Causes of Fatigue Changes in thyroid function typically cause fatigue. An underactive thyroid results in a general decrease in the activity of the body including the brain which would rather sleep. An overactive thyroid initially may cause an increase in energy but the energy wasted in the process results in a decrease in total energy stores including the loss of protein and this eventually results in fatigue and loss of stamina and strength. There are, however, many other causes of fatigue. Anemias of which there are many causes result in decreased oxygen delivery to cells throughout the body and thus fatigue.
Any chronic disease affecting the heart, kidneys, gut, and other organs can result in fatigue.
Overwork, lack of sleep, depression, and boredom all sap a person's energy and result in fatigue. These latter factors are probably the commonest causes of fatigue and certainly more common than changes in thyroid function. Gain A decrease in thyroid function results in a decrease in the consumption of energy and as a result more energy is available to be stored. Weight gain is, therefore, typical of an underactive thyroid. There are many other causes for weight gain.