Testosterone

Testosterone is a steroid hormone made from cholesterol. In men, most is produced by the Leydig cells in the testes, while women produce much smaller amounts in the ovaries and adrenal glands. The amount the body produces is regulated by signals from the hypothalamus and pituitary gland in the brain.

Most testosterone in the blood circulates bound to transport proteins: sex hormone-binding globulin (SHBG) and albumin. A small amount that is not bound to protein, called free testosterone, acts in tissues. Some testosterone is converted by 5-alpha-reductase into dihydrotestosterone (DHT), a more potent androgen, and some is converted by aromatase into estradiol.

Its effects fall into two broad categories.

• Androgenic effects — It is involved in secondary sex characteristics and reproductive functions, including genital differentiation during fetal development, voice changes and facial hair at puberty, and sperm production.
• Anabolic effects — It helps synthesize proteins in tissues such as muscle and bone. It is involved in muscle mass and strength, bone mineral density, and red blood cell production, and also affects libido and vitality. (Muscle protein synthesis, Bone mineral density)

These anabolic effects are why testosterone is often mentioned in body care. Controlled studies have confirmed that administering testosterone from an external source at levels above the physiological range increases muscle mass and strength. This is explained through pathways that increase muscle protein synthesis via androgen receptors and involve the activity of satellite cells that repair muscle.

Testosterone levels fluctuate substantially throughout the day. They are generally highest in the early morning and fall toward the evening. This means the time of the blood draw matters when interpreting test results, and measurements taken at different times are difficult to compare directly. (Circadian rhythm)

The relationship with sleep is also well known. One study found that when young, healthy men were restricted to 5 hours of sleep a day for one week, their daytime testosterone levels fell noticeably (Leproult & Van Cauter, JAMA 2011). Chronic sleep deprivation is described as unfavorable for recovery because it tends to raise the stress hormone cortisol and lower testosterone and growth hormone. (Sleep hygiene, Growth hormone and IGF-1)

Testosterone and growth hormone rise for a while after resistance exercise. This led to the widespread idea that "the greater the hormone rise immediately after exercise, the more muscle you build." In a large-scale study, there was no clear association between the magnitude of hormone changes in the tens of minutes immediately after exercise and subsequent gains in strength or muscle size (West et al., 2012). Muscle growth is influenced more strongly by the local mechanical stimuli received by muscle cells and the resulting intracellular signals. (Mechanotransduction, Muscle hypertrophy, Training adaptation)

The explanation, then, is that it is better to focus on the basics—progressively increasing the load, eating enough, and sleeping enough—rather than on ways to raise hormone levels during exercise.

Prolonged energy deficiency, excessive training volume, or accumulating psychological stress are known to tend to lower testosterone. This has led to ongoing attempts to use the testosterone-to-cortisol ratio as an indicator of training load or overtraining syndrome. However, large individual differences and substantial variability in measurements mean it is not yet suitable as a basis for everyday decisions. (Energy availability and relative energy deficiency in sport, Overtraining and recovery)

In men, blood testosterone levels have been observed to tend to decline gradually each year from around their 30s–40s onward. However, factors such as weight gain, chronic illness, and reduced physical activity also play a role, so it is difficult to attribute the decline to age alone. Levels vary widely even among people of the same age.

In women, too, testosterone is involved in muscles, bones, and libido. Blood levels are much lower than in men, and changes around menopause vary considerably between individuals.

• Get enough sleep. Regular sleep of around 7 hours a day helps support hormonal rhythms.
• Combine resistance exercise with walking. Exercise that uses your muscles supports overall metabolic health and conditions for recovery. (Resistance training)
• Avoid eating too little. Prolonged, extreme dietary restriction affects hormonal balance. Aim for a balanced intake of protein and nutrients. (Vitamin D)
• Relieve stress. Prolonged tension raises cortisol and slows recovery.
• Read supplement advertisements with a level head. There are many types of so-called "testosterone boosters," and results are mixed. There are reports of levels recovering in people with deficiencies after taking zinc or vitamin D, but this is a matter of replenishing missing nutrients.

Testosterone testing and replacement therapy are carried out through medical consultation. In South Korea, testosterone is treated as a prescription medicine. Blood is generally drawn in the morning, and reference ranges vary by laboratory and measurement method. If fatigue, reduced libido, or reduced muscle strength persist, a common approach is to look not only at testosterone levels but also at sleep, weight, medications, and other medical conditions.