Lethargy, also known as malaise, is a symptom of hypothyroidism and can be treated with a blood test for thyroid function. After hypothyroidism is confirmed, the treatment is based on thyroid hormone supplementation, and the symptoms will disappear when the thyroid function is restored. It can also be seen after prolonged strenuous activities or heavy physical labor, mostly accompanied by muscle aches and pains, and after a period of rest, the weakness and aches can be relieved. It can also be caused by anemia, which can be relieved by oral blood and iron supplements for a period of time in mild cases, or by blood transfusion in severe cases.