All About Testosterone
What is testosterone?
Testosterone molecule
Testosterone is a steroid hormone from the androgen group.
The androgens are a group of steroids that have anabolic (aka growth) and/or masculinizing effects in both males and females. Testosterone is the most important androgen in humans.
Testosterone regulates libido, energy, immune function, muscle development and bone health.
How is testosterone made?
In males, Leydig cells in the testes synthesize testosterone. In females, the ovaries and adrenal glands synthesize a much smaller amount of testosterone.
Testosterone secretion is controlled by GNRH, which is released by the hypothalamus in pulses. These pulses stimulate the pituitary gland to secrete LH. LH causes the enzymatic conversion of cholesterol into testosterone in the Leydig cells. Indeed, “cholesterol” has a bad rap in the mass media, but in fact all of our steroid hormones begin with cholesterol. (See chart below.)
Synthesis of steroid hormones. Click to enlarge.
Testosterone, like other hormones, is regulated by a feedback loop. If the body thinks there’s too much, it “turns off the tap” at the source (i.e. in the brain) or converts the excess to something else such as estradiol or DHT.
Testosterone feedback loop in men
Why is testosterone so important?
One of testosterone’s major roles is to control muscle growth.
Androgen exposure throughout early development determines the number and size of motor units. Number, size, and physiological characteristics of motor units determine the size and physiological characteristics of muscle fibres. Thus, the ability of muscle to hypertrophy (get bigger) in adulthood may be determined in part by intrauterine androgen exposure.
Exercise and testosterone
Exercise-associated increases in testosterone may amplify training-induced muscle growth.
Stimulation of beta-adrenergic receptors encourages testosterone synthesis and release in a dose dependent fashion — the more stimulation, the more synthesis. Thus, increases in plasma concentrations of testosterone are relative to the intensity of exercise. The harder you train, at least with resistance exercise or metabolic conditioning, the more testosterone you get.
Scientists think that sympathetic nerve activity and catecholamine secretion — which occur to a much greater extent during, say, a 20-rep squat set than, say, a pleasant stroll — stimulate testosterone during exercise.
However, exercise type also matters. Extended bouts of endurance exercise seem to suppress testosterone.
Sex differences in testosterone release
Moderate to high intensity exercise increases plasma concentrations of testosterone in a sex-specific fashion. In other words, intense exercise causes testosterone release, more so in men than in women.
Plasma testosterone increases occur in men after various forms of exercise, as long as that exercise is high intensity. On the other hand, women respond to intense exercise with very small and/or delayed testosterone increases, or even no testosterone increases at all.
What you should know about testosterone
Sex differences in testosterone levels
Normal serum total testosterone levels vary from person to person over time, but in general men have much more than women. Typical ranges:
Male: 230-1000 ng/dL
Female: 28-80 ng/dL
Testosterone and age
Testosterone levels increase during puberty from <20 ng/dL to around 300 – 1200 ng/dL at full maturity.
Serum testosterone is secreted in pulses and is circadian. In the second half of puberty, levels are elevated more at night than during the day. During puberty, increases in circulating testosterone and estradiol induce a 1 ½ to 3-fold amplification of pulsatile growth hormone secretion.
Conversely, testosterone levels decline as we age.
How testosterone may change with advancing age
Test-osterone
One blood sample is enough to establish circulating testosterone levels, although sometimes doctors may also test salivary testosterone, and if you’re competing in the Olympics, you may find yourself giving a urine test for androgens as well.
Since 98% of testosterone is bound to carrier proteins in the serum (sex hormone binding globulin or SHBG), alterations in these protein levels will change total testosterone levels.
SHBG is produced in the liver and its production is increased by estrogens and hyperthyroidism. SHBG is decreased by androgens, advancing age, and hypothyroidism. Thus, tests may also look for changes in SHBG.
