Hypothyroidism (//; from hypo- meaning under or reduced, plus thyroid), often called underactive thyroid or low thyroid and sometimes hypothyreosis, is a common endocrine disorder in which the thyroid gland does not produce enough thyroid hormone. It can cause a number of symptoms, such as tiredness, poor ability to tolerate cold, and weight gain. In children, hypothyroidism leads to delays in growth and intellectual development, which is called cretinism in severe cases. The diagnosis of hypothyroidism, when suspected, can be confirmed with blood tests measuring thyroid-stimulating hormone (TSH) and thyroxine levels.

Worldwide, too little iodine in the diet is the most common cause of hypothyroidism. In countries with enough dietary iodine, the most common cause of hypothyroidism is the autoimmune condition Hashimoto's thyroiditis. Less common causes include the following: previous treatment with radioactive iodine, injury to the hypothalamus or the anterior pituitary gland, certain medications, a lack of a functioning thyroid at birth, or previous thyroid surgery.

Hypothyroidism can be treated with manufactured levothyroxine; the dose is adjusted according to symptoms and normalization of the thyroxine and TSH levels. In Western countries, hypothyroidism occurs in 0.30.4% while subclinical hypothyroidism, a milder form of hypothyroidism characterized by normal thyroxine levels and an elevated TSH level, is thought to occur in 4.38.5%. Dogs are also known to develop hypothyroidism and in rare circumstances cats and horses can also have the disorder.

People with hypothyroidism often have no or only mild symptoms. Numerous symptoms and signs are associated with hypothyroidism, and can be related to the underlying cause, or a direct effect of having not enough thyroid hormones.[1][2] Hashimoto's thyroiditis may present with the mass effect of a goiter (enlarged thyroid gland).[1]

Delayed relaxation after testing the ankle jerk reflex is a characteristic sign in hypothyroidism and is associated with the severity of the hormone deficit.[4]

Man with myxedema or severe hypothyroidism showing an expressionless face, puffiness around the eyes and pallor

Additional finding include swelling of the arms and legs and significant ascites.

Myxedema coma is a rare but life-threatening state of extreme hypothyroidism. It may occur in those who are known to have hypothyroidism when they develop another illness, but it can be the first presentation of hypothyroidism. The illness is characterized by very low body temperature without shivering, confusion, a slow heart rate and reduced breathing effort. There may be physical signs suggestive of hypothyroidism, such as skin changes or enlargement of the tongue.[5]

Even mild or subclinical hypothyroidism has been associated with impaired fertility and an increased risk of miscarriage.[6] Hypothyroidism in early pregnancy, even with limited or no symptoms, may increase the risk of pre-eclampsia, offspring with lower intelligence, and the risk of infant death around the time of birth.[6][7]Women are affected by hypothyroidism in 0.30.5% of pregnancies.[7] Subclinical hypothyroidism during pregnancy has also been associated with gestational diabetes and birth of the baby before 37 weeks of pregnancy.[8]

Newborn children with hypothyroidism may have normal birth weight and height (although the head may be larger than expected and the posterior fontanelle may be open). Some may have drowsiness, decreased muscle tone, a hoarse-sounding cry, feeding difficulties, constipation, an enlarged tongue, umbilical hernia, dry skin, a decreased body temperature and jaundice.[9] A goiter is rare, although it may develop later in children who have a thyroid gland that does not produce functioning thyroid hormone.[9] A goiter may also develop in children growing up in areas with iodine deficiency.[10] Normal growth and development may be delayed, and not treating infants may lead to an intellectual impairment (IQ 615 points lower in severe cases). Other problems include the following: large scale and fine motor skills and coordination, reduced muscle tone, squinting, decreased attention span, and delayed speaking.[9]Tooth eruption may be delayed.[11]

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Hypothyroidism - Wikipedia, the free encyclopedia