When your thyroid gland is unable to produce enough thyroid hormone, you have a condition called hypothyroidism. Hypothyroidism is more common in females than males, and the risk increases with age, according to the National Institutes of Health. Thyroid hormones regulate your body metabolism. When you lack sufficient amounts, nearly every body system is affected, resulting in a wide range of symptoms. Treatment involves daily oral doses of synthetic thyroid hormone.

Fatigue and Cold Intolerance

Metabolism is the name given to the chemical processes that convert food and oxygen into the energy your body needs to sustain life. Hypothyroidism slows your metabolism. According to an August 2012 article published in "American Family Physician," the most common symptoms of hypothyroidism are constant fatigue and intolerance to cold 234. These symptoms occur because a slow metabolism associated with insufficient thyroid hormone reduces your body's production of energy and heat.

Weight Gain and Constipation

When your metabolism slows, you may gain weight even if you are eating the same number of calories you did before the hypothyroidism. Weight gain alone is not necessarily an indication of hypothyroidism, but if you experience unexplained weight gain along with other symptoms, your doctor may want to check your thyroid 4. Constipation can occur as a slower metabolism causes food and waste to move through your intestines more slowly.

Dry Skin, Brittle Nails and Thin Hair

Your skin, hair and nails may also exhibit characteristic changes when you have hypothyroidism. Skin can become dry and -- in extreme cases -- your face may become puffy. Nails may become brittle and break easily and your hair can become dry and coarse. Hair thinning can also be a sign of hypothyroidism.

Joint, Muscle and Cardiovascular Problems

Chronic pain and weakness in your joints and muscles can indicate an underactive thyroid. As thyroid hormone levels drop, muscle cramps can occur, and limbs can feel numb. You might develop an unsteady gait. Your heart rate typically becomes slow. Some people with hypothyroidism develop diastolic hypertension, meaning your lower blood pressure number may be higher than is desirable.

Mental Changes

Because hypothyroidism robs the brain of the energy it needs to function, difficulty concentrating, memory loss and headaches can occur. Depression can be an indicator of hypothyroidism in women, especially after menopause. After giving birth, a woman with hypothyroidism is at greater risk for postpartum depression.

Reproductive Problems

Irregular periods or heavy blood flow during your periods can occur with hypothyroidism. The condition may also interfere with your menstrual cycle, causing fertility problems. Thyroid hormone replacement therapy generally reverses the temporary infertility associated with hypothyroidism.