Chronic iodine excess does not increase the incidence of hyperthyroidism: a prospective community-based epidemiological survey in China.
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.


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 microg/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).


CONCLUSIONS
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.
Yang
Fan
F
Department of Endocrinology and Metabolism, First Affiliated Hospital, China Medical University, No.155 Nanjing Bei St., Heping District, Shenyang, China.
Shan
Zhongyan
Z
Teng
Xiaochun
X
Li
Yushu
Y
Guan
Haixia
H
Chong
Wei
W
Teng
Di
D
Yu
Xiaohui
X
Fan
Chenling
C
Dai
Hong
H
Yu
Yang
Y
Yang
Rong
R
Li
Jia
J
Chen
Yanyan
Y
Zhao
Dong
D
Mao
Jinyuan
J
Teng
Weiping
W
D000293:Adolescent; D001323:Autoantibodies / Q000097:blood; D001327:Autoimmune Diseases / Q000503:physiopathology; D002681:China / Q000453:epidemiology; D004032:Diet; D005260:Female; D005500:Follow-Up Studies; D006042:Goiter / Q000150:complications; D006801:Humans; D006980:Hyperthyroidism / Q000453:epidemiology* / Q000209:etiology / Q000503:physiopathology; D015994:Incidence; D007453:Iodide Peroxidase / Q000276:immunology; D007455:Iodine / Q000008:administration & dosage / Q000652:urine*; D008297:Male; D009752:Nutritional Status; D011446:Prospective Studies; D012307:Risk Factors; D013961:Thyroid Gland / Q000503:physiopathology; D013997:Time Factors
D001323:Autoantibodies; D007455:Iodine; D007453:Iodide Peroxidase