Testosterone Deficiency: Blood Tests, Symptoms & When to Test

What Is Testosterone Deficiency?

Testosterone deficiency, also known as male hypogonadism, occurs when the body does not produce enough testosterone. It can be caused by problems with the testes (primary hypogonadism), the pituitary gland or hypothalamus (secondary hypogonadism), or a combination of both. Late-onset hypogonadism — an age-related decline in testosterone — is increasingly recognised as a clinical entity, though it remains a topic of debate regarding when treatment is appropriate.

The British Society for Sexual Medicine (BSSM) published comprehensive guidelines in 2017 for the diagnosis and management of testosterone deficiency, and these remain the standard reference in UK practice.

Symptoms of Low Testosterone

Testosterone deficiency produces a range of symptoms that often develop gradually and may be attributed to ageing or stress:

• Persistent fatigue and low energy
• Reduced libido and erectile dysfunction
• Low mood, irritability, or depression
• Loss of muscle mass and strength
• Increased body fat, particularly around the abdomen
• Decreased bone mineral density (risk of osteoporosis)
• Poor concentration and “brain fog”
• Hot flushes and night sweats (in severe deficiency)
• Reduced body and facial hair growth

Because these symptoms are non-specific, blood tests are essential to confirm whether testosterone levels are genuinely low before considering treatment.

Total Testosterone

Total testosterone is the first-line blood test. The sample must be taken before 10am due to the circadian rhythm of testosterone secretion. According to the BSSM:

• Below 8 nmol/L: testosterone deficiency is likely. Treatment should be considered if symptoms are present.
• 8–12 nmol/L: borderline zone. Free testosterone or calculated free testosterone should be measured to clarify.
• Above 12 nmol/L: testosterone deficiency is unlikely to be the cause of symptoms.

A diagnosis should not be made on a single blood test. BSSM guidelines require two separate morning measurements showing low testosterone, taken at least 4 weeks apart.

Free Testosterone and SHBG

If total testosterone is borderline (8–12 nmol/L), measuring sex hormone-binding globulin (SHBG) allows calculation of free testosterone. SHBG is a protein that binds testosterone, making it inactive. Conditions that raise SHBG (ageing, liver disease, hyperthyroidism, anticonvulsants) can cause a normal total testosterone level to mask a low free testosterone.

Conversely, obesity, type 2 diabetes, and hypothyroidism lower SHBG, which can make total testosterone appear lower than the true free testosterone level. Calculated free testosterone below 0.225 nmol/L supports a diagnosis of testosterone deficiency.

LH, FSH, and Prolactin

Once low testosterone is confirmed, the next step is to determine the cause:

• Elevated LH and FSH indicate primary hypogonadism — the testes are failing and the pituitary is trying to compensate. Causes include Klinefelter syndrome, testicular injury, chemotherapy, and mumps orchitis.
• Low or normal LH and FSH in the presence of low testosterone indicate secondary hypogonadism — the pituitary gland is not producing enough stimulating hormones. Causes include pituitary tumours, obesity, opioid use, and excessive exercise.
• Prolactin should be measured to rule out a prolactinoma, which is a treatable pituitary tumour that suppresses gonadotropin production.

Additional Investigations

Depending on the clinical picture, your doctor may also request:

• Thyroid function tests — hypothyroidism can mimic testosterone deficiency symptoms.
• FBC — to check for anaemia (testosterone deficiency can cause mild anaemia) and as a baseline before TRT.
• HbA1c or fasting glucose — type 2 diabetes is strongly associated with low testosterone.
• Lipid profile — metabolic syndrome commonly coexists with hypogonadism.
• PSA — baseline prostate screening before starting TRT (mandatory per BSSM guidelines).
• DEXA scan — if osteoporosis is suspected.

Testosterone Replacement Therapy (TRT) Monitoring

If TRT is initiated, regular blood monitoring is essential to ensure safety and efficacy. The BSSM recommends the following monitoring schedule:

• 3 months after starting: testosterone level (to check dose adequacy), FBC (haematocrit must stay below 0.54), PSA, and liver function.
• 6 months: repeat the above panel.
• 12 months and annually thereafter: testosterone, FBC, PSA, lipid profile, HbA1c, and liver function.

The most important safety concern during TRT is polycythaemia (elevated red blood cell count), which increases the risk of blood clots. If the haematocrit rises above 0.54, the TRT dose should be reduced or temporarily suspended. PSA monitoring ensures early detection of any prostate changes.

Getting Tested at Home

Because testosterone blood tests must be taken in the morning, having a mobile phlebotomist visit your home before 10am is one of the most convenient ways to get tested. There is no need to take time off work or sit in a waiting room. Whether you are investigating symptoms for the first time or monitoring TRT, home testing makes regular blood work straightforward. Visit our home blood test page to learn more.