An under-active thyroid, or hypothyroidism, is usually caused by the body developing antibodies against its thyroid gland. As a result, you feel weak and lethergic. There may be hair loss, flaky skin, a deeper voice and you are also likely to gain weight. It needs to be diagnosed by a doctor. Treatment is by hormone therapy.
I suffer from an under-active thyroid. It can be very debilitating as you feel cold, weak, tired,depressed and bloated. It also affects periods and bowel movements. It's more common in women and more so in middle age. In the western world it is not believed to be caused by iodine deficiency.