Thyroid autoimmunity in endemic goitre caused by excessive iodine intake

Effect of excess iodine intake on thyroid diseases in different populations: A systematic review and meta-analyses including observational studies

Background

Although several reports concerning the association of iodine excess and thyroid disease have appeared, no systematic review of the association between iodine excess intake and thyroid diseases, especially hyperthyroidism and hypothyroidism, has yet been reported.

Method

We conducted a systematic search of Ovid MEDLINE, PubMed, Cochrane Central Register of Controlled Trials databases, Ichushi-Web and CiNii database for intervention trials and observational studies. Search terms were constructed from related words for excess AND iodine intake or excretion AND thyroid hormones or diseases AND study designs. After considering the qualitative heterogeneity among studies, a meta-analysis was conducted and odds ratios and 95% confidence intervals (CI) were estimated in random-effects models. A protocol was registered with PROSPERO (No. CRD42015028081).

Results

50 articles were included, including three intervention trials, six case-control studies, six follow-up studies and 35 cross-sectional studies. Three cross-sectional studies in adults included in meta-analysis. Odds ratio of overt and subclinical hypothyroidism between excess and adequate populations were 2.78 (CI:1.47 to 5.27) and 2.03 (CI:1.58 to 2.62) in adults, respectively. Source of excess iodine status was mainly iodized salt or water in included studies.

Conclusion

Although universal salt iodization has improved goiter rates, chronic exposure to excess iodine from water or poorly monitored salt are risk factors for hypothyroidism in free-living populations. Monitoring of both iodine concentration in salt as well as the iodine concentration in local drinking water are essential to preventing thyroid diseases. Hypothyroidism should be also carefully monitored in areas with excess iodine. Because of the low quality and limited number of included studies, further evidence and review are required.

Effect of iodine restriction on thyroid function in subclinical hypothyroid patients in an iodine-replete area: a long period observation in a large-scale cohort

BACKGROUND

Excessive iodine intake is related to a higher prevalence of hypothyroidism, including subclinical hypothyroidism (SCH), in iodine-replete areas. This study aimed to evaluate the effect of iodine restriction on thyroid function in SCH patients in an iodine-replete area and analyze the relationship between serum thyrotropin (TSH) levels and iodine intake.

METHODS

The study consisted of 146 consecutive patients who were diagnosed with SCH at the Samsung Medical Center between 2010 and 2012. Urinary iodine concentration (UIC) was measured in 82 patients. Of these, 20 patients with UIC < 300 μg/L were excluded, and 62 patients with UIC ≥ 300 μg/L were educated about the restriction of iodine-rich foods. Following the first follow-up visit, these patients were divided into two groups based on the UIC level: group A (well-controlled iodine intake, UIC < 300 μg/L, n = 40), and group B (poorly controlled iodine intake, UIC ≥ 300 μg/L, n = 22). The remaining 64 patients did not restrict iodine rich foods (group C). The 82 patients with measured UICs were reevaluated every 3-6 months, and the median follow-up was 13 months (range 3-32 months). Thyroid function and UIC were measured at each visit. The correlation between serum TSH level and UIC was determined for the 82 patients in whom UIC was measured.

RESULTS

Following 3-6 months of iodine restriction, the serum TSH levels significantly decreased in group A (from 9.0 mU/L to 4.7 mU/L; p < 0.01). In addition, the serum free thyroxine (T4) levels in group A significantly increased (from 1.11 ± 0.23 ng/dL to 1.18 ± 0.17 ng/dL; p < 0.05). However, there were no significant changes in serum TSH or free T4 levels in groups B and C. Serum TSH levels significantly correlated with UIC (r = 0.33, p < 0.01).

CONCLUSION

Iodine restriction may normalize or, at the very least, decrease serum TSH levels in SCH patients, and serum TSH levels are strongly correlated with UIC. Therefore, restriction of iodine intake could be a primary treatment option in SCH patients in an iodine-replete area.

