Testosterone is a hormone involved in the development, reproduction and day-to-day functioning of the body. It is best known as the male sex hormone, but that description is only part of the story. Both men and women produce testosterone; only the average amount and its role differ. Among other things, the hormone helps with the building and maintenance of muscle and bone tissue, the production of red blood cells, the development of sex characteristics and sexual functioning. Moreover, testosterone never works alone. It is part of a carefully regulated interplay between the brain, hormone glands, organs and body tissues. On this page we explain step by step what testosterone precisely is, where it comes from and what it does in the body. Clear, factual and without immediately turning every hormonal hiccup into a drama.

Testosterone in brief

Testosterone is a sex hormone from the androgen group.
The body makes testosterone from cholesterol.
In men, most production takes place in the testicles.
In women, the ovaries, adrenal glands and conversion in other tissues are involved.
The brain and pituitary gland control production via a hormonal regulatory system.
Testosterone plays a role in bones, muscles, red blood cells, reproduction and sexual functioning.
In the blood, testosterone is largely bound to proteins. A small fraction circulates freely.
A testosterone level fluctuates throughout the day and must always be assessed in the proper context.

What kind of hormone is testosterone?

Hormones are chemical messengers. They are produced in one place, travel through the blood and transmit a signal elsewhere in the body. Testosterone belongs to the androgens, a group of hormones involved in male sexual development and reproduction. It is also a steroid hormone. That might sound as though it automatically refers to anabolic substances from the gym, but that is not the case. Here, ‘steroid’ simply describes the chemical structure of the hormone. The body produces testosterone itself, using cholesterol as a raw material. After the hormone reaches a target tissue, it can bind to an androgen receptor within a cell. That contact influences which proteins the cell produces and therefore how the tissue functions. The effect varies depending on the location. A muscle cell reacts differently from a cell in a hair follicle, bone or reproductive organ. One hormone can therefore influence multiple systems at the same time.

Where is testosterone produced?

In men, the vast majority of testosterone is produced in the Leydig cells of the testes. The adrenal glands also provide a small amount of androgens, which can be further converted in other tissues. In women, the ovaries and adrenal glands are involved in production. Testosterone can also be produced through the conversion of precursors, such as androstenedione, in various body tissues. The amount of testosterone in the blood is, on average, clearly higher in men than in women, but the hormone is important for both. Production does not work like a tap that is constantly open to the exact same degree. The body sends signals in pulses and constantly adjusts production. Age, biological sex, time of day and various physical conditions can therefore influence the measured value. Testosterone is therefore not a fixed supply, but part of a living system that is constantly making adjustments.

How does the body regulate the production of testosterone?

The control starts in the brain. The hypothalamus releases the hormone GnRH in small pulses. That signal goes to the pituitary gland, a hormone gland at the base of the brain. The pituitary gland then produces, among other things, luteinising hormone, usually abbreviated to LH. In men, LH travels via the blood to the testes and stimulates the Leydig cells there to produce testosterone. FSH, another hormone from the pituitary gland, works together with testosterone in the formation of sperm cells. The system regulates itself via negative feedback. When sufficient testosterone is circulating, the hypothalamus and pituitary gland receive the signal to slow down control. If the level drops, stimulation can increase again. You can compare it to a thermostat, although the body is more complicated than the heating at home. This network is called the hypothalamic-pituitary-gonadal axis, often abbreviated to the HPG axis.

What does testosterone do during development?

Testosterone plays a role even before birth. During the development of a male foetus, testosterone and substances derived from it contribute to the formation of internal and external male sex characteristics. After birth, production remains relatively low throughout much of childhood. Around puberty, the HPG axis becomes much more active and production increases significantly. This contributes to changes such as growth of the penis and testicles, development of body and facial hair, a deeper voice and an increase in muscle and bone mass. Sperm production also gets underway. Not every body goes through these changes at the exact same time or at the same pace. Genetic predisposition, general health and other hormones also play a role. Testosterone therefore provides important developmental signals, but does not carry out this work entirely on its own.

What does testosterone do in an adult body?

After puberty, testosterone continues to be involved in various processes. It supports the maintenance of muscle mass and strength, and contributes to the building and strength of bones. It also stimulates the production of red blood cells in the bone marrow. In men, testosterone is necessary for normal sperm production, for which other hormones and cells in the testes are also indispensable. In addition, it plays a role in libido and sexual functioning. The hormone also influences fat distribution and may be involved in mood, motivation and cognitive processes. The latter does require nuance. How someone feels is never determined by a single hormone. Sleep, stress, physical health, relationships, medication and lifestyle all play a part. Testosterone is important, but not a remote control for energy, self-confidence or behaviour. Its influence must therefore always be viewed as part of overall physical and mental functioning.

Is testosterone only important for men?

