These side effects may include: 1  Feeling hot all the time and increased sweating. 2  Fever. 3  An increased appetite, along with weight loss. 4  Heart palpitations. 5  A rapid heart rate (tachycardia) 6  Heart failure, chest pain, or a heart attack. 7  Difficult or painful breathing. 8  Insomnia (see Levothyroxine and Insomnia) 9  Emotional changes.