Some athletes have used human growth hormone in an attempt to increase their muscle size and strength. Any increase in muscle mass is probably a result of increased connective tissue which does not contribute to strength. The strength increases reported by some athletes who have taken HGH may have actually been the result of anabolic steroids which have been found in seized stocks of the hormone.
Human growth hormone functions by forming a complex with its receptor protein called HGH-bp (-bp for binding protein). Binding of human growth hormone to its receptor is required for regulation of normal human growth and development. In the above and below pictures, the receptor molecule (a membrane protein) is shown in red and green and the Human Growth Hormone is blue. The relationship between the hormone and its receptor is very much like that of an enzyme and its substrate. They must fit together, hand in glove, in order to function.
Other protein hormones such as Glucagon and Insulin regulate the blood sugar levels in your blood. These hormones also have membrane bound protein receptors that bind specifically to their ligand (any molecule that binds to another).
The Protein receptor complex shown above is actually found as part of the plasma membrane as shown on the left. The signal (hormone - in blue) is received outside the cell, and then "the message is relayed" to the inside of the cell. This is followed by some cascade of events which leads to some sort of cellular action. The type of response or action inside the cell is particular to each type of hormone.
Not all the hormones
are proteins . Remember that Testosterone and several others are lipid