Fatigue, low mood, poor concentration, reduced libido, weight gain, poor recovery. That list sends a great many men looking at testosterone. It is also, almost word for word, the presentation of several other conditions — some of which are considerably more common.
Testing testosterone alone means examining one candidate and leaving the rest unexamined. Worse, some of these conditions actively suppress testosterone, so a low result can be a downstream consequence rather than the cause.
Underactive thyroid
Hypothyroidism causes fatigue, weight gain, low mood, cold intolerance and cognitive fog. It is common, particularly in women but far from rare in men, and it is diagnosed with a simple blood test. It also lowers SHBG, which complicates the interpretation of any testosterone result taken at the same time.
Iron deficiency
Iron deficiency causes profound fatigue, breathlessness on exertion, poor concentration and low exercise tolerance long before it produces anaemia. A normal full blood count does not exclude it — ferritin can be severely depleted while haemoglobin holds up. In men, unexplained iron deficiency also warrants investigation for a source of blood loss.
Obstructive sleep apnoea
This one deserves particular attention, because it both mimics and causes low testosterone. Repeated overnight oxygen desaturation suppresses the pituitary signal that drives testosterone production. Treat the apnoea and testosterone often recovers on its own.
Depression
The overlap here is close to complete: low mood, anhedonia, fatigue, poor concentration, disturbed sleep, reduced libido. Depression is common, treatable, and frequently reframed as a hormonal problem — which is more palatable but delays effective treatment. The two can coexist, and each can worsen the other.
Obesity and insulin resistance
Adipose tissue converts testosterone to oestradiol and suppresses the pituitary signal, producing genuine secondary hypogonadism. Substantial weight loss reliably raises testosterone — one of the few interventions that treats the cause rather than replacing the output.
Medication
- Opioids — potent suppressors of the pituitary signal, even at modest long-term doses
- Long-term corticosteroids
- Some antidepressants — where reduced libido may be a drug effect rather than a hormonal one
- Anabolic steroid use, past or present — suppression can persist long after stopping
Sources
NICE Clinical Knowledge Summaries: Tiredness/fatigue in adults
First-line investigation of unexplained fatigue in primary care.
BSSM guidelines on adult testosterone deficiency, 2022
Assessment for reversible and functional causes before initiating therapy.
