosterone in men

This is why you need blood tests — not just a testosterone level, but a comprehensive panel that investigates all the common mimics.

The Blood Tests You Need

A proper evaluation of suspected low testosterone requires more than a single testosterone reading. The BSSM guidelines recommend the following:

Core hormone panel

• Total testosterone — measured in the morning (before 11am) when levels peak
• Free testosterone — the biologically active fraction; important because SHBG (sex hormone-binding globulin) can bind testosterone and reduce availability
• SHBG — high SHBG means more testosterone is bound and unavailable
• LH and FSH — distinguish between primary hypogonadism (testicular failure) and secondary hypogonadism (pituitary or hypothalamic cause)
• Prolactin — elevated prolactin suppresses testosterone; high levels warrant further investigation

Essential additional tests

• Thyroid function (TSH, Free T4) — to rule out hypothyroidism
• Full blood count — low testosterone may cause mild anaemia; TRT can cause polycythaemia
• HbA1c and fasting glucose — to screen for diabetes and insulin resistance
• Liver function — to assess SHBG context and metabolic health
• Iron studies (ferritin, serum iron, TIBC) — to exclude iron deficiency
• Vitamin D — to identify deficiency contributing to symptoms
• PSA — a baseline before considering testosterone replacement

Understanding Your Results

In the UK, testosterone is measured in nmol/L. Here's how to interpret the numbers:

The BSSM defines testosterone deficiency as a total testosterone consistently below 8 nmol/L with symptoms. Levels between 8–12 nmol/L are a grey zone where symptoms and free testosterone levels guide clinical decisions. Levels above 12 nmol/L are generally considered normal, though some men experience symptoms even within the "normal" range if their free testosterone is low due to high SHBG.

Critical point: A single testosterone reading is not diagnostic. Levels fluctuate day to day and are affected by sleep, stress, illness, and time of measurement. The BSSM recommends confirming low testosterone with at least two morning samples before making a diagnosis.

Causes of Low Testosterone

Understanding why your testosterone is low matters because it determines the treatment approach:

Primary hypogonadism (testicular origin)

• Ageing — testosterone declines by approximately 1–2% per year after age 30
• Testicular damage — from infection (mumps orchitis), injury, or surgery
• Genetic conditions — Klinefelter syndrome, Y-chromosome microdeletions
• Varicocele — enlarged veins in the scrotum affecting testicular function

Secondary hypogonadism (brain/pituitary origin)

• Obesity — excess body fat converts testosterone to oestrogen via aromatase activity; one of the most common and reversible causes
• Chronic illness — diabetes, liver disease, and kidney disease all suppress testosterone
• Medications — opioids, corticosteroids, and some antidepressants can reduce testosterone
• Pituitary tumours — rare but important to exclude, particularly if prolactin is elevated
• Obstructive sleep apnoea — disrupts GnRH release and suppresses testosterone production

Treatment Options

Treatment depends on the cause, severity, and your individual circumstances.

Lifestyle interventions (first line)

For men with borderline testosterone levels (8–12 nmol/L), lifestyle changes can produce meaningful improvements:

• Weight loss — losing 5–10% of body weight can raise testosterone by 2–3 nmol/L in overweight men
• Resistance training — compound exercises (squats, deadlifts, presses) stimulate testosterone more effectively than cardio alone
• Sleep optimisation — 7–9 hours of quality sleep is critical; testosterone is produced during deep sleep
• Stress management — chronically elevated cortisol directly inhibits testosterone synthesis
• Alcohol reduction — regular heavy drinking suppresses testosterone

Testosterone replacement therapy (TRT)

When lifestyle changes are insufficient and total testosterone is consistently below 8 nmol/L (or free testosterone is low with symptoms), testosterone replacement therapy may be recommended.

Options available in the UK include:

• Testosterone gel (Testogel, Tostran) — applied daily to the skin; provides stable levels
• Testosterone injections — Sustanon (every 2–3 weeks) or Nebido (every 10–14 weeks); injections provide reliable dosing
• Testosterone patches — less commonly used due to skin irritation

TRT: What to Expect

If you start TRT, improvements don't happen overnight. The Endocrine Society guidelines outline a typical timeline:

• 2–4 weeks: Improved energy and mood (often the first noticeable change)
• 3–6 weeks: Increased libido and sexual function
• 3–6 months: Improvements in body composition — increased lean mass, reduced body fat
• 6–12 months: Full metabolic and psychological benefits

TRT requires ongoing monitoring. You'll need blood tests every 3–6 months initially, checking testosterone levels, haematocrit (red blood cell concentration), PSA, and liver function. The BSSM guidelines emphasise that TRT is a long-term commitment — stopping therapy returns testosterone to pre-treatment levels.

Important considerations

• TRT suppresses sperm production — if fertility is a concern, alternatives like hCG (human chorionic gonadotropin) or clomiphene citrate should be discussed
• TRT can increase haematocrit — requiring regular monitoring to avoid polycythaemia (excessively thick blood)
• Prostate monitoring is essential — while TRT doesn't cause prostate cancer, it may stimulate growth of pre-existing disease

Lifestyle Changes That Help

Whether or not you pursue TRT, these evidence-based strategies support healthy testosterone levels:

• Prioritise sleep — aim for 7–9 hours; treat sleep apnoea if present
• Maintain a healthy weight — obesity is the single most modifiable risk factor for low testosterone
• Lift weights regularly — resistance training 3–4 times per week has the strongest evidence
• Manage stress — chronic stress raises cortisol and suppresses testosterone
• Limit alcohol — even moderate consumption can reduce testosterone over time

Low testosterone is real, it's common, and it's treatable. But diagnosis starts with the right blood tests — not assumptions. If the symptoms resonate, get tested. The answers are in your blood.

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