Low testosterone
Several factors can suppress testosterone output and ultimately reproductive function. These factors include:
- chronically low calorie intake (>20% below basal needs) and chronically high calorie intake (especially if obesity results)
- low nutrient intake and vitamin/mineral deficiency
- low fat intake
- depression
- drug use
- overtraining
- limited sexual activity
- stress and anxiety
- aging
- obesity and other metabolic disorders
- overuse of hormonal contraception (in women)
- chronic illness/infection
- poor, minimal, and disrupted sleep (including sleep apnea)
Both men and women can suffer from low testosterone. Symptoms of low testosterone include:
- low energy, fatigue, and lethargy — loss of “mojo”
- decreased strength and work capacity
- low sexual desire, lack of sexual responsiveness and weaker orgasms or difficulty achieving orgasm
- loss of lean body mass, including muscle and bone density, along with an increase in body fat
- increased cardiovascular risk (including poor blood lipid profile), higher blood pressure
Supplementing testosterone
Many athletes supplement testosterone for bigger muscles and/or better athletic performance. Excessively high doses of testosterone, taken over a long period, can result in:
- acne and oilier skin
- mood changes ranging from hostility to euphoria (though evidence is mixed regarding the prevalence and details, as well as individual susceptibility)
- hair growth (in women) or hair loss
- masculinization of facial features, voice deepening (in women)
- breast growth in males as excess testosterone converts to estrogen
- testicular atrophy in males
- disruption of menstrual cycles in females; increased incidence of reproductive disorders
- certain types of muscle and connective tissue damage
- increased cardiovascular disease risk, including increased blood pressure and heart myopathies along with deep venous thrombosis and embolisms
However, endocrinologists are now starting to prescribe testosterone therapeutically, either for replacement (e.g. in older men and women) or to treat symptoms of many chronic degenerative diseases. The side effects of excessive testosterone listed above generally do not apply to therapeutic and replacement doses.
Summary and recommendations
To maximize testosterone levels for muscle growth, recovery and health:
- Engage in regular, intense exercise sessions
- Don’t severely restrict calories to more than 20% below base needs
- Make sure you are consuming enough micro- and macronutrients
- Engage in safe, regular sexual activity (yes, PN says go get some action!)
- Avoid medications/drugs
- Get adequate sleep, 7-9 hours per night
- Control stress and anxiety levels
Further resources
Male Hormones: Adjustment or Replacement
For extra credit
Testosterone facilitates spontaneous growth hormone secretion.
Sympathetic nerve stimulation increases testosterone synthesis and release.
Testosterone levels increase during the summer months.
Sexual arousal increases LH pulses (which increases testosterone levels).
Alcohol, aspirin, marijuana, codeine, lots of sugar, and opioids can decrease testosterone levels.
Diets higher in protein, cholesterol, and fat tend to maintain testosterone levels.
Eating 1-2 servings of whole, unprocessed soy foods doesn’t seem to have any negative effect on testosterone levels. However, high levels of soy intake may be a problem.
References
EndoText.org Endocrinology of Male Reproduction.
Beers MH, Berkow R eds. Merck Manual. 17th ed. Merck Research Laboratories. Whitehouse Station, NJ. 1999.
Murray RK, Granner DK, Mayes PA, Rodwell VW, eds. Harper’s Illustrated Biochemistry. 26th ed. McGraw Hill. 2003.
Borer KT. Exercise Endocrinology. Human Kinetics. Champaign, IL. 2003.
Harvey RA, Champe PC eds. Pharmacology 2nd ed. Lippincott Williams & Wilkins. 2000.
Almeida OP, et al. Low free testosterone concentration as a potentially treatable cause of depressive symptoms in older men. Arch Gen Psychiatry 2008;65:283-289.
Goh VH & Tong TY. Sleep, sex steroid hormones, sexual activities, and aging in Asian men. J Androl 2009;Aug 14 Epub.
Uchida MC, et al. Hormonal responses to different resistance exercise schemes of similar total volume. J Strength Cond Res 2009;23:2003-2008.
Hackney AC. Effects of endurance exercise on the reproductive system of men: the “exercise-hypogonadal male condition”. J Endocrinol Invest 2008;31:932-938.
Rocha JS, et al. Mild calorie restriction does not affect testosterone levels and testicular gene expression in mutant mice. Exp Biol Med (Maywood) 2007;232:1050-1063.
Traish AM, et al. The dark side of testosterone deficiency: I. Metabolic syndrome and erectile dysfunction. J Androl 2009;30:10-22.