[Ultrasonographic evaluation of the thyroid, determination of ioduria and iodine concentration in kitchen salt used by schoolchildren in Ribeirão Preto, São Paulo, Brazil]

OBJECTIVE AND METHODS

Previous studies have pointed out that Ribeirão Preto, São Paulo, is an area of borderline iodine deficiency, later becoming a region of excessive iodination. In a reevaluation of the iodine status of the city in 2007-2008, 300 schoolchildren aged 8 to 10 years were evaluated, from 3 schools of 3 different socioeconomic levels who were submitted to the determination of iodine concentration in urine samples and in kitchen salt and thyroid ultrasound.

RESULTS AND CONCLUSION

Among boys and girls ioduria was higher in the school of highest socioeconomic level compared to the others, with a significant difference in the ages 8 and 10 years for boys with ranges : p < 0,0001 and p = 0,0106, respectively; and in the ages 8 and 9 years for girls with ranges: p = 0,0024 and p = 0,0154, respectively. Median iodine concentrations in samples of kitchen salt ranged from 26.6 to 27.8 mg iodine/kg and did not differ between the schools studied. Ultrasound evaluation revealed that 15,6% of thyroids examined were hypoechoic, suggesting that these glands may present changes in the architecture not reflected yet in their volume.

Role of iodine in thyroid physiology

Adequate levels of iodine, a trace element variably distributed on the earth, are required for the synthesis of the thyroid hormones thyroxine (T₄) and triiodothyronine (T₃). The iodide cycle consists of a series of transport, oxidation and coupling steps in thyroid follicular cells to produce thyroid hormone. The sodium/iodide symporter (NIS) transports iodide into the thyrocyte. Competitive inhibitors of NIS, such as perchlorate and thiocyanate, can decrease the entrance of iodide into the follicular cell. Pendrin is the primary protein that is responsible for iodide efflux out of the thyrocyte and into the follicular lumen. T₄ is deiodinated in target tissues to produce the active form of thyroid hormone, T₃, and other metabolites. Exposure to excessive iodine or chronic iodine deficiency may result in various clinical disorders. The Wolff-Chaikoff effect and Jöd-Basedow phenomenon describe mechanisms of thyroid autoregulation and dysregulation, respectively, during iodine excess. Population studies have determined that iodine deficiency exists in approximately 38% of the world's population, is the leading cause of preventable mental retardation, and is of particular concern to women and their infants. Finally, the unique role of iodine utilization in thyroid physiology has applications in many important clinical areas.

On the importance of selenium and iodine metabolism for thyroid hormone biosynthesis and human health

The trace elements iodine and selenium (Se) are essential for thyroid gland functioning and thyroid hormone biosynthesis and metabolism. While iodine is needed as the eponymous constituent of the two major thyroid hormones triiodo-L-thyronine (T3), and tetraiodo-L-thyronine (T4), Se is essential for the biosynthesis and function of a small number of selenocysteine (Sec)-containing selenoproteins implicated in thyroid hormone metabolism and gland function. The Se-dependent iodothyronine deiodinases control thyroid hormone turnover, while both intracellular and secreted Se-dependent glutathione peroxidases are implicated in gland protection. Recently, a number of clinical supplementation trials have indicated positive effects of increasing the Se status of the participants in a variety of pathologies. These findings enforce the notion that many people might profit from improving their Se status, both as a means to reduce the individual health risk as well as to balance a Se deficiency which often develops during the course of illness. Even though the underlying mechanisms are still largely uncharacterised, the effects of Se appear to be exerted via multiple different mechanisms that impact most pronounced on the endocrine and the immune systems.

Changes in thyroid morphology and function in children in western Poland as a result of intensified iodine prophylaxis

AIM

To determine the changes in thyroid pathology resulting from obligatory salt iodization in a group of children aged 8-12 years from the rural and urban areas of Wielkopolska Region, Poland.

