Abstract
Urinary iodine concentration (UIC) is highly affected by recent iodine intake and can only be used to determine iodine status in populations and not in individuals. Serum thyroglobulin (Tg) is a suitable marker of the short-term iodine status in a population because it increases when iodine supply to the thyroid is low (1). Thyrotropin (TSH) was never considered a reliable marker of iodine deficiency. Recently, two articles published in Thyroid referred to the role of Tg as a marker of iodine status in pregnancy (2) and the role of human chorionic gonadotropin (hCG) in thyroid function (3). In the first of these, Bath et al. studied prospectively pregnant women with negative thyroid antibodies and concluded that “Tg is a more sensitive biomarker of iodine status in pregnancy than is TSH.” Moreover, although the authors stated in the title: “Thyroglobulin as a Functional Biomarker of Iodine Status in a Cohort Study of Pregnant Women” and concluded that “Tg shows promise as a long-term marker of iodine status in pregnant women,” they did not identify a reliable cutoff for this marker assessing its reliability regarding the UIC (and not TSH). In 2016, we published a cross-sectional study conducted in thyroid antibody–negative pregnant women (4). In that study, we found that “…Tg cannot be considered as a valid marker of iodine deficiency in pregnancy, at least in a mildly iodine-deficient environment…” even for individual subjects. We concluded that this is probably due to the presence of hCG, especially during the first trimester, as the α subunit is identical, the β subunits are highly homologous, and the receptors are also related to that of TSH and there is therefore cross-reactivity between them. The results of Korevaar et al. published in the same issue of Thyroid strongly support our hypothesis. They found that “…human chorionic gonadotropin is associated with the risk of (subclinical) hyperthyroidism…” due to the overstimulation of the secretion of hormones thyroid, mainly in the first trimester of gestation when hCG is particularly increased. In summary, TSH was never used as a marker of iodine deficiency. Moreover, the use of Tg levels as an iodine biomarker during the first three months of pregnancy when adequate dietary iodine intake is of outmost importance is questioned. Such use would require validation of gestational, week-specific thresholds.