An underactive thyroid, known as hypothyroidism, is caused by an inadequate production of thyroid hormone.
hypothyroidism hy·po·thy·roid·ism (hī'pō-thī'roi-dĭz'əm)
Insufficient production of thyroid hormones.
A pathological condition resulting from thyroid insufficiency, which may lead to cretinism or myxedema.
An abnormality of the thyroid gland characterized by insufficient production of thyroid hormone, which can result in a decreased basal metabolic rate, causing weight gain and fatigue. Compare hyperthyroidism.