POPULATION AND METHODS

The survey was conducted on 1215 children, of both sexes, 402 of whom were examined in 1992 (before salt iodization), 408 in 2000, and 405 in 2005 (after salt iodization beginning in 1996). Thyroid ultrasound, urinary iodine, FT4, FT3, TSH and antithyroid antibodies were measured.

RESULTS

A significant drop in goiter cases was observed (35.4% in 1992 vs 6.3% in 2005), coupled with a marked increase of urinary iodine. There were also changes in ultrasonography and elevated levels of antibodies.

CONCLUSIONS

The study proves the high efficacy of iodine prophylaxis. Despite a growing number of children with elevated antithyroid antibody titers, only a slight increase of autoimmune thyroid disorders was observed.

Variable influences of iodine on the T-cell recognition of a single thyroglobulin epitope

We have previously shown that iodotyrosyl formation within certain innocuous thyroglobulin (Tg) peptides confers on them immunopathogenic properties. In this report, we generated a panel of T-cell hybridoma clones specific for the immunogenic 16 mer Tg peptide p179 (amino acids 179-94) or its iodinated analogue (I-p179), with a view to examining the effects of a single iodine atom at the Y192 amino acid residue on T-cell recognition. We found that the peptide p179 was subdominant, and its binding to both A(k) and E(k) molecules was not significantly influenced by iodine. T-cell receptor (TCR) engagement was unaffected by the bulky iodine atom in two clones that responded to both analogues but it was sterically hindered in two other clones that recognized only p179. One clone was reactive only to I-p179, suggesting that the iodine atom is an integral part of its TCR ligand. Truncation analysis localized the determinant seen by all clones within the 11 mer peptide p184 (amino acids 184-194), suggesting that the cross-reactive clones were not activated by a minimal epitope lacking Y192 and that the negative influence of iodine was not the result of a flanking residue effect. These results demonstrate, at the clonal level, variable influences of a single iodine atom on the recognition of a single Tg peptide. Iodination of tyrosyl-containing, immunopathogenic Tg peptides may have unpredictable effects at the polyclonal level, depending on the extent of iodination at the particular site, and the relative number or effector function of autoreactive T-cell clones that are switched on or off by the neoantigenic determinant.

High prevalence of thyroid dysfunction and autoimmune thyroiditis in adolescents after elimination of iodine deficiency in the Eastern Black Sea Region of Turkey

In the present study we evaluated the effects of iodine intake on the prevalence of thyroid dysfunction, autoimmunity, and goiter in two regions with different iodine status after two years of iodization in Turkey. In total 1733 adolescent subjects were enrolled into the study (993 from an iodine-sufficient area--the Eastern Black Sea Region (group 1) and 740 from an iodine-deficient area--Middle Anatolia (group 2)). We measured free thyroxine (FT(4)), thyrotropin (TSH), antithyroid peroxidase antibodies (Anti-TPO), antithyroglobulin antibodies (Anti-Tg), and urinary iodine (UI), and examined the thyroid gland by ultrasound. Median urinary iodine excretion was found to be significantly different in group 1 and group 2 (139 micro/l vs 61micro/l, p < 0.001). Hyperthyroidism was more frequent in group 1 (3.6% vs 0.7%; p < 0.001), but the hypothyroidism rate was similar between groups (1.8% vs 1.4 %; p>0.05). The percentage of anti-Tg positive subjects was found to be 17.6% in group 1 and 6.4% in group 2; that of anti-TPO positive subjects was 4.3% in group 1 and 1.5% in group 2. The prevalence of antithyroid antibody (anti-Tg and/or anti-TPO) positivity was significantly higher in group 1 than in group 2 (18.52% vs 6.62%; p < 0.001). Thyroid volumes of the hyperthyroid subjects in both groups were significantly higher than hypo- and euthyroid subjects. In conclusion, iodine supplementation in Turkey has resulted in the elimination of iodine deficiency in the Eastern Black Sea Region, and this has been accompanied by an increase in the prevalence of autoimmune thyroiditis and thyroid dysfunction.

