Underactive thyroid is known as hypothyroidism. It is a fairly common condition. Fortunately it is easily treated. I recommend discussing your concerns with your doctor
. He or she can easily test your thyroid function with a simple blood test that is far more sensitive than symptoms (that is, one can catch the disease earlier).
In general, the thyroid gland (located in the neck) is a gland that produces the thyroid hormone. The thyroid hormone is the "energy hormone" -- it facilitates the energy use by all cells in your body. Therefore symptoms of hypothyroidism are decreased cell function.
These symptoms include: Fatigue. Feeling cold more often. Constipation (decreased activity of the colon cells pushing the food down). Depression
is common. Decreased sweating. Dry skin. Hair loss
. Interestingly, loss of the outside part of the eyebrows are common. Change in periods may occur in women. The thyroid gland often enlarges in many low thyroid conditions--resulting in a neck bulge (known as a goiter).
Severe hypothyroidism can result in heart failure and even coma. Myxedema coma is a specific type of coma that occurs with hypothyroidism that also results in a puffy nature to the skin.
Beyond symptoms, there are physical exam signs your doctor can find. These include low heart rate, "hung up reflexes", decreased bowel sounds and many others. There are also changes seen in your blood work--for example, your cholesterol can increase.
Discuss your concerns with your doctor.