Testosterone is the male sex hormone. It is found in both genders and throughout many different species. Testosterone serves a variety of functions in the body such as sexual and bodily development, metabolic and behavioral influence, and much more. It is important to know what increases and decreases this hormone, as well as when levels are too high or low in your body.
What Is Testosterone?
Testosterone is the male sex hormone. It is produced primarily in the testicles in males and the ovaries in females. This steroid hormone has extensive effects on sexual development, body composition, and behavior. This hormone in excess or deficiency can cause a variety of diseases, and both dietary and behavioral factors affect its levels.
1) Triggers Puberty
While testosterone grows throughout age before puberty, a significant jump occurs at around age 10. Testosterone increases are correlated with early stages of puberty [1, 2, 3, 4].
Increasing testosterone can cause early puberty and also hasten the onset of puberty in children with delayed puberty [5, 6, 7].
Testosterone leads to increases in insulin-like growth factor 1 (IGF-1) and growth hormone, both of which are pivotal in childhood growth [8, 9, 10].
The hormone causes men to grow beards and increase the size of the skull and face [11, 12].
Testosterone can improve protein synthesis, in combination with a minimum level of growth hormone .
Experimental research in animals suggests that testosterone is necessary for proper neurobehavioral development, especially before birth and in early infancy .
Furthermore, a study on Chinese boys found a correlation between testosterone levels and fluid intelligence (problem-solving skills) during certain stages of puberty .
2) Reduces Body Fat and Improves Metabolic Health
Low testosterone is associated with obesity, metabolic syndrome, type 2 diabetes and insulin resistance [15, 16].
Testosterone levels are associated with fat loss, and testosterone treatment can decrease fat percentage .
Testosterone treatment decreased total fat mass while increasing fat-free mass in 26 elderly men .
In a clinical trial on 39 men with lowered bioavailable testosterone levels and type 2 diabetes, testosterone replacement therapy improved body composition .
In another trial on 32 men with low testosterone, testosterone replacement improved the metabolic profile .
3) Increases Sexual Function
A baseline testosterone level (~12 nmol/L) allows for healthy intercourse, sexual drive, and induced nocturnal erections in men [21, 22].
Adding testosterone beyond the apparent baseline does not continue to increase these characteristics based on 2 meta-analyses [21, 23].
However, treating men with low testosterone helps to repair these functions, and promoted other functions such as bone mineral density and body composition (see above) in 2 clinical trials on almost 400 men [24, 25].
Testosterone therapy improved well-being, mood, and sexual function in a clinical trial on 34 premenopausal women with low libido and low testosterone .
Oral and vaginally applied testosterone & estrogen improved sexual function in 2 clinical trials on 150 postmenopausal women [27, 28].
In a clinical trial on 71 women who had their uterus removed, testosterone treatment resulted in better sexual function, leaner body mass, and increased strength .
Almost 200 men with sexual dysfunction (ED) associated with type 2 diabetes reported improved sexual function and mood when treated with testosterone replacement in a trial .
High volume, moderate-intensity aerobic exercise for at least 200 minutes a week showed great improvements in sexual function and testosterone levels in a clinical trial on 90 obese men .
Testosterone increased the expression of dopamine transporters (DAT) and vesicular monoamine transporter (VMAT), both of which allow dopamine to promote sexual desire, in adolescent rats. This neurotransmitter is a possible mechanism for testosterone’s effects .
4) Increases Bone Mineral Density
Low testosterone is associated with male osteoporosis .
Testosterone increased bone mineral density in the spine in a meta-analysis of 29 clinical trials and over 1,000 men .
In a clinical trial on 57 postmenopausal women, testosterone treatment prevented bone loss and increased hip bone mineral density .
5) Reduces the Risk of Heart Disease
A meta-analysis of 29 clinical trials and 1,000 men found that testosterone reduces total cholesterol .
Testosterone and cholesterol are long known to be inversely proportional. In pigs fed high fat and cholesterol diets, those that were castrated for testosterone deficiency developed much higher levels of LDL, as well as the PCSK9 enzyme (an enzyme that binds to an LDL receptor). This presents a possible mechanism for the reason behind this inverse relationship .
