Universal screening of thyroid dysfunction in the pregnant population

Universal screening for thyroid dysfunction in pregnant women is an extremely complex subject which has stirred up broad debate and opposing positions regarding the convenience or inconvenience of performing universal versus selective screening. The recently published consensus document advocates universal screening because adequate evidence to justify it is considered to be available. Some of our arguments are debated in the letter of G. GiménezPérez published in this issue. As regards the prevalence of hypothyroidism, the values given under the Stagnaro et al. reference (0.3--0.5%) are based on studies by Casey et al. and Allan et al. In both these studies, prevalence was estimated based on previously calculated or established reference values (RVs) for TSH. In the Casey study, clinical hypothyroidism was defined as a TSH level above the 97.5th percentile and a free thyroxine value below the 2nd percentile. These criteria were met by 0.2% of the total sample. In the Allan et al. study, pregnant women with TSH levels higher than 10 mU/mL (0.4% of all study patients) were considered to have clinical hypothyroidism. Thus, the prevalence estimated in both studies corresponded to women who were hypothyroid at the time of the control. The document discusses the possibility that prevalence may be higher and, actually, Blatt et al. (the first edition, in an electronic version, was published in 2011) found a 2.4% prevalence of clinical hypothyroidism using a TSH cut-off point of 2.5 IU/mL, based on specific RVs for each trimester of pregnancy. Very few studies are available in Spain on the prevalence of thyroid dysfunction in pregnant women. This is why mention is made of the results of the study conducted in Asturias of more than 2,000 women. While it is true that the study has not been published and the reference is to the abstract of a conference, its relevance lies in the fact that, in addition to recruiting a large population, the prevalence of hypothyroidism was calculated based on their own RFs and was found to be 1.96%. G. Giménez-Pérez states that the most adequate estimate of the prevalence of clinical hypothyroidism in this population is 0.23%, as reported by Lazarus. In accordance to the American Thyroid Association (ATA) recommendations for thyroid dysfunction screening in pregnancy and using this latter prevalence figure, in 2010 there may have been in Spain approximately 290 women with clinical hypothyroidism younger than 30 years who did not meet other criteria for screening. Regarding the screening test. The document discusses in depth the limitations of immunoassays for measuring