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Yang, Li
Chronic iodine excess does not increase the incidence of hyperthyroidism: a prospective community-based epidemiological survey in China.Yang F, Shan Z, Teng X, Li Y, Guan H, Chong W, Teng D, Yu X, Fan C, Dai H, Yu Y, Yang R, Li J, Chen Y, Zhao D, Mao J, Teng W. Eur J Endocrinol. 2007 Apr;156(4):403-8. [abstract only]
"OBJECTIVE: An increasing incidence of hyperthyroidism has been observed when iodine supplementation has been introduced to an iodine-deficient population. Moreover, the influence of chronic more than adequate or excessive iodine intake on the epidemiological features of hyperthyroidism has not been widely and thoroughly described. To investigate the influences of different iodine intake levels on the incidence of hyperthyroidism, we conducted a prospective community-based survey in three communities with mild-deficient, more than adequate (previously mild deficient iodine intake), and excessive iodine intake.
SUBJECTS AND METHODS: In three rural Chinese communities, a total of 3761 unselected inhabitants aged above 13 years participated in the original investigation and 3018 of them received identical examinations after 5 years. Thyroid function, levels of thyroid peroxidase antibody (TPOAb), thyroglobulin antibody and urinary iodine excretion were measured and thyroid ultrasound examination was also performed.
RESULTS: In three communities, median urinary iodine excretion was 88, 214, and 634 mug/l (P<0.05) respectively. The cumulative incidence of hyperthyroidism was 1.4, 0.9, and 0.8% (P>0.05) respectively. Autoimmune hyperthyroidism was predominant in thyroid hyperfunction in all the three cohorts. Either positive TPOAb (>50 U/ml) or goiter in original healthy participants was associated with the occurrence of unsuspected hyperthyroidism in 5 years (logistic regression, OR=4.2 (95% CI 1.7-8.8) for positive TPOAb, OR=3.1 (95% CI 1.4-6.8) for goiter).
CONCLUSION: Iodine supplementation may not induce an increase in hyperthyroidism in a previously mildly iodine-deficient population. Chronic iodine excess does not apparently increase the risk of autoimmune hyperthyroidism, suggesting that excessive iodine intake may not be an environmental factor involved in the occurrence of autoimmune hyperthyroidism."
Effect of different iodine intake on the prevalence of hypothyroidism in 3 counties in China.Shan ZY, Li YS, Wang ZY, Jin Y, Guan HX, Hu FN, Teng XC, Yang F, Gao TS, Wang WB, Shi XG, Tong YJ, Chen W, Teng WP. Chin Med J (Engl). 2005 Nov 20;118(22):1918-20.
" In this study, we found that the prevalence of hypothyroidism varied from 0% to 1.8% in the entire population, 0.3%-1.8% in women and 0%-0.7% in men. The prevalence of hypothyroidism increased with increasing iodine supply.［2-4］ The prevalence of hypothyroidism in the elderly increased from 0.8% to 1.5% and 7.6% with the urinary iodine level increasing from 72 μg/g creatinine to 100 μg/g creatinine and 513 μg/g creatinine, respectively.［2］
"Elevated level of TSH is regarded as an early index to assess impaired thyroid function. Subclinical hypothyroidism is a definite risk factor for overt hypothyroidism and coronary heart diseases. The reported prevalence of subclinical hypothyroidism is 2.5%-10.4% in general population, 4%-17% in women, and 2%-7% in men.［5］ In this study, the effect of iodine on the prevalence of subclinical hypothyroidism was similar to that of hypothyroidism, and the prevalence of subclinical hypothyroidism in the elderly was significantly higher in the area with 150 μg/L of UI than that with 38 μg/L UI.［6］"
Epidemiological survey on the relationship between different iodine intakes and the prevalence of hyperthyroidism.Yang F, Teng W, Shan Z, Guan H, Li Y, Jin Y, Hu F, Shi X, Tong Y, Chen W, Yuan B, Wang Z, Cui B, Yang S. Eur J Endocrinol. 2002 May;146(5):613-8.
"OBJECTIVE: To investigate the effect of different levels of iodine intake on the prevalence of hyperthyroidism and the impact of universal salt iodization on the incidence of hyperthyroidism.
DESIGN: A comparative cross-sectional and longitudinal survey was conducted in three areas with borderline iodine deficiency, mild iodine excess (previously mild iodine deficiency) and severe iodine excess. Universal salt iodization had been introduced 3 years previously except in the area with borderline iodine deficiency.
METHODS: In total 16 287 inhabitants from three areas answered a questionnaire concerning the history of thyroid disease. Among them 3761 unselected subjects received further investigations including thyroid function, thyroid autoantibodies, thyroid ultrasonography and urinary iodine excretion.
RESULTS: Among areas with median urinary iodine excretion of 103 microg/l, 375 microg/l and 615 microg/l (P<0.05), the prevalence of hyperthyroidism did not differ significantly (1.6%, 2% and 1.2%). The prevalence of subclinical hyperthyroidism was higher in areas with borderline iodine deficiency and mild iodine excess than in the area with severe excess iodine intake (3.7%, 3.9% and 1.1%, P<0.001). The prevalence of Graves' disease and its proportion in hyperthyroidism did not differ among areas. The incidence of hyperthyroidism did not significantly increase after the introduction of universal salt iodization.
CONCLUSION: Different iodine intakes under a certain range do not affect the prevalence and type of hyperthyroidism. Subclinical hyperthyroidism is more prevalent in the iodine deficient area than in the severe iodine excessive area. In the area with mild iodine deficiency, the introduction of universal salt iodization may not be accompanied by an increased incidence of hyperthyroidism."
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