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Testosterone is the principal androgen produced primarily by the Leydig cells of the testes in males and, in smaller quantities, by the ovaries and adrenal cortex in females. It governs sexual development, muscle mass, bone density, and red blood cell production, and modulates mood, motivation, and cognitive function via effects on the central nervous system.
In clinical practice, 'normal testosterone' depends on whether total, free, or bioavailable fractions are measured; lab-to-lab reference ranges vary considerably.
How it works
Testosterone synthesis is governed by the hypothalamic-pituitary-gonadal (HPG) axis. The hypothalamus releases gonadotrophin-releasing hormone (GnRH) in pulses every one to three hours, which prompts the anterior pituitary to secrete luteinising hormone (LH). LH in turn signals the testicular Leydig cells to synthesise testosterone. As circulating testosterone rises, it feeds back to suppress both GnRH and LH, maintaining concentrations within a defined range.2
Of total circulating testosterone, approximately 54% is tightly bound to sex hormone-binding globulin (SHBG) and biologically inactive; around 44% binds loosely to albumin; only 2–3% circulates as free hormone capable of entering target cells and activating androgen receptors.2 This distinction matters clinically: total testosterone alone can appear normal while free testosterone is low, particularly when SHBG is elevated.
Testosterone acts both directly through the intracellular androgen receptor and as a prohormone. In peripheral tissues, 5-alpha-reductase converts it to dihydrotestosterone (DHT), a more potent androgen with particular effects on prostate and scalp; in adipose and neural tissue, aromatase converts it to oestradiol, which mediates bone mineralisation and libido in males.1 In the central nervous system, testosterone and its metabolites modulate dopaminergic signalling in the mesolimbic reward pathway, with androgen receptors expressed in the amygdala, hippocampus, and prefrontal cortex.4
Example
An athlete maintaining consistent training and nutrition begins experiencing fatigue, reduced drive, and slower recovery. Lab work shows total testosterone in the low-normal range. The practical culprit: chronically shortened sleep. Testosterone is secreted predominantly during slow-wave and REM stages; persistent sleep restriction suppresses nightly output, pushing levels down even in otherwise healthy individuals. No training programme compensates for that accumulated hormonal deficit.
Sleep is not peripheral to testosterone status; for nightly synthesis, it is the substrate.
Why it matters
For performance-oriented individuals, testosterone matters on two timescales. Acutely, resistance exercise raises circulating levels; this transient spike contributes to satellite cell activation and muscle protein synthesis. Chronically, it is resting testosterone, not the post-workout surge, that drives long-term hypertrophic adaptation.13 A 2023 meta-analysis of 16 randomised trials found six months of testosterone therapy produced significant increases in hip bone density and lean mass, quantifying the anabolic margin at stake when levels are suppressed.
Clinically, the picture is more contested than popular discourse suggests. The Endocrine Society defines biochemical hypogonadism as a consistently low morning total testosterone (approximately 300 ng/dL), accompanied by symptoms such as fatigue, low libido, and depressed mood; total testosterone alone is insufficient when SHBG is abnormal.2 Cognitive benefits from testosterone therapy remain uncertain: controlled trials show inconsistent effects on verbal memory and executive function, and the evidence does not support supplementation for cognitive preservation alone.4
What is a normal testosterone level for men?+
The Endocrine Society defines the lower limit of normal at approximately 300 ng/dL for morning total testosterone. This threshold indicates hypogonadism, not optimal performance. Total testosterone must be interpreted alongside free testosterone, SHBG, and clinical symptoms before any clinical conclusion is drawn.{{cite:10.1210/jc.2018-00229}}
What does testosterone do in the body?+
Testosterone regulates a broad range of physiological processes: it drives development of secondary sex characteristics, promotes muscle protein synthesis and bone mineralisation, stimulates red blood cell production, and modulates mood and motivation through androgen receptors in the limbic system and prefrontal cortex.{{cite:10.1210/edrv-8-1-1}}
Does testosterone affect mood and motivation?+
Yes. Testosterone and its metabolites modulate dopaminergic signalling in the mesolimbic reward pathway, influencing competitive drive, approach behaviour, and general motivation. Androgen receptors are expressed in the amygdala, hippocampus, and prefrontal cortex. The relationship is real but more nuanced than straightforward dose-response thinking suggests.{{cite:10.1097/wnn.0000000000000104}}
How does testosterone affect muscle and bone?+
Testosterone promotes muscle protein synthesis and activates satellite cells following resistance exercise, supporting hypertrophic adaptation. For bone, it is converted to oestradiol via aromatisation, the primary driver of bone mineralisation in males. A meta-analysis of randomised trials confirms significant gains in lean mass and hip bone density with testosterone therapy.{{cite:10.1016/j.eprac.2023.04.013}}
