Just like food, training brings about some important hormonal responses in our organism as well, responses that we can compare to a natural “pharmacological effect”. 
Physical activity, according to duration and intensity, can increase or diminish the production of several hormones, which in their turn modify metabolic responses to effort, towards progressive adaptation. 

Levels of Testosterone, the main muscular anabolic hormone naturally produced both in males and females, increase by 20-40% during and immediately after either a strengthening or endurance focused training session when compared to values at rest. But if the effort is very high and prolonged over 4-5 hours Testosterone tends to decrease, even much below normal levels, for a period that might extend up to 3 or 4 days. 

Estrogen hormones have a very similar behavior, as they are decisive in female athletes in order to maintain a good bone mineralization. 
Estrogens, for women as well as men, are powerful inductors in the production of GH, another hormone very much influenced by physical activity. 

GH, or Growth Hormone, has anabolic action on all tissues, not only muscles, thus favoring the regeneration of organs such as liver, kidneys, heart, skin, cartilage; it also has an effect of lipolysis, favoring the mobilization of fats from adipose tissue and their use as fuel under effort. 

30 minutes of aerobic exercise close to anaerobic threshold level are sufficient to increase GH concentration in the blood from 5 mM/l (average value at rest) to 70 mM/l immediately after the effort. 

Cortisol and Aldosterone are produced by adrenal glands as a response to “stress”. 
Their effect in training is decisive for water retention, increasing the plasmatic volume in blood and prevent dehydration from exertion. 

Both hormones improve peripheral oxygenation, augmenting O2 transfer from red blood cells to muscle cells. 
Cortisol also favors the use of fats, preserves sugars and reduces inflammations. 
Their concentrations under effort increase considerably in comparison with rest conditions levels; without proper recovery strategies, the catabolic effect of cortisol in excess could nullify many beneficial effects of training itself.

 

Production of Erythropoietin is also stimulated by training, being a little unclear whether because of hypoxia consequent to effort or because of increased red blood cells turnover in athletes. 
Altitude training enhances such response, as long as supported by adequate protein intake. 

Thyroid Hormones (activators of mitochondrion functions) and Glucagon (glycolytic and lipolytic action) are both stimulated by physical exercises, while Insulin gets a decrease under effort. 

All of these hormonal responses, together with some other less known or unknown ones (yet not less important), act naturally in perfect synchronism, just like the instruments of an orchestra: any attempt of manipulation from the outside inevitably alters such symphony, with deleterious medium and long term effects on the athlete’s performances. 

Exogenous administration of even just one of these hormones does inhibit the production of all the other ones.