Claim: Hypothyroidism can cause poor ability to tolerate heat.

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 goitre . Untreated hypothyroidism during pregnancy can lead to delays in growth and intellectual development in the baby , which is called cretinism .   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 : 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 -LRB- TSH -RRB- and thyroxine levels .   Prevention at the population level has been with the universal salt iodization . Hypothyroidism can be treated with levothyroxine . The dose is adjusted according to symptoms and normalization of the thyroxine and TSH levels . Thyroid medication is safe in pregnancy . While a certain amount of dietary iodine is important , excessive amounts can worsen certain types of hypothyroidism .   Worldwide about one billion people are estimated to be iodine deficient ; however , it is unknown how often this results in hypothyroidism . In the United States , hypothyroidism occurs in 0.3 -- 0.4 % of people . 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 in the United States . Hypothyroidism is more common in women than men . People over the age of 60 are more commonly affected . Dogs are also known to develop hypothyroidism and in rare cases cats and horses can also have the disorder . The word `` hypothyroidism '' is from Greek hypo - meaning `` reduced '' , thyreos for `` shield '' , and eidos for `` form . ''