Urinary iodine concentration of pregnant women and female adolescents as an indicator of excessive iodine intake in Sri Lanka

BACKGROUND

Mild deficiencies and excesses of iodine have deleterious effects in both females and males. The iodine status of the population after implementation of the universal salt iodization program in Sri Lanka is not known.

OBJECTIVE

This cross-sectional study was carried out to assess the iodine status of pregnant women and female adolescents, with urinary iodine concentration used as the measure of outcome.

METHODS

The participants were 100 women in the first trimester of pregnancy and 99 female adolescents in Kuliyapitiya, Kurunegala District, North-Western Province, Sri Lanka. The urinary iodine concentration was measured in a casual urine sample from each subject. The iodate contents of salt samples collected from households of the adolescents participating in the study were also measured.

RESULTS

The median urinary iodine concentration of 185.0 microg/L and the prevalence of values under 50 microg/L of only 1% among the pregnant women indicate adequate iodine intake and optimal iodine nutrition. The median urinary iodine concentration (213.1 microg/L) among female adolescents indicates a more than adequate iodine intake and a risk of iodine-induced hyperthyroidism. Approximately 8% and 4% of the adolescents and pregnant women, respectively, had urinary iodine concentrations in the range of mild iodine deficiency (51 to 100 microg/L). More than half of the adolescents (56%) and 39% of the pregnant women had urinary iodine concentrations higher than optimal. The median iodine content in salt samples was 12.7 ppm. Only 20.2% of the samples were adequately iodized, and 10.1% of the samples had very high iodine levels.

CONCLUSIONS

Female adolescents and pregnant women had no iodine deficiency, but a considerable proportion of them, especially female adolescents, were at risk for iodine-induced hyperthyroidism. There is thus a need for proper monitoring of the salt iodization program to achieve acceptable iodine status.

Health effects classification and its role in the derivation of minimal risk levels: Reproductive and endocrine effects

The Agency for Toxic Substances and Disease Registry (ATSDR) derives health-based guidance values called minimal risk levels (MRLs) to assist with assessment of risks posed by exposures to hazardous chemicals. From the total of 326 MRLs currently posted on ATSDR's web site (www.atsdr.cdc.gov), 14 and 5 MRLs are based on reproductive and endocrine endpoints, respectively. The paper also describes the ranking of effects into less serious and serious categories according to ATSDR's Guidance for Developing Toxicological Profiles, endpoints used for the MRLs derivation, and the use of uncertainty factors.

High thyroid volume in children with excess dietary iodine intakes

BACKGROUND

There are few data on the adverse effects of chronic exposure to high iodine intakes, particularly in children.

OBJECTIVE

The objective of the study was to ascertain whether high dietary intakes of iodine in children result in high thyroid volume (Tvol), a high risk of goiter, or both.

DESIGN

In an international sample of 6-12-y-old children (n = 3319) from 5 continents with iodine intakes ranging from adequate to excessive, Tvol was measured by ultrasound, and the urinary iodine (UI) concentration was measured. Regressions were done on Tvol and goiter including age, body surface area, sex, and UI concentration as covariates.

RESULTS

The median UI concentration ranged from 115 microg/L in central Switzerland to 728 microg/L in coastal Hokkaido, Japan. In the entire sample, 31% of children had UI concentrations >300 microg/L, and 11% had UI concentrations >500 microg/L; in coastal Hokkaido, 59% had UI concentrations >500 microg/L, and 39% had UI concentrations >1000 microg/L. In coastal Hokkaido, the mean age- and body surface area-adjusted Tvol was approximately 2-fold the mean Tvol from the other sites combined (P < 0.0001), and there was a positive correlation between log(UI concentration) and log(Tvol) (r = 0.24, P < 0.0001). In the combined sample, after adjustment for age, sex, and body surface area, log(Tvol) began to rise at a log(UI concentration) >2.7, which, when transformed back to the linear scale, corresponded to a UI concentration of approximately 500 microg/L.

CONCLUSIONS

Chronic iodine intakes approximately twice those recommended-indicated by UI concentrations in the range of 300-500 microg/L-do not increase Tvol in children. However, UI concentrations >/=500 microg/L are associated with increasing Tvol, which reflects the adverse effects of chronic iodine excess.

Avaliação ultra-sonográfica da tireóide e determinação da iodúria em escolares de diferentes regiões do Estado de São Paulo

A iodação do sal é eficiente no combate às doenças decorrentes da deficiência de iodo, sendo empregada em todo território nacional desde 1995. O Estado de São Paulo é considerado uma área ioda-suficiente. Para avaliar a ingesta de iodo, foram selecionados aleatoriamente 844 escolares entre 6 e 14 anos, de seis regiões do Estado. Foram avaliados, sob o ponto de vista ecográfico da tireóide, 423 meninos e 421 meninas. Os volumes da glândula tireóide elevaram-se progressivamente com a idade, guardando correlação positiva e significativa com a superfície corporal. Cerca de 1,6% dos escolares apresentava bócio. Nódulos, cistos, hemiagenesia foram detectados em 1,4% dos examinados. Notamos excessiva excreção urinária de iodo nesta população, cerca de 53% eliminou acima de 300<FONT FACE=Symbol>µ</FONT>g Iodo/L e valores acima de 600<FONT FACE=Symbol>µ</FONT>gI/L foram encontrados em 21% dos escolares. As amostras de sal doméstico apresentavam valores entre 28,1 e 63,3mg Iodo/kg de sal. Concluímos que a população escolar do Estado de São Paulo apresenta excessiva ingestão diária de iodo, a qual, extrapolada para a população em geral, pode induzir várias alterações da função tireóidea, como hipertiroidismo subclínico (em idosos) e tireoidite crônica autoimune na população adulta, em geral.

Enhanced iodination of thyroglobulin facilitates processing and presentation of a cryptic pathogenic peptide

Increased iodine intake has been associated with the development of experimental autoimmune thyroiditis (EAT), but the biological basis for this association remains poorly understood. One hypothesis has been that enhanced incorporation of iodine in thyroglobulin (Tg) promotes the generation of pathogenic T cell determinants. In this study we sought to test this by using the pathogenic nondominant A(s)-binding Tg peptides p2495 and p2694 as model Ags. SJL mice challenged with highly iodinated Tg (I-Tg) developed EAT of higher severity than Tg-primed controls, and lymph node cells (LNC) from I-Tg-primed hosts showed a higher proliferation in response to I-Tg in vitro than Tg-primed LNC reacting to Tg. Interestingly, I-Tg-primed LNC proliferated strongly in vitro against p2495, but not p2694, indicating efficient and selective priming with p2495 following processing of I-Tg in vivo. Tg-primed LNC did not respond to either peptide. Similarly, the p2495-specific, IL-2-secreting T cell hybridoma clone 5E8 was activated when I-Tg-pulsed, but not Tg-pulsed, splenocytes were used as APC, whereas the p2694-specific T cell hybridoma clone 6E10 remained unresponsive to splenic APC pulsed with Tg or I-Tg. The selective in vitro generation of p2495 was observed in macrophages or dendritic cells, but not in B cells, suggesting differential processing of I-Tg among various APC. These data demonstrate that enhanced iodination of Tg facilitates the selective processing and presentation of a cryptic pathogenic peptide in vivo or in vitro and suggest a mechanism that can at least in part account for the association of high iodine intake and the development of EAT.

Prevalence of hypothyroidism and diabetes mellitus in elderly kibbutz members

The aim of this study was to determine the prevalence of hypothyroidism and diabetes mellitus (DM) in elderly (aged 65-92 years) kibbutz members in Northern Israel.

METHOD

The medical records of 1096 elderly (642 females and 454 males) residing in 11 kibbutzim were reviewed for data regarding thyroid function tests (TSH and FT4) and fasting blood glucose. Fasting blood glucose levels above 7.8 mmol/l was considered diagnostic for diabetes mellitus.

RESULTS

The prevalence of hypothyroidism was 14% (9.7% in males and 18.2% in females) and that of DM was 11.5% (12.1% in males and 11.1% in females). In 74% of the diabetics the diagnosis was made after the age of 60 years. Distribution of treatment modalities in diabetics was as follows: diet only 42%. oral hypoglycemic agents 52% and Insulin 6%. Subclinical hypothyroidism (serum TSH levels above 4.5 mU/L with normal FT4 levels) was detected in 38% of all the hypothyroid subjects.

CONCLUSION

The data suggest that diabetes mellitus and primary hypothyroidism are common disorders in elderly subjects. DM in the elderly can usually be handled with diet and oral hypoglycemic drugs. Since the clinical features of hypothyroidism in the elderly are often atypical, we suggest that elderly subjects should be screened for hypothyroidism.

Goitre in southern Chinese children in Hong Kong

In a 9-year retrospective review of goitre in Chinese children in Hong Kong, 84 children and adolescents were identified. There was a female predominance with a male:female ratio of 1:5.5. The majority of patients (95%) were found to have diffuse enlargement of the thyroid gland. Forty-nine were found to have Graves' disease. The mean age of presentation of patients with Graves' disease was 11.34 years. Forty-three of them were found to have thyroid autoantibodies. Only two cases of autoimmune thyroiditis with thyroid autoantibodies were diagnosed. The other causes of goitre include simple colloid goitre, multinodular goitre and suppurative thyroiditis. The spectrum of disease causing goitre in Chinese children is thus quite different from that reported in Caucasian children in developed countries. Further studies are necessary to define the genetic or environmental factors resulting in the observed differences.

The effect of treatment with levothyroxine or iodine on thyroid size and thyroid growth stimulating immunoglobulins in endemic goitre patients

OBJECTIVE

We assessed the effect of levothyroxine or iodine on thyroid size and on thyroid growth stimulating immunoglobulins in endemic goitre patients.

DESIGN

Levothyroxine or iodine was given orally in an open randomized prospective study (100 and 200 micrograms respectively).

PATIENTS

Thirty-seven euthyroid patients with diffuse iodine deficiency goitres and thyroid growth stimulating immunoglobulins were studied.

MEASUREMENTS

Thyroid size, thyroid growth stimulating immunoglobulins (mitosis arrest assay), basal TSH, free T3, free T4, thyroid anti-microsomal antibodies, antithyroglobulin antibodies, anti-TSH receptor antibodies and urinary iodine excretion were measured.

RESULTS

Thyroid size decreased significantly in both groups, in the levothyroxine group more than in the iodine treated group. Thyroid growth stimulating immunoglobulins levels also decreased significantly in both groups. Between groups there was no statistically significant difference. A statistically significant correlation between thyroid growth stimulating immunoglobulins reduction profiles and goitre size reduction could not be established. TSH levels became suppressed in the levothyroxine group while the T4 values rose; in the iodine treated group TSH levels stayed constant as did T4. None of the patients developed thyroid microsomal or thyroglobulin auto-antibodies and/or hyperthyroidism during the treatment.

CONCLUSIONS

Levothyroxine as well as iodine was effective in reducing thyroid size as well as thyroid growth stimulating immunoglobulins levels in endemic goitre patients. Since in both groups TSH levels were not related to thyroid size reduction, other factors than TSH suppression must be responsible for the observed thyroid size reduction. Iodine itself by virtue of its antiproliferative action on thyrocytes may have had a direct action on the goitre reduction during iodine treatment; however, the levothyroxine dose, containing less iodine, had a similar effect. A complicated picture hence emerges with regard to factors involved in the shrinkage of iodine deficiency goitre during thyroxine or iodine therapy. These findings indicate that TSH and thyroid growth promoting immunoglobulins are not the only influences on the size of endemic goitres, although it cannot be excluded that these two factors contribute to influence the pathogenetic process.

