Fatigue is one of the most common and key signs of low thyroid. 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, making you feel exhausted for seemingly no reason. If you are constantly tired even though you have slept enough hours and did not engage in any strenuous activity, check your thyroid gland.