Iodine deficiency: female menstrual cycle to conception

Iodine is an inorganic substrate essential for thyroid hormone (TH) synthesis. Iodine deficiency (ID) is a global public health problem, and estimates of the extent of the problem were last updated in 2007. Two bilion people in the general population have been estimated to be exposed to insufficient iodine intake, so that this debilitating health issue affects almost one in three individuals globally. Iodine deficiency is still considered to be a public health in 47 countries and both children and child-bearing age women represent subset populations particularly exposed to the most severe consequences of iodine deficiency. More than 250,000 children (30% of school age children population in the world) have insufficient iodine intake, while, unfortunately, no sufficient data are at present available in both pregnant and fertile women (de Benoist et al. 2008). Thyroid hormone is essential for a proper gonad maturation and gametogenesis in mammals of both sex. When iodine intake is inadequate thyroid hormone production can be deeply affected, resulting in various degrees of thyroid failure ranging from isolated hypothyroxinemia to the more severe condition of either subclinical or overt hypothyroidism. During the fetal life, the development of male and female reproductive tracts does not seem to be influenced by THs, which conversely seem to play a crucial role later on, in sexual maturation and gonad development. Hypothyroidism can result in a marked delay in sexual development in males and, conversely, in precocious puberty, menstrual disturbances and reduced fertility in females (Jannini et al. 1995, Longcope et al. 1996, Zhang et al. 1997). So far, only a few studies specifically focused on the relationship between iodine deficiency and reproduction. Indeed, the vast majority of the investigations were aimed at evaluating the relationship between hypothyroidism (not necessarily iodine-deficiency related) and reproduction. As a consequence this chapter will be aimed at reviewing the effects of iodine deficiency/hypothyroidism on female reproductive life up to conception. The importance of thyroid hormones in contributing to the stability of the feto-placental unit and in ensuring normal neurological development in the fetus, once pregnancy has occurred, will not be discussed, being beyond the scope of this review.