Iodine is an essential element needed to produce thyroid hormone, which is necessary for metabolism and fetal neurodevelopment. Iodine deficiency thus causes impairment in thyroid hormone synthesis, resulting in goiter, hypothyroidism, congenital hypothyroidism, and impaired cognitive development. Globally, iodine deficiency is the most prevalent and preventable cause of intellectual disability.

Goiter is enlargement of the thyroid gland, most commonly caused by chronic iodine deficiency. Iodine deficiency results in constant thyrotropin (TSH) release from the pituitary gland, stimulating unregulated thyroid growth. Enlargement tends to be diffuse in children and nodular in adults.

Hypothyroidism from iodine intake below 50 µg/day presents as normal or low T3, low T4, and variably elevated TSH. Clinical symptoms are the same as in hypothyroidism due to other causes.

Congenital hypothyroidism, also known as congenital iodine deficiency syndrome, is caused by severe iodine deficiency during gestation and presents as severe intellectual disability with other neurological or physical defects. Inadequate iodine intake early in pregnancy results in neurological deficits such as deaf-mutism, gait disturbances, and spasticity in the infant. Iodine deficiency late in pregnancy and postnatally results in growth retardation and physical signs of hypothyroidism such as dry skin and delayed reflex relaxation in the infant. The skin manifestations in this syndrome can improve with thyroid hormone supplementation, while the neurological effects of congenital hypothyroidism cannot.

The prevalence of iodine deficiency in the United States has decreased since table salt iodization began in the 1920s. With the reduction of iodine in dairy products, increased use of non-iodized salt by the food industry, and recommendations to avoid salt to control blood pressure, iodine deficiency can still present in certain individuals. Vegans and pregnant individuals are at highest risk. In the United States, pregnant, non-Hispanic Black women were the only ethnic group with an inadequate median urinary iodine concentration, of less than 150 µg/L, from 2009-2010.

In developing countries and/or those without national iodine supplementation programs, the risk of iodine deficiency is higher. These countries include Angola, Burkina Faso, Finland, Haiti, Israel, Italy, Sudan, Ukraine, and Vietnam.