The under-activation of the thyroid gland is called hypothyroidism. This condition is very harmful and causes many more health problems.
The symptoms associated with hypothyroidism are;
profound fatigue
slow heart beat
lower blood pressure
chronic anemia
chronic constipation
stunted growth
pale or yellowish dry, itchy skin
thick nails that break or split easily

Even less obvious symptoms may include:
reduction in body hair, including eyebrows and groin
bloating or puffiness around the eyes
"feeling cold all the time"
heavy, irregular menstruation with clotting
history of miscarriage, low birth weight babies or early delivery
weak muscles or muscle cramps
vision disturbances, including "flickering" in the peripheral field
decreased sexual desire
retarded sexual development
memory loss
These symptoms vary from one individual to another. Most people don’t feel many of these. The extreme fatigue associated with underactive thyroid can result in depression, impaired mental processes, and emotional disturbances. Frequent infections are often noted. The cardiac system can also be affected, causing irregular heartbeat, slow pulse rate and fluctuations in blood pressure. But if you found the problem, get treated immediately.