The hormone might also prevent the formation of atheromas (plaque-induced degeneration of arteries) and progression to acute coronary syndrome .
6) Increases Risk-Taking
Various measures of risk-taking propensity have shown that high testosterone and low cortisol led to risk-taking in financial markets for both sexes [37, 38, 39, 40, 41].
Changes in salivary testosterone can predict future risk-taking behavior .
More specifically, some studies suggest that risk-taking increases in certain situations, and not others – namely, in those of known possibilities and strategic decision making .
Another study on almost 300 adolescent girls found no relation between testosterone and risk-taking. In contrast, a strong association was observed between estradiol (E2) and risk-taking (E2 is aromatized from testosterone) .
Similar to aggression, orbitofrontal cortex (impulse control) could be responsible for the relationship between testosterone and risk-taking based on another study on over 250 adolescent boys and girls .
7) Increases Competitiveness, Aggression, and Protectiveness
Studies show a strong association between testosterone levels and physical and verbal aggression/dominance .
Women with high testosterone have a strong sense of intrasexual competitiveness, as seen in a study on 136 women .
Male rats with low androgen levels show less rough/competitive play behaviors than those with normal levels .
A possible mechanism to explain this link between testosterone and aggressiveness is a reduced activity in the orbitofrontal cortex of the brain, which handles impulse control .
Beyond inherent testosterone levels, it is difficult to predict the effects of external testosterone on qualities such as aggression. For instance, a study on 30 men with low testosterone levels found that testosterone treatment reduced negative mood and increased vigor .
Mice studies suggest that the timing of peak plasma testosterone during pregnancy and the first few days of birth can predict possible aggressive behavior in adulthood .
8) Increases Muscle Strength
A meta-analysis of 29 clinical trials and over 1,000 men found that testosterone treatment increases muscle strength .
9) May Increase Attraction to Femininity
Changes in salivary testosterone levels in men contributed to the strength of men’s reported attraction to femininity in women’s faces in a clinical trial on 29 men .
10) May Increase Monogamy in Men
In male mice who were in relationships, testosterone decreased their desire for other females .
11) May Increase Red Blood Cells
Via the 5α-reductase enzyme, testosterone becomes dihydrotestosterone. This hormone increases red blood cell creation and maintains proper blood iron levels. Testosterone had the same function in a clinical trial on 60 men .
12) May Improve Some Cognitive Parameters and Mood
Testosterone is associated with risk-taking, which is in turn associated with abstract reasoning ability and fluid intelligence (Raven matrices). One study on 188 college students (male and female) showed that a substantial part of the effect of testosterone on attitude to risk is mediated by abstract reasoning ability .
Sex hormones have an effect on the memory of facts and knowledge. Testosterone was associated with activation of the left prefrontal lobe of the brain in 2 small studies on 48 people (men and women) [56, 57].
Low testosterone is associated with depression .
13) May Help Autoimmune Disease and Pain
Androgens such as testosterone could have an influence on T cell differentiation based on a study in rats .
Testosterone inhibited the binding of substance P to its receptor in a cell-based study. Substance P is associated with inflammation and pain .
Autoimmune diseases may be associated with low blood testosterone concentrations, as seen in a small study on 17 people .
Testosterone reduced temporomandibular joint (connecting the cheekbone to the jaw bone) pain in rats .
14) May Be Associated with Prostate Cancer
Low testosterone was associated with prostate cancer in a study on almost 500 men. However, note that the study didn’t establish low testosterone as the cause of this disease. Other environmental and genetic factors might have contributed .
High Testosterone Side Effects
The following conditions are commonly associated with high testosterone levels, but this single symptom is not enough for a diagnosis. Work with your doctor to discover what underlying condition might be causing your unusually high testosterone levels and to develop an appropriate plan to improve your health:
- Adverse effects on the cardiovascular system, such as high blood pressure [64, 65, 66, 67]
- Polycystic ovary syndrome in women [68, 69]
- Borderline personality disorder (BPD) 
- Non-gender specific cancer aggressiveness 
- Anxiety disorders in children of mothers exposed to excess androgens 
Anabolic Steroid Side Effects
The following side effects have been observed in male taking anabolic steroids to increase athletic performance. Never take anabolic steroids without consulting it with your doctor and always follow your treatment plan.
