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. The main cause for weight gain, however, is still not understood. In North America almost everyone eats more calories than needed each day for normal metabolism of the cells. Fortunately most of us can waste these extra calories probably by burning them off in the form of heat. People who start to put on weight lose the mechanism for wasting extra calories.