In the developing world, iodine deficiency is a major cause of hypothyroidism. While less common in the developed world, mild to moderate iodine deficiency is common in some southern and western European countries.[4] Global efforts to universally iodise salt have diminished the magnitude of this problem; however, iodine intake is still low in many countries and in high-risk individuals, such as pregnant women.[5][6] The prevalence of overt hypothyroidism in the general population ranges between 0.2% and 5.3% in Europe.[7] The incidence of primary hypothyroidism in the UK is estimated to be 0.41% per year for women and 0.06% per year in men.[8] The prevalence of overt primary hypothyroidism in the US is 0.3%, and of sub-clinical hypothyroidism is 4.3%.[9] The prevalence of overt and sub-clinical hypothyroidism is higher in white people (5.1%) than in black people (1.7%), or Hispanic people (4.2%).[9] The prevalence is higher in women and increases with age. It ranges from 4% in women aged 18-24 years, to 21% in women older than 74 years, and 3% to 16% in men of the same age groups.[10]

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