Testosterone is a steroid hormone which is predominantly made in the male testes. The hormone is stimulated and controlled by luteinising hormone (LH) which is produced by the pituitary gland in the brain. Women also produce a small amount of testosterone in their ovaries and adrenal glands.
Free testosterone is the amount of testosterone which is not bound to either sex hormone binding globulin or albumin. Instead, it is freely travelling around the body in the blood without being bound to another protein.
Testosterone has an important role in the human body affecting the brain, bone and muscle mass, fat distribution, the heart and blood vessels, energy levels and sexual function.
The secretion of testosterone follows a circadian rhythm in men i.e. the highest levels are witnessed in the early hours of the morning. As men age, their testosterone levels decrease which can result in a reduced libido, lower bone density, reduced muscle mass and reduced cognitive function. The levels of testosterone in men are as follows:
In early adulthood, testosterone is essential for growth and the regulation of the prostate gland. Testosterone is carried to the prostate gland via the blood where it is then converted to dihydrotestosterone which encourages prostatic cell growth. 
Measuring free testosterone is more difficult than measuring total testosterone. As free testosterone is unbound to any other protein is also referred to as bioavailable testosterone because it is easily available for use by the body. Whilst the measurement of total testosterone is not a reliable index of free testosterone, when considered in the context of SHBG concentrations this measurement is useful in the clinical context. 
High levels of testosterone in women can be associated with Polycystic Ovary Syndrome (PCOS). The condition affects how the ovaries work and can cause unwanted symptoms including:
PCOS affects 1 in 5 women who are of reproductive age and can have some psychological side effects, like:
The condition is chronic and usually first manifests in a women’s teenage years. The condition can also result in reduced fertility which can also lead to psychological symptoms. 
Individuals who have a naturally higher level of testosterone maybe more motivated to take part in exercise and train. Higher levels of testosterone are more beneficial in sports which require strength and power.
Low levels of testosterone in men are associated with an increased risk of diabetes, cardiovascular disease, osteoporosis and a reduced libido.
Testosterone is an important hormone with regards to exercise and exercise training. This is largely due to the fact it is has a major role in growth and maintenance of muscle and bone. Studies have shown that the type of exercise such as strength work, can increase levels of testosterone in the body. 
Eating the right diet can help to control some of the symptoms and risks associated with PCOS. Heart disease and diabetes are consequences of PCOS and so you should incorporate an increased amount of fruit and vegetables into your diet, and limit the amount of sugary and fatty foods you eat.
Eat a diet which has a low glycaemic index may also be beneficial. Low GI diets incorporate foods which cause your blood sugar to rise slowly can reduce the symptoms of PCOS. Foods include:
Low GI diets and foods may help to alleviate the symptoms of PCOS because many women who have the condition are resistant to insulin. Insulin is a hormone which helps the body to use the energy from food.
Low testosterone levels have been associated with low protein intake. Insulin is associated with SHBG levels. Protein intake has been shown to increase insulin levels and insulin has been shown to decrease SHBG levels which in turn can increase the amount of testosterone.
As men age, serum testosterone levels decrease, particularly between the ages of 40 and 50. Men who are in their 70s have around a 40% lower testosterone level than men in their 20s. The age-related decline in testosterone is associated with a lower quality of life, a decrease in muscle mass and osteoporosis. Studies in middle-aged men show that moderate to vigorous exercise can see greater improvements in testosterone levels and in turn improves cardiopulmonary fitness.
Being overweight can increase the severity of the symptoms associated with PCOS in women. Physical activity can help with weight loss but perhaps, more importantly, it can improve the body’s response to insulin.7
Weight loss in PCOS patients is also beneficial for all other symptoms of the condition. Exercise should be encouraged and a plan should be devised with your GP.
Further studies in women have shown that individuals who have a higher free testosterone level have a significant competitive advantage over their rivals. Testosterone is a performance-enhancing androgen and can have beneficial effects on muscle and body mass as well as red blood cell development.
All these tests include Testosterone (total). Select the test that suits your personal needs.
 Tyagi, V., Scordo, M., Yoon, R, S., Liporace, F, A and Wissner Greene, L. (2017). Revisiting the Role of Testosterone: Are We Missing Something? Rev Urol: 19(1), pp 16-24.
 Alvarado, L, C. (2011). Total Testosterone in Young Men is More Closely Associated than Free Testosterone with Prostate Cancer Disparities. Ther Adv Urol: 3(3), pp 99-106.
 Mueller, A., Dittrich, R., Cupisti, S., Beckmann, M, W and Binder, H. (2006). Is It Necessary to Measure Free Testosterone to Assess Hyperandrogenemia in Women? The Role of Calculated Free and Bioavailable Testosterone. Exp Clin Endocrinol Diabetes: 114(4), pp 182-187.
 Ehrmann, D, A. (2005). Polycystic Ovary Syndrome. The New England Journal of Medicine: 352, pp 1223-36.
 Teede, H., Deeks, A and Moran, L. (2010). Polycystic Ovary Syndrome: A Complex Condition with Psychological, Reproductive and Metabolic Manifestations that Impacts on Health Across the Lifespan. BMC Medicine: 8.
 Shores, M, M., Smith, N, L., Forsberg, C, W., Anawalt, B, D and Matsumoto, A, M. (2012). Testosterone Treatment and Mortality in Men with Low Testosterone Levels. J Clin Endocrinol Metab: 97(6), pp 2050-2058.
 Lane, A, R and Hackney, A, C. (2014). Relationship Between Salivary and Serum Testosterone Levels in Response to Different Exercise Intensities. Hormones.
 The Association of UK Dietitians. (2016). Polycystic Ovary Syndrome (PCOS). Available at: https://www.bda.uk.com/foodfacts/pcos.pdf
Longcope, C., Feldman, J, B., McKinlay, B and Araujo, A, B. (2000). Diet and Sex Hormone-Binding Globulin. The Journal of Clinical Endocrinology and Metabolism: 85(1), pp 293-296.
 Hawkin, V, H., Foster-Schubert, K and Chubak, J et al. (2008). Effect of Exercise of Serum Sex Hormones in Men: A 12 Month Randomized Clinical Trial. Med Sci Sports Exerc: 40(2), pp 223-233.
 Khademi, A., Alleyassin, A., Aghahisseini, M., Tabataeefar, L and Amini, M. (2010). The Effect of Exercise in PCOS Women Who Exercise Regularly. Asian Journal of Sports Medicine: 1(1), pp 35-40.
 Bermon, A and Garnier, P, Y. (2017). Serum Androgen Levels and Their Relation to Performance in Track and Field: Mass Spectrometry Results from 2127 Observations in Male and Female Elite Athletes. Br J Sports Med:0, pp 1-7.