Hypothyroidism and isolated hypothyroxinemia in pregnancy, from physiology to the clinic.

Many changes occur in the physiology of the maternal thyroid gland to maintain an adequate level of thyroid hormones (THs) at each stage of gestation during normal pregnancy, however, some factors can produce low levels of these hormones, which can alter the onset and progression of pregnancy. Deficiency of THs can be moderate or severe, and classified as overt or clinical hypothyroidism, subclinical hypothyroidism, and isolated hypothyroxinemia. Overt hypothyroidism has been reported in 0.3-1.9% and subclinical hypothyroidism in approximately 1.5-5% of pregnancies. With respect to isolated hypothyroxinemia, the frequency has been reported in approximately 1.3% of pregnant women, however it can be as high as 25.4%. Worldwide, iodine deficiency is the most common cause of hypothyroidism, however, in iodine-sufficient countries like the United States, the most common cause is autoimmune thyroiditis or Hashimoto's thyroiditis. The diagnosis and timely treatment of deficiency of THs (before or during the first weeks of gestation) can significantly reduce some of the related adverse effects, such as recurrent pregnancy loss, preterm delivery, gestational hypertension, and alterations in the offspring. However, so far there is no consensus on the reference levels of thyroid hormones during pregnancy to establish the diagnosis and there is no consensus on universal screening of women during first trimester of pregnancy to identify thyroid dysfunction, to give treatment and to reduce adverse perinatal events, so it is necessary to carry out specific studies for each population that provide information about it.