No. Women also produce testosterone, only on average in lower amounts. The hormone contributes to the maintenance of bone and muscle tissue and plays a role in sexual desire and general physical functioning. The difference between men and women lies mainly in the average concentrations, the ratio between hormones and the way different tissues respond to them. The familiar labels ‘male hormone’ and ‘female hormone’ are therefore useful as a brief explanation, but biologically speaking a bit crude. Men do not exclusively have testosterone and women do not exclusively have oestrogens. Every body uses a combination of hormones, in different proportions and with its own dynamics.

What is the difference between total and free testosterone?

Most testosterone does not travel loosely through the blood. It is bound to transport proteins, mainly SHBG, fully known as sex hormone-binding globulin, and albumin. A small percentage circulates as free testosterone. Total testosterone is the sum of the bound and free fractions. Free testosterone can easily be available to tissues, whereas albumin-bound testosterone is relatively loosely bound and is often also considered bioavailable. SHBG holds testosterone more tightly. The amount of SHBG can vary per person and is influenced by age, liver function, thyroid hormones, weight, medication and other hormonal conditions, among other things. As a result, two people with a similar total testosterone level can still have a different amount of available testosterone. Conversely, an abnormal result does not automatically indicate what someone notices physically. A doctor therefore looks not only at a figure, but if necessary also at SHBG, free testosterone and the broader medical context.

What happens to testosterone in the tissues?

Testosterone can bind directly to an androgen receptor, but the body can also convert the hormone into other active hormones. Which pathway is used varies by tissue.

Conversion to DHT

Using the enzyme 5-alpha-reductase, testosterone can be converted into dihydrotestosterone, usually called DHT. DHT binds more strongly to the androgen receptor and plays a role in, amongst other things, prostate tissue and body and facial hair. Read more about the relationship between testosterone, the prostate and blood tests in the article about the PSA level and ageing.

Conversion to oestradiol

Via the enzyme aromatase, testosterone can be converted into oestradiol, in adipose tissue and bones, among other places. Oestradiol is also important in men, for example for bone health and parts of sexual function. You can read more about this conversion in the article about oestradiol in men.

Does testosterone change during the day and with age?

A testosterone level is not a static figure. For many men, production follows a circadian rhythm, with generally higher levels in the morning and lower levels later in the day. This difference is usually more pronounced in younger men. In addition, release fluctuates in short pulses. Sleep, acute illness, diet, intense exercise, medication and the timing of the blood draw can also influence a measurement. From adulthood onwards, testosterone levels can change gradually. However, age alone does not tell whether a level is appropriate or abnormal. People naturally differ, and laboratories may use different measurement methods and reference ranges. Therefore, a single test result is not a final conclusion. Especially when investigating a possible deficiency, the standard approach is to look at symptoms as well as repeated morning measurements. This provides a more reliable picture than a single blood draw at a random time.

How is testosterone measured?

Testosterone is usually measured with a blood test. In men, an initial measurement is typically done in the morning, because the level is often highest then and reference ranges are tailored to that. When a value turns out low, repetition on a different morning is important. After all, a single measurement can be influenced by normal fluctuations, poor sleep, illness or other temporary factors. Depending on the situation, SHBG, free testosterone, LH and FSH may also be investigated. These values help to understand how much testosterone is available and how the hormonal regulation is proceeding. The result must be assessed using the laboratory's reference range and with attention to age, health, medication and any symptoms. A figure outside the limit is not automatically a diagnosis. Conversely, a figure within the limit does not rule out every hormonal issue. Measurement provides information. Expert interpretation gives meaning.

What values can a laboratory measure?

A testosterone test does not always consist of a single figure. Depending on the medical question, a doctor may have several values determined. Together, these show how much testosterone is circulating, which part is available, and how the regulation from the brain and gonads is functioning.

lab value	What does the laboratory measure?	Why is this relevant?
Total testosterone	All the testosterone in the blood: both free and bound to transport proteins.	This is usually the first value used to assess the amount of circulating testosterone.
SHBG	The amount of sex hormone-binding globulin, the protein that tightly binds testosterone.	SHBG helps to explain why total testosterone does not always correspond to the amount of testosterone available to tissues.
Free testosterone	The small fraction of testosterone that is not bound to a transport protein. This can be measured or calculated.	This value can provide additional information when total testosterone and the clinical picture do not match well or when SHBG is abnormal.
LH	The signalling hormone from the pituitary gland that stimulates the Leydig cells in the testes in men.	The combination of LH and testosterone helps determine whether an abnormality lies in the testicles or in the control from the brain.
FSH	A hormone from the pituitary gland that is involved in men in the function of the Sertoli cells and the formation of sperm cells.	FSH provides additional information on testicular function and may be relevant in cases of fertility concerns.

Which combination is needed varies per person. The values are therefore not assessed as separate report marks, but in conjunction with the time of testing, any symptoms, medication, age and other health information.

What does a testosterone level indicate and what does it not?

