Exploration of the safe upper level of iodine intake in euthyroid Chinese adults: a randomized double-blind trial.
BACKGROUND
The beneficial health effects associated with Universal Salt Iodization are well known. Yet, little is known about the possible adverse health effects in people with high iodine intake and the safe daily intake upper limit in the Chinese population.


OBJECTIVE
The objective of this study was to explore the safe upper level of total daily iodine intake among adults in China.


METHODS
A 4-wk, double-blind, placebo-controlled, randomized controlled trial was conducted in 256 euthyroid adults. Participants were randomly assigned to 12 intervention groups with various iodine supplement doses ranging from 0 to 2000 μg/d. Total iodine intake included iodine from both supplements and diet. Multiple outcome measures were used to evaluate possible adverse effects, including thyroid function, thyroid size, and urinary iodine.


RESULTS
The mean iodine intake from the diets and salt intake of the participants were 105 ± 25 and 258 ± 101 μg/d, respectively. In comparison with the placebo group, all iodide-supplemented groups responded with significant increases in median urinary iodine concentrations (P < 0.05) and in thyroid-stimulating hormone concentration (P < 0.05). Thyroid volume decreased after 4 wk in the high-iodine intervention groups (1500-2000 μg). Subclinical hypothyroidism appeared in the groups that received 400 μg I (5%) and 500-2000 μg I (15-47%).


CONCLUSIONS
This study showed that subclinical hypothyroidism appeared in the participants who took the 400-μg I supplement, which provided a total iodine intake of ∼800 μg/d. Thus, we caution against a total daily iodine intake that exceeds 800 μg/d in China and recommend further research to determine a safe daily upper limit.
Department of Nutrition and Food Hygiene, School of Public Health, Tianjin Medical University, China.
Zhongna
Z
Sang
Peizhong Peter
PP
Wang
Zhaixiao
Z
Yao
Jun
J
Shen
Beth
B
Halfyard
Long
L
Tan
Na
N
Zhao
Yuntang
Y
Wu
Shuo
S
Gao
Jian
J
Tan
Jiayu
J
Liu
Zupei
Z
Chen
Wanqi
W
Zhang
D000328:Adult; D002681:China / Q000453:epidemiology; D019587:Dietary Supplements / Q000009:adverse effects*; D004311:Double-Blind Method; D004334:Drug Administration Schedule; D005260:Female; D006801:Humans; D007037:Hypothyroidism / Q000097:blood / Q000139:chemically induced* / Q000453:epidemiology; D015994:Incidence; D007454:Iodides / Q000008:administration & dosage / Q000009:adverse effects; D007455:Iodine / Q000008:administration & dosage / Q000009:adverse effects* / Q000652:urine; D008297:Male; D009929:Organ Size; D012016:Reference Values; D017673:Sodium Chloride, Dietary / Q000008:administration & dosage; D013961:Thyroid Gland / Q000033:anatomy & histology / Q000187:drug effects*; D013972:Thyrotropin / Q000097:blood*; D014131:Trace Elements / Q000008:administration & dosage / Q000009:adverse effects* / Q000652:urine; D055815:Young Adult
D007454:Iodides; D017673:Sodium Chloride, Dietary; D014131:Trace Elements; C034024:iodized salt; D013972:Thyrotropin; D007455:Iodine