Worldwide about one billion people are estimated to be iodine-deficient; however, it is unknown how often this results in hypothyroidism.[10] In the United States, hypothyroidism occurs in 0.3–0.4% of people.[7] 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.[7] Hypothyroidism is more common in women than in men.[3] People over the age of 60 are more commonly affected.[3] Dogs are also known to develop hypothyroidism, as are cats and horses, albeit more rarely.[11] The word "hypothyroidism" is from Greek hypo- meaning reduced, thyreos for shield, and eidos for form.[12]