The role of iodine in the development of autoimmune thyroiditis

Thyroid cancer, thyroiditis and dietary iodine: a review based on the Salta, Argentina model

Factors that should be considered when studying the effect of dietary iodine in the development of thyroid cancer include pathological criteria, diagnostic techniques, screening programs, radioactive fallout, and standard of medical care in the studied population. In most surveys, papillary carcinoma forms the largest group of thyroid malignancies, both before and after iodine prophylaxis where an increase in the papillary:follicular carcinoma ratio is also noted. Undifferentiated carcinomas decrease after salt prophylaxis. In Salta, Argentina, the increasing incidence of clinically significant papillary thyroid cancer and the decrease of undifferentiated carcinoma after iodine prophylaxis are probably due to better access to health centers and consequent earlier detection of differentiated precursor tumors. Autoimmune focal and diffuse or Hashimoto's thyroiditis are linked to dietary iodine. Pathological studies made in different regions indicate that these types of thyroiditis occur more frequently in areas of iodine sufficiency than in areas of iodine deficiency, and increase after iodine prophylaxis both in non-goitrous and iodine-deficient areas like Salta, Argentina. An increase of lymphocytic thyroiditis could be linked to an increased incidence of primary thyroid lymphoma, and thyroiditis is more commonly associated with papillary carcinoma than with other types of thyroid follicular or C-cell derived carcinomas regardless of iodine intake.

High prevalence of thyroid dysfunction in adult patients with beta-thalassemia major submitted to amiodarone treatment

Amiodarone may induce hyper- or hypothyroidism. Patients with beta-Thalassemia Major (beta-Thal) have an increased prevalence of primary hypothyroidism and often require amiodarone for hemosyderotic cardiomyopathy. Aim of this study was to retrospectively evaluate thyroid function in beta-Thal adult patients on long-term amiodarone. The study group consisted of twenty-two (21 males, 1 female; age: 23-36 yr) beta-Thal patients submitted to long-term (3-48 months) amiodarone therapy from January 1991 to July 1996. Controls included 73 beta-Thal patients (23 males and 50 females aged 25-35 yr) not treated with amiodarone. In all cases serum free thyroid hormones, thyrotropin and thyroid autoantibodies were evaluated. A higher prevalence of overt hypothyroidism (5/22 [22.7%]) as compared to controls (3/73 [4.1%], p=0.02) was found in beta-Thal patients < or = 3 months after starting amiodarone, while the prevalence of subclinical hypothyroidism was similar in amiodarone-treated (18.2%) and untreated (15%) beta-Thal patients. Overt hypothyroidism resolved spontaneously after amiodarone withdrawal in 1 case, while the remaining patients were maintained euthyroid on amiodarone by L-thyroxine administration. After 21-47 months of amiodarone therapy, 3 patients (13.6%) developed thyrotoxicosis (2 overt and 1 subclinical), which remitted shortly after amiodarone withdrawal. No case of hyperthyroidism was observed in beta-Thal controls (p=0.012 vs amiodarone-treated patients). In conclusion, amiodarone administration is often associated in adult beta-Thal patients to a rapid progression of the pre-existing subclinical hypothyroidism, but transient thyrotoxicosis may also be observed after a longer period of therapy. These findings should be carefully considered in the management of these patients.

Thyroid iodide transporter: local sequence homologies with thyroid autoantigens

Here we show the existence of local amino acid (aa) sequence homologies between rat thyroid iodide transporter (Na+/l- symporter or NIS), whose gene was recently cloned, and known human thyroid autoantigens [thyroglobulin (Tg), thyroid peroxidase (TPO) and thyrotropin receptor (TSHR)] NIS sequences corresponding to the fourth (aa 264-282) and fifth extracellular loop (aa 386-414) are 15 to 40% identical and 30 to 60% similar to sequences corresponding to known or putative epitopes of Tg, TPO and TSHR. The sixth extracellular loop (aa 465-485) beared homology (44% identity, 52% similarity) only to a region of Tg which flanks one of its immunodominant domains. Sequences of thyroid autoantigens other than NIS shared homology, especially Tg and TPO. We conclude that in all likelihood NIS is an additional thyroid antigen, which shares common epitopes with the other thyroid autoantigens. Addendum: A study in abstract form appeared after submission of our paper finds experimental evidence for the antigenicity of two extracellular segments (aa 262-280 and 468-487) and of a portion of the intracellular C-terminus (aa 560-579).