Low testosterone is one of the most searched terms in men’s health — and one of the most inconsistently used. In clinical practice it means something specific. In much of the content men encounter online, it means something considerably broader: a symptom list that includes fatigue, low mood, reduced libido, and weight gain, accompanied by the suggestion that a blood test and testosterone treatment will address it.
The gap between those two framings matters, because the clinical picture is more nuanced than either a symptom checklist or a single number can capture. This article explains what low testosterone actually means, how it is properly diagnosed, and why the same number on a blood test can mean very different things in different men.
What “low” means
Testosterone levels in men follow a normal distribution — there is a wide range of values that are considered within the normal reference interval, and that range shifts with age. A result described as “low” by a lab report is typically below the lower limit of the population reference range. In most assays, this is around 8–10 nmol/L for total testosterone, though thresholds vary between laboratories and country guidelines.
A number below the reference range is a starting point, not a diagnosis. Several things determine whether that number is clinically meaningful:
- Was it measured correctly? Testosterone follows a diurnal rhythm, peaking in the morning and falling across the day. A sample taken at 3pm may read lower than one taken at 8am from the same man on the same day. Guidelines recommend measuring testosterone between 7 and 11am, ideally fasting, and confirming any low result on a second occasion before acting on it.[1]
- Is it free or total testosterone? Most of the testosterone circulating in the blood is bound to proteins, primarily sex hormone-binding globulin (SHBG), and is biologically inactive. Free testosterone — the unbound fraction — is what actually acts on tissues. In men with elevated SHBG (common in older men and those with liver disease), total testosterone may read low while free testosterone remains adequate. The reverse is also possible. A complete picture often requires both.
- Are there symptoms that correspond to the low level? A low number without symptoms is not the same clinical situation as a low number with symptoms that cannot be explained by anything else. The distinction matters for whether treatment is appropriate.
- What is the patient’s original baseline? The main problem with using an arbitrary cut off to diagnose low testosterone is that few patients actually know what their baseline testosterone levels are at their peak (usually in the early years of puberty). As a result, someone with a baseline of 40 units of testosterone dropping to 15 units (25 unit drop) will come in to the clinic with symptoms and still be told that he is “normal” and does not need Testosterone Replacement Therapy (TRT) while a patient with a baseline of 11 units who drops to 9 units (2 unit drop) will, in most clinics, be immediately diagnosed with low testosterone and started on TRT regardless of the severity of his symptoms.
Two different causes — two different conditions
Low testosterone has two broad categories of cause, and they are clinically distinct. Testosterone is regulated by an internal framework within the body as the Hypothalamus (Brain)/ Pituitary Gland (Gland situated in the neck) / Gonads (Testicles) HPG Axis. The causes of low testosterone hence is categorized based on where the dysfunction in the HPG Axis is.
Primary hypogonadism refers to a problem at the level of the testes — the testes are not producing adequate testosterone despite receiving appropriate signals from the hypothalamus and pituitary. Causes include genetic conditions (such as Klinefelter syndrome), prior trauma, infection, or testicular damage from chemotherapy or radiation. The pituitary compensates by producing elevated LH and FSH. This is a structural, typically permanent condition that usually requires testosterone replacement.
Functional (secondary) hypogonadism refers to suppression of testosterone production by factors outside the testes — obesity, chronic illness, sleep apnoea, metabolic syndrome, certain medications, or psychological stress. The HPG axis is intact but being suppressed. This form is common, often reversible, and frequently does not require testosterone replacement — addressing the underlying driver is the more appropriate first response.[2]
This distinction is one of the most clinically important and least discussed in the content men typically encounter. Most online material treats low testosterone as a single condition with a single treatment. In practice, a man with functional hypogonadism driven by visceral obesity may restore his testosterone to normal range through weight loss without ever needing exogenous testosterone. This is because Testosterone is a fat-soluble hormone, therefore there is too much fat in the body, the testosterone gets sequestered into the fat rather that circulating in the blood stream where it is supposed to be. However, losing weight will not yield this same result for a man with primary hypogonadism from a genetic condition.