Testosterone Circadian Rhythm
Testosterone is higher in the morning than at night according to a study of 66 men. It’s about 32% higher in the morning for 30-year-olds, but the cycle becomes more constant as men age, getting to only 8% higher in the morning at 70 years old .
For this reason, testosterone levels should be tested in the morning .
This testosterone cycle is also found in women. Young girls (even as young as 5) and boys experience an increase in testosterone on their way to puberty [75, 76].
Daily rhythms of testosterone are associated with parenting. In an observational study on 341 parents with two children (per couple), higher evening testosterone in mothers was associated with more sensitivity to the oldest and youngest child .
Also for mothers, more daily changes in testosterone were associated with less sensitivity to both children and less respect for the youngest child.
What Decreases Testosterone?
The following conditions and substances may reduce testosterone levels:
- Increasing age 
- Stress 
- Heavy acute alcohol drinking 
- Sleep disorders 
- Obesity 
- Inflammation (IL-6, and prostaglandins A1, A2, and E2) [82, 83, 84]
- Dibutyryl cAMP (increases aromatase) 
- Severe traumatic brain injury affecting the pituitary gland 
- Conditions such as diabetes, hemochromatosis, HIV/AIDS, non-alcoholic liver disease, mumps, meningitis, syphilis, and other infections [86, 87]
- Methadone (opioid-dependence medication) 
- Soy-phytoestrogens (molecules that imitate estrogens) 
- Broad beans 
- Anything containing quercetin, genistein, and flavone 
- Statins (HMG-CoA reductase inhibitors) 
What Increases Testosterone?
If your testosterone levels are too low, the most important thing is to work with your doctor to find out what’s causing this high value and to treat any underlying conditions. The additional lifestyle changes listed below are other strategies you may want to discuss with your doctor. None of them should ever be implemented in place of what your doctor recommends or prescribes!
Doing 4 hours of moderate (55-70% maximum heart rate) exercise per week increased testosterone by 250% compared to doing ~2 hours of exercise in 90 obese men. Administering pure lactate (a by-product of exercise) had a similar effect [93, 94].
Resistance training seems to be especially effective, as seen in two clinical trials on 41 people [95, 96].
2) High-Protein and Healthy-Fat Diets
A high protein diet and supplementing oleuropein (a compound from extra virgin oil) increased testosterone by 300% in rats while halving cortisol .
Low-fat diets are associated with reduced testosterone levels. For instance, one study of 30 men showed a reduction of 12% after 8 weeks on a low-fat, high-fiber diet [98, 99]
3) Reducing Stress
Unusually high levels of the stress hormone cortisol reduced testosterone levels in 2 studies on 24 men [100, 101].
4) Improving Sleep Quality
In an observational study on over 1,300 men, low sleep quality was associated with reduced testosterone levels .
Sleeping 5 hours per night was associated with a 15% reduction of this hormone, while sleeping only 4 hours was linked to borderline testosterone deficiency in a study on 12 elderly men [102, 103].
5) Getting More Vitamin D
In an observational study on over 2,000 men, high vitamin D levels were associated with increased testosterone. Supplementation with this vitamin (3,000 IU per day) increased testosterone by 25% in a trial on 165 young to middle-aged overweight, deficient men [104, 105].
You can increase your dietary intake of this vitamin, take supplements, or get moderate sun exposure.
6) Taking Vitamin and Mineral Supplements
Supplementing with the following vitamins and minerals improved testosterone levels in clinical trials:
- Zinc [106, 107, 108, 109, 110, 111, 112]
- Calcium 
- Boron 
- Magnesium 
- Vitamin B9 
- Vitamin A 
- Vitamin C [117, 118]
- Vitamin E 
7) Taking Testosterone Boosters
Supplementing with DHEA increased testosterone by 200% in middle-aged men and by about 180% in young men in a clinical trial on 16 men .
Some herbal extracts that increased testosterone levels in clinical trials include: