Disfunción tiroidea subclínica en la gestación

The impact of gut microbiota on autoimmune thyroiditis and relationship with pregnancy outcomes: a review

Autoimmune thyroiditis (AITD) is a T-cell-mediated, organ- specific autoimmune disease caused by interactions between genetic and environmental factors. Patients with AITD show thyroid lymphocyte infiltration and an increase in the titer of thyroid autoimmune antibodies, thereby altering the integrity of thyroid follicle epithelial cells and dysregulating their metabolism and immune function, leading to a decrease in multi-tissue metabolic activity. Research has shown that patients with AITD have a significantly higher risk of adverse pregnancy outcomes, such as infertility and miscarriage. Levothyroxine(LT4) treatment can improve the pregnancy outcomes of normal pregnant women with thyroid peroxidase antibodies(TPOAb) positivity, but it is not effective for invitro fertilization embryo transfer (IVF-ET) in women with normal thyroid function and positive TPOAb. Other factors may also influence pregnancy outcomes of patients with AITD. Recent studies have revealed that the gut microbiota participates in the occurrence and development of AITD by influencing the gut-thyroid axis. The bacterial abundance and diversity of patients with Hashimoto thyroiditis (HT) were significantly reduced, and the relative abundances of Bacteroides, fecal Bacillus, Prevotella, and Lactobacillus also decreased. The confirmation of whether adjusting the composition of the gut microbiota can improve pregnancy outcomes in patients with AITD is still pending. This article reviews the characteristics of the gut microbiota in patients with AITD and the current research on its impact in pregnancy.

Thyroid diseases during pregnancy: a number of important issues

The most common thyroid diseases during pregnancy are hyper- and hypothyroidism and their variants including isolated hypothyroxinemia (hypo-T4), autoimmune thyroid disease (AITD) and different types of goiter. AITD represents the main cause of hypothyroidism during pregnancy ranging in prevalence between 5 and 20% with an average of 7.8%. The incidence of isolated hypo-T4 is about 150 times higher compared to congenital hypothyroidism. Prevalence of Graves' disease (GD) ranges between 0.1% and 1% and the Transient Gestational Hyperthyroidism Syndrome between 1 and 3%. Thyroid stimulating hormone (TSH) is a sensitive marker of thyroid dysfunction during pregnancy. Normal values have been modified recently with a downward shift. Thus, the upper normal range is now considered to be 2.5 mUI/mL in the first trimester and 3.0 mUI/mL for the remainder of pregnancy. Most studies have shown that children born to women with hypothyroidism during gestation had significantly lower scores in neuropsychological tests related to intelligence, attention, language, reading ability, school performance and visual motor performance. However, some studies have not confirmed these findings. On the other hand, multiple retrospective studies have shown that the risks of maternal and fetal/neonatal complications are directly related to the duration and inadequate control of maternal thyrotoxicosis. The latter is associated with a risk of spontaneous abortion, congestive heart failure, thyrotoxic storm, preeclampsia, preterm delivery, low birth weight and stillbirth. Despite the lack of consensus among professional organizations, recent studies, which are based on sophisticated analyses, support universal screening in all pregnant women in the first trimester for thyroid diseases.

Controversies in endocrinology: On the need for universal thyroid screening in pregnant women

There is a well-known controversy among scientific societies regarding the recommendation to screen for thyroid dysfunction (TD) during pregnancy. Although several studies have shown an association between maternal subclinical hypothyroidism and/or hypothyroxinemia with obstetric problems and/or neurocognitive impairment in the offspring, there is only limited evidence on the possible positive effects of thyroxine (T4) treatment in such cases. Despite the scarcity of this evidence, there is a widespread agreement among clinicians on the need for treatment of clinical hypothyroidism during pregnancy and the risks that could arise due to therapeutic abstention. As maternal TD is a quite prevalent condition, easily diagnosed and for which an effective and safe treatment is available, some scientific societies have proposed to assess thyroid function during the first trimester of pregnancy and ideally before week 10 of gestational age. Given the physiologic changes of thyroid function during pregnancy, hormone assessment should be performed using trimester-specific reference values ideally based on locally generated data as geographic variations have been detected. Screening of TD should be based on an initial determination of TSH performed early during the first trimester and only if abnormal should it be followed by either a free or total T4 measurement. Furthermore, adequate iodine supplementation during pregnancy is critical and if feasible it should be initiated before the woman attempts to conceive.

[Detection of thyroid dysfunction in pregnant women: universal screening is justified]

There is a controversy among different scientific societies in relation to the recommendations on whether universal screening for the detection of thyroid dysfunction during gestation should be performed or not. Although various studies have shown an association between subclinical hypothyroidism or hypothyroxinemia with obstetric problems and/or neurocognitive impairment in the offspring, no evidence on the possible positive effects of treatment of such conditions with thyroxin has been demonstrated so far. However, there is a general agreement about the need for treatment of clinical hypothyroidism during pregnancy and the risks of not doing so. Because it is a common, easily diagnosed and effectively treated disorder without special risk, the working Group of Iodine Deficiency Disorders and Thyroid Dysfunction of the Spanish Society of Endocrinology and Nutrition and Spanish Society of Gynaecology and Obstetrics recommends an early evaluation (before week 10) of thyroid function in all pregnant women. Given the complex physiology of thyroid function during pregnancy, hormone assessment should be performed according to reference values for each gestational trimester and generated locally in each reference laboratory. Thyrotropin determination would be sufficient for screening purposes and only if it is altered, free thyroxin or total thyroxin would be required. Adequate iodine nutrition is also highly recommended before and during pregnancy to contribute to a normal thyroid function in the pregnant women and fetus.