Hypothyroidism or underactive thyroid or low thyroid:
Hypothyroidism, also called underactive thyroid or low thyroid, is a common disorder of the endocrine system in which the thyroid gland does not produce enough thyroid hormone. It can cause a number of symptoms, such as poor ability to tolerate cold, a feeling of tiredness, constipation, depression, and weight gain. Occasionally there may be swelling of the front part of the neck due to goiter. Untreated hypothyroidism during pregnancy can lead to delays in growth and intellectual development in the baby, which is called cretinism. More severe disease is known as myxedema. It is diagnosed by thyroid function tests especially based on TSH levels. It can be managed by hormone replacement therapy after ruling out all the other causes of hypothyroidism.
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http://pediatrics.aappublications.org/content/117/6/2290 update of newborn screening and therapy for congenital hypothyroidism.american academy of pediatrics, susan r. Rose, american thyroid association, rosalind s. Brown, lawson wilkins pediatric endocrine society. June 2006, volume 117 / issue 6