Recent reports of the World Health Organization show iodine deficiency to be a worldwide occurring health problem. As iodine status is based on median urinary iodine excretion, even in countries regarded as iodine sufficient, a considerable part of the population may be iodine deficient. Iodine is a key element in the synthesis of thyroid hormones and as a consequence, severe iodine deficiency results in hypothyroidism, goiter, and cretinism with the well known biochemical alterations. However, it is also known that iodine deficiency may give rise to clinical symptoms of hypothyroidism without abnormality of thyroid hormone values. This led us to the hypothesis that iodine deficiency may give rise to subtle impairment of thyroid function leading to clinical syndromes resembling hypothyroidism or diseases that have been associated with the occurrence of hypothyroidism. We describe several clinical conditions possibly linked to iodine deficiency, a connection that has not been made thus far. In this paper we will focus on the relationship between iodine deficiency and obesity, attention deficit hyperactivity disorder (ADHD), psychiatric disorders, fibromyalgia, and malignancies.