Fatigue is a very important sign of low thyroid,” Dr. Hall says. Since hyperthyroidism increases your metabolism, many individuals initially have a lot of energy, according to the American Association of Clinical Endocrinologists. However, as the hyperthyroidism continues, the body tends to break down, so being tired is very common. If you are constantly tired after sleeping enough hours and not doing a strenuous activity, check your thyroid gland.