A testosterone level shows how much of the hormone was present in the blood at the time of the blood draw. That is useful information, but not a complete summary of someone's health. For example, the value does not independently indicate how sensitive tissues are to testosterone, how much hormone is converted locally, or what caused a change. Nor can you deduce personality, athletic performance or degree of ‘masculinity’ from it. According to medical guidelines, establishing a testosterone deficiency in men requires both appropriate symptoms or characteristics and repeatedly and clearly low blood values. This distinction prevents a normal fluctuation from immediately being regarded as a medical condition. Anyone wanting to understand a result therefore benefits more from a careful discussion and targeted assessment than from comparing a single figure with that of a friend, influencer or random table on the internet. Hormones simply do not like hasty conclusions.

Frequently asked questions about testosterone

Is testosterone the same as an anabolic steroid?

Testosterone is naturally a steroid hormone with both androgenic and anabolic effects. ‘Androgenic’ refers to effects on male sex characteristics. ‘Anabolic’ means that the hormone can support the building up of tissue, such as muscle and bone tissue. Anabolic steroids are manufactured variants of testosterone or substances with a similar effect. They can be used medically, but are also used without a medical indication to influence muscle growth or performance. That non-medical use is different from the normal production of testosterone by the body and also different from treatment under medical supervision. The same chemical family therefore does not mean the same use, the same dosage or the same risk. The medical application and the amount determine which effects and risks are relevant.

Does more testosterone always have more effect?

No. Hormones work within a balance and more is not automatically better. Tissues react via receptors and can only process signals in a certain way. Moreover, extra testosterone can be converted into DHT or oestradiol, and exogenous administration can suppress the body's own hormonal regulation and sperm cell production. The goal of a healthy hormonal system is therefore not the highest possible level, but an appropriate balance for the individual body. Normal values also vary between individuals and laboratories. A value at the upper end of a reference range does not automatically make someone stronger, more energetic or healthier than someone with a value in the middle. A testosterone result only gains meaning alongside other measurements, physical characteristics, any symptoms and medical history.

Can you feel testosterone?

You do not feel a hormone flowing directly through the body. You can, however, notice the effects of processes involving testosterone, for example changes during puberty or fluctuations in sexual desire. Yet experiences such as fatigue, reduced strength, low mood or concentration problems are not specific to testosterone. They can have countless causes, ranging from sleep deprivation and stress to medication or another physical condition. Based on feeling alone, therefore, you cannot reliably determine whether a testosterone level is low, normal or high. Conversely, an abnormal laboratory value does not necessarily predict precisely how someone feels. Anyone who really wants to know how the hormonal system is doing needs a combination of a good consultation, medical assessment and, where appropriate, a blood test.

What is the difference between testosterone and oestrogen?

Testosterone and oestrogens are different groups of sex hormones, but both men and women produce both. The average amounts and relative proportions vary depending on biological sex, life stage and individual, with the body being able to convert some of the testosterone into oestradiol. You can read more about how they work together in the article on oestradiol in men.

When is a blood test for testosterone useful?

Blood tests can be useful when a doctor suspects a hormonal abnormality based on symptoms, physical characteristics or medical history. In men, guidelines advise against diagnosing a potential deficiency based solely on a questionnaire or a single blood test. There must be appropriate symptoms or characteristics, and the testosterone level must be clearly low on repeated morning measurements. Sometimes additional values are measured to investigate how the regulation from the brain and testicles is functioning. In women, testosterone testing may be requested for other reasons, for example in cases of signs of an excess of androgens. Which test is appropriate therefore depends on the individual and the question behind the investigation. Having an isolated value measured independently may satisfy curiosity, but without context it sometimes mainly generates new questions. Good diagnostics do not start with a number, but with the right question.

Testosterone is one part of a larger whole

Testosterone is an important hormone, but it never works in isolation from the rest of the body. The brain regulates production, transport proteins determine how much is available, and tissues convert or respond to the hormone. Age, health and the timing of measurement colour the picture further. That does not make testosterone a vague or mysterious subject. It simply means that a simple conclusion is usually oversimplified. Anyone who understands how the system works can better contextualise information about hormones and take a more critical look at big promises or quick diagnoses. If you have questions about your own hormone levels or health, personal investigation is the logical next step. At IKARIA Clinics we combine a medical assessment with targeted blood tests and explanations in plain language. Not to look for a problem behind every number, but to make clear what the results mean in your situation.

Sources used for this article:

Society for Endocrinology, Testosterone: https://www.yourhormones.info/hormones/testosterone/
NCBI Bookshelf, Physiology, Testosterone: https://www.ncbi.nlm.nih.gov/books/NBK526128/
Endocrine Society, Testosterone Therapy for Hypogonadism Guideline Resources: https://www.endocrine.org/clinical-practice-guidelines/testosterone-therapy
Newcastle Hospitals Laboratories, Testosterone, serum: https://laboratories.newcastle-hospitals.nhs.uk/test-directory/testosterone-serum/

What is testosterone?

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