Influence of iodine on the reference interval of TSH and the optimal interval of TSH: results of a follow-up study in areas with different iodine intakes.
OBJECTIVE
The aim of the present study was to evaluate whether the status of iodine nutrition influences the TSH concentration in a selected Chinese reference population according to the criteria proposed by National Academy of Clinical Biochemistry (NACB) and regular thyroid ultrasonography, to establish a new reference interval of TSH based on the wide variation of iodine nutrition in populations, and to identify an optimal interval of TSH by following up the cohort with normal TSH concentrations at baseline.


METHODS
The study was conducted in Panshan, Zhangwu and Huanghua, the regions with mildly deficient, more than adequate and excessive iodine intake, respectively. Of the 3761 unselected subjects who were enrolled at baseline, 2237 met the criteria for a reference population. Of 3048 subjects with normal serum TSH at baseline, 2727 (80.0%) participated in the 5-year follow-up study. TSH and thyroid autoantibodies in serum and iodine in urine were measured, and B-mode ultrasonography of the thyroid was performed.


RESULTS
In the reference population, there was a urinary iodine-related increment of serum TSH levels (r = 0.21, P = 0.000), and the mean levels of TSH in Panshan, Zhangwu and Huanghua were 1.15, 1.28 and 1.93 mIU/l, respectively (P = 0.000), corresponding to the rising regional iodine intake. Based on the overall data, we obtained a reference interval of 0.3-4.8 mIU/l. TSH concentrations obtained in the follow-up study correlated well with those at baseline (r = 0.58, P = 0.000). A baseline serum TSH > 1.9 mIU/l was associated with an increased incidence of development of supranormal TSH and a baseline serum TSH < 1.0 mIU/l was associated with an increased incidence of subnormal TSH development.


CONCLUSIONS
Iodine nutrition is an important factor associated with TSH concentration even in the rigorously selected reference population. Baseline TSH of 1.0-1.9 mIU/l is an optimal interval with the lowest incidence of abnormal TSH in 5 years.
Department of Endocrinology and Metabolism, and Institute of Endocrinology, The First Affiliated Hospital of China Medical University, Shenyang, Liaoning, China.
Haixia
H
Guan
Zhongyan
Z
Shan
Xiaochun
X
Teng
Yushu
Y
Li
Di
D
Teng
Ying
Y
Jin
Xiaohui
X
Yu
Chenling
C
Fan
Wei
W
Chong
Fan
F
Yang
Hong
H
Dai
Yang
Y
Yu
Jia
J
Li
Yanyan
Y
Chen
Dong
D
Zhao
Xiaoguang
X
Shi
Fengnan
F
Hu
Jinyuan
J
Mao
Xiaolan
X
Gu
Rong
R
Yang
Wei
W
Chen
Yajie
Y
Tong
Weibo
W
Wang
Tianshu
T
Gao
Chenyang
C
Li
Weiping
W
Teng
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002681:China; D019587:Dietary Supplements; D004435:Eating / Q000502:physiology*; D005260:Female; D005500:Follow-Up Studies; D005843:Geography; D006801:Humans; D007455:Iodine / Q000008:administration & dosage / Q000502:physiology* / Q000652:urine; D008297:Male; D008875:Middle Aged; D009747:Nutritional Physiological Phenomena; D012016:Reference Values; D017673:Sodium Chloride, Dietary / Q000008:administration & dosage / Q000494:pharmacology; D013972:Thyrotropin / Q000097:blood* / Q000592:standards*; D055815:Young Adult
D017673:Sodium Chloride, Dietary; D013972:Thyrotropin; D007455:Iodine