Whether low testosterone is functional or pathological changes the entire treatment approach. That distinction requires a proper workup by a doctor.
The symptom question
The symptoms most commonly associated with low testosterone — fatigue, reduced libido, low mood, difficulty maintaining muscle mass, weight gain, poor sleep, reduced morning erections — are real consequences of genuinely low testosterone. They are also common consequences of poor sleep, metabolic dysfunction, stress, depression, and ageing in general.
The three symptoms that are most specific to low testosterone — meaning they are more likely to reflect true androgen deficiency rather than other causes — are reduced morning erections, reduced libido, and erectile dysfunction.[1] Even these are not exclusive to testosterone deficiency, but their presence alongside a confirmed low level strengthens the clinical picture considerably.
The less specific symptoms — fatigue, low mood, difficulty concentrating — have a long differential diagnosis. Attributing them to testosterone without ruling out other causes is a clinical shortcut that can result in men receiving treatment they do not need, while the actual cause goes unaddressed.
Fatigue, low mood and reduced drive have a long list of causes. Attributing them to testosterone without ruling out the others is a shortcut that can lead to the wrong treatment – or treatment when none is needed.
What proper diagnosis looks like
A proper assessment for suspected low testosterone involves more than a single blood test. It includes:
- A clinical history covering symptoms, their onset, and their impact — and a careful assessment of what else might explain them
- Two morning testosterone measurements on separate occasions, with SHBG to allow calculation of free testosterone where indicated
- LH and FSH to distinguish primary from functional hypogonadism
- Assessment of metabolic health — glucose handling, lipids, body composition — since metabolic syndrome and obesity are both causes and consequences of low testosterone
- A review of medications and lifestyle factors that may be suppressing testosterone
The result of this assessment is not simply a number and a treatment decision. It is an understanding of why testosterone is low and whether addressing the cause directly — through weight loss, sleep improvement, or metabolic treatment — is the right first step before considering testosterone replacement.
When testosterone replacement is and is not the answer
Testosterone replacement therapy (TRT) is an appropriate treatment for men with confirmed, symptomatic testosterone deficiency that is not explained by a reversible cause. It is not appropriate as a first response to a low number without symptoms, as a treatment for fatigue or low mood that has not been otherwise investigated, or as a performance supplement for men with normal levels.
For men whose low testosterone is driven by functional causes — particularly obesity — the evidence is clear that weight loss substantially reverses the suppression.[2] Treating the obesity first is not the conservative option; in many cases it is the more effective one, and it does not carry the risks of exogenous testosterone including fertility suppression, haematocrit elevation, and the requirement for indefinite monitoring.
A low testosterone number is a starting point, not a diagnosis. If you have had a result that concerns you – or symptoms you have been attributing to other causes – speak to a doctor who can access what is actually driving it.
For men in whom TRT is appropriate, the conversation about treatment, monitoring, and the implications for fertility is a separate and important one. The next article in this series covers what TRT actually involves.
- Wu FC, Tajar A, Beynon JM, Pye SR, Silman AJ, Finn JD, O’Neill TW, Bartfai G, Casanueva F, Forti G, Giwercman A, Huhtaniemi IT, Kula K, Punab M, Boonen S, Vanderschueren D; European Male Aging Study Group. Identification of late-onset hypogonadism in middle-aged and elderly men. New England Journal of Medicine. 2010;363(2):123–135. doi:10.1056/NEJMoa0911101. PMID: 20554979.
- Muir CA, Wittert GA, Handelsman DJ. Approach to the patient: low testosterone concentrations in men with obesity. Journal of Clinical Endocrinology and Metabolism. 2025;110(9):e3125–e3130. doi:10.1210/clinem/dgaf137. PMID: 40052430.