Thyroid autoimmunity and iodine nutritional status

TOPIC: Thyroid autoimmunity in schoolchildren

Title: Thyroid autoimmunity in schoolchildren in a long-standing iodine sufficient area: correlation with gender, pubertal stage and maternal autoimmunity.

Authors: Kaloumenou I, Mastorakos G, Alevizaki M, Duntas LH, Mantzou E, Ladopoulos C, Antoniou A, Chiotis D, Papassotiriou I, Chrousos GP, & Dacou-Voutetakis C.

Reference: Thyroid 18: 747-754, 2008

Summary

Background

Thyroid autoimmunity (TAI) is clearly linked to genetic background & gender. In order to establish a possible relationship between the onset of TAI & gender, TAI & maternal thyroid autoimmunity, the authors examined the prevalence of TAI in children & adolescents

Patients & Methods

Thyroid function, thyroid auto-antibodies (TPO-Ab, TG-Ab) and thyroid ultrasonographic studies were performed in 440 healthy schoolchildren (200 boys & 240 girls, aged 5-18 years), as well as in randomly assigned 143 mothers, in an iodine-replete region.

Results

Prevalence of positive TPO-Ab & TG-Ab was 4.6% and 4.3% respectively, being higher in girls at Tanner stage II-V compared with pre-pubertal girls (8.2% vs . 2.2%). Also, positive TPO-Ab & TG-Ab titers (in girls) were associated with significantly larger thyroid volumes. Hypoechogenicity was detected in 52.6% & 36.8% of the children positive for TPO-Ab & TG-Ab, respectively. Autoimmune thyroiditis (defined as the combination of positive antibodies and hypoechoic gland) was present in 2.5% of the children. Mothers of children positive for TPO-Ab presented TPO-Ab more frequently.

Conclusions

The findings demonstrate that positive TPO-Ab in children is associated significantly with maternal thyroid autoimmunity. The development of autoimmune thyroiditis may possibly emerge only after puberty.

Commentary

Kaloumenou et al. report a relatively high incidence of confirmed autoimmune thyroiditis (AIT) in schoolchildren in an area with long-standing iodine sufficiency. In this article, the median urinary iodine excretion was 309 µg/g creatinine (range: 44 – 1.018 µg/g). These values thus represent relative iodine excess in this population. Another study in Greece by Zois et al. (Thyroid, 2006) also compared the prevalence of AIT in schoolchildren before & after introduction of iodized salt. After the fortification programme was implemented, there was a significant increase in the prevalence of subjects with thyroid antibodies, subclinical hypothyroidism and glandular hypoechogenicity. In another study conducted in Sri Lanka, 367 school girls were examined before & 5 years after a salt iodination programme. A high incidence of TG-Ab was reported after fortification. Three years later, these children frequently tested positive for TG-Ab & TPO-Ab (Premawardana in Eur J Endocrinol, 2000). In Brazil after 5 years of excess iodine, Duarte et al. (article in press, 2009) examined 843 children with a median urinary iodine excretion of 306 µg/L. Grade 4 thyroid hypoechogenicity was found in ~8% of children, suggesting AIT. In adults, 3.018 subjects living in China participated into a 5 year follow-up study. The cumulative incidence of AIT was highest in the region with the highest iodine intake (Teng in NEJM, 2006). Finally, Camargo (Eur J Endocrinol 2008) documented a marked increase of AIT (up to 16.5%) in a population study of individuals with excess nutritional iodine for 5 years in Brazil.

Taken together, these studies suggest that more than adequate or excessive iodine intake may trigger the process of thyroid autoimmunity in genetically susceptible subjects. Indeed, Kaloumenou et al. (2008) elegantly demonstrated that mothers with positive TPO-Ab & TG-Ab have children with abnormally high positive thyroid antibodies. In summary, the present population study in schoolchildren provides an epidemiologic basis for both genetic background and excess nutritional iodine in the process of initiating thyroid autoimmunity.

Summary and commentary prepared by Geraldo Medeiros-Neto (Related to Chapter 7 & 20 of TDM)