Effect of iodine intake on thyroid diseases in China.
BACKGROUND
Iodine is an essential component of thyroid hormones; either low or high intake may lead to thyroid disease. We observed an increase in the prevalence of overt hypothyroidism, subclinical hypothyroidism, and autoimmune thyroiditis with increasing iodine intake in China in cohorts from three regions with different levels of iodine intake: mildly deficient (median urinary iodine excretion, 84 microg per liter), more than adequate (median, 243 microg per liter), and excessive (median, 651 microg per liter). Participants enrolled in a baseline study in 1999, and during the five-year follow-up through 2004, we examined the effect of regional differences in iodine intake on the incidence of thyroid disease.


METHODS
Of the 3761 unselected subjects who were enrolled at baseline, 3018 (80.2 percent) participated in this follow-up study. Levels of thyroid hormones and thyroid autoantibodies in serum, and iodine in urine, were measured and B-mode ultrasonography of the thyroid was performed at baseline and follow-up.


RESULTS
Among subjects with mildly deficient iodine intake, those with more than adequate intake, and those with excessive intake, the cumulative incidence of overt hypothyroidism was 0.2 percent, 0.5 percent, and 0.3 percent, respectively; that of subclinical hypothyroidism, 0.2 percent, 2.6 percent, and 2.9 percent, respectively; and that of autoimmune thyroiditis, 0.2 percent, 1.0 percent, and 1.3 percent, respectively. Among subjects with euthyroidism and antithyroid antibodies at baseline, the five-year incidence of elevated serum thyrotropin levels was greater among those with more than adequate or excessive iodine intake than among those with mildly deficient iodine intake. A baseline serum thyrotropin level of 1.0 to 1.9 mIU per liter was associated with the lowest subsequent incidence of abnormal thyroid function.


CONCLUSIONS
More than adequate or excessive iodine intake may lead to hypothyroidism and autoimmune thyroiditis.
Teng
Weiping
W
Department of Endocrinology and Metabolism, First Affiliated Hospital, China Medical University, Shengyang, China. twpendocrine@yahoo.com.cn
Shan
Zhongyan
Z
Teng
Xiaochun
X
Guan
Haixia
H
Li
Yushu
Y
Teng
Di
D
Jin
Ying
Y
Yu
Xiaohui
X
Fan
Chenling
C
Chong
Wei
W
Yang
Fan
F
Dai
Hong
H
Yu
Yang
Y
Li
Jia
J
Chen
Yanyan
Y
Zhao
Dong
D
Shi
Xiaoguang
X
Hu
Fengnan
F
Mao
Jinyuan
J
Gu
Xiaolan
X
Yang
Rong
R
Tong
Yajie
Y
Wang
Weibo
W
Gao
Tianshu
T
Li
Chenyang
C
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001323:Autoantibodies / Q000097:blood; D002681:China / Q000453:epidemiology; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007037:Hypothyroidism / Q000139:chemically induced* / Q000453:epidemiology; D015994:Incidence; D007455:Iodine / Q000008:administration & dosage* / Q000009:adverse effects / Q000652:urine; D008297:Male; D008875:Middle Aged; D011795:Surveys and Questionnaires; D013959:Thyroid Diseases / Q000453:epidemiology* / Q000517:prevention & control; D013961:Thyroid Gland / Q000000981:diagnostic imaging; D013963:Thyroid Hormones / Q000097:blood / Q000276:immunology; D013967:Thyroiditis, Autoimmune / Q000139:chemically induced / Q000453:epidemiology; D014463:Ultrasonography
D001323:Autoantibodies; D013963:Thyroid Hormones; D007455:Iodine