Hypothyroidism, often called underactive thyroid or low thyroid and sometimes hypothyreosis, 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, and weight gain. In children, hypothyroidism leads to delays in growth and intellectual development, which is called cretinism in severe cases.

Worldwide, too little iodine in the diet is the most common cause of hypothyroidism. In countries with enough iodine in the diet, 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. The diagnosis of hypothyroidism, when suspected, can be confirmed with blood tests measuring thyroid-stimulating hormone (TSH) and thyroxine levels.

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 0.3–0.4% of people while subclinical hypothyroidism, a milder form of hypothyroidism characterized by normal thyroxine levels and an elevated TSH level, is thought to occur in 4.3–8.5% of people. Dogs are also known to develop hypothyroidism and in rare cases cats and horses can also have the disorder. The word hypothyroidism is from hypo- meaning under or reduced, plus thyroid.

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