Simplified measurement of protein-bound iodine with epithermal neutron activation analysis

A refined method for the measurement of protein-bound iodine (PBI) in blood serum was demonstrated by the use of epithermal neutron activation analysis (ENAA). PBI in ammonium sulfate-precipitated serum protein, after epithermal neutron activation, was determined by high resolution gamma-ray spectrometry. From our results, the PBI concentration was 67.8 +/- 2.2 ng/mL. Good agreement was obtained with published data, ranging from 40 to 80 ng/mL, which had been obtained using different analytical techniques. The validity of these techniques for PBI has been born out by a very good accuracy and simplicity without temperature dependence.

Screening for thyroid diseases in an iodine sufficient area with sensitive thyrotrophin assays, and serum thyroid autoantibody and urinary iodide determinations

OBJECTIVE

The present study was designed to investigate the prevalence of thyroid dysfunction and its relation to thyroid autoantibodies and urine iodide concentration in apparently healthy people residing in Sapporo, a city of northern Japan, where the iodine intake is high.

DESIGN AND SUBJECTS

Serum TSH and thyroid autoantibodies, and urine iodide were measured in 4110 people (2931 men and 1179 women) (age 45.6 +/- 10.3 years (mean +/- SD)) who were recruited at the hospital for medical examinations.

RESULTS

The thyroid autoantibodies were positive in 6.4% of males and 13.8% of females with an age-related increase. Of the people with positive antibodies, 87.2% had normal TSH values (0.15-5.0 mU/l) as measured by a sensitive assay. The prevalence of unsuspected hyperthyroidism as defined by suppressed TSH values was 0.61%, of which 64% was diagnosed as Graves' disease based on positive thyrotrophin receptor antibody results. The prevalence of unsuspected hypothyroidism, as evidenced by supranormal TSH, was 0.68% for males and 3.13% for females with an age-related increase. Of those with hypothyroidism, 45.5% were autoantibody positive. The overall prevalence of Hashimoto's thyroiditis was 13.11% for females and 6.15% for males. The urine iodide levels of hypothyroidism with a positive autoantibody of 38.5 (17.7-83.9) mumol/l and a negative autoantibody of 34.9 (17.9-67.9) mumol/l were both significantly higher than that of normal subjects (26.9 (14.6-49.6) mumol/l) (P < 0.01). When iodine intake was restricted for 6-8 weeks for hypothyroid subjects, the elevated TSH and thyroglobulin and low free T4 levels were reversed in the autoantibody negative but not in the positive group.

CONCLUSIONS

This study provides further information on the prevalence of thyroid dysfunction and autoimmune thyroid diseases in an iodine sufficient area. In addition, it suggests that more than half of the patients with unsuspected hypothyroidism were negative for autoantibodies and that the excessive iodine intake may be involved in causing latent hypothyroidism.

The problem of the nodular goiter

Nodular goiter is a worldwide problem involving millions of persons. Endemic goiter, and associated cretinism, is totally preventable by ensuring an adequate dietary iodine intake and eliminating malnutrition and dietary goitrogens. Therapy, on the other hand, is difficult in that the goiters often do not regress and the cretinoid changes are irreversible. Nonendemic goiter due to autoimmune thyroid disease, genetic defects in thyroid hormone biosynthesis, and environmental goitrogens or neoplasia is not usually preventable. The usual therapy, involving TSH suppression by administration of L-thyroxine orally, will frequently bring about regression of early, diffuse goiters but is often ineffective in bringing about regression of large, multinodular goiters. In these patients, surgical removal of the goiter may be necessary for alleviation of obstructive symptoms. Further research is needed to elucidate the factors involved in the development of these multinodular goiters and to control the autocrine and paracrine factors involved in nodule growth.