|
|
||||||||
ORIGINAL RESEARCH COMMUNICATION |
1 From the Human Nutrition Laboratory, Swiss Federal Institute of Technology, Zürich, Switzerland (MBZ and SYH); the Department of Pediatrics, Asahikawa Medical College, Asahikawa, Japan (YI and KF); and the Department for Growth and Development, University Children's Hospital, Zürich, Switzerland (LM).
2 Supported by the Swiss Federal Institute of Technology and the Swiss Foundation for Nutrition Research, the Japanese Growth Science Foundation, Merck KGaA (Darmstadt, Germany), the Medical Research Council (Cape Town, South Africa), and the World Health Organization. 3 Reprints not available. Address correspondence to M Zimmermann, Human Nutrition Laboratory, Swiss Federal Institute of Technology Zürich, Seestrasse 72/Postfach 474, CH-8803 Rüschlikon, Switzerland. E-mail: michael.zimmermann{at}ilw.agrl.ethz.ch.
| ABSTRACT |
|---|
|
|
|---|
Objective: The objective of the study was to ascertain whether high dietary intakes of iodine in children result in high thyroid volume (Tvol), a high risk of goiter, or both.
Design: In an international sample of 612-y-old children (n = 3319) from 5 continents with iodine intakes ranging from adequate to excessive, Tvol was measured by ultrasound, and the urinary iodine (UI) concentration was measured. Regressions were done on Tvol and goiter including age, body surface area, sex, and UI concentration as covariates.
Results: The median UI concentration ranged from 115 µg/L in central Switzerland to 728 µg/L in coastal Hokkaido, Japan. In the entire sample, 31% of children had UI concentrations >300 µg/L, and 11% had UI concentrations >500 µg/L; in coastal Hokkaido, 59% had UI concentrations >500 µg/L, and 39% had UI concentrations >1000 µg/L. In coastal Hokkaido, the mean age- and body surface areaadjusted Tvol was
2-fold the mean Tvol from the other sites combined (P < 0.0001), and there was a positive correlation between log(UI concentration) and log(Tvol) (r = 0.24, P < 0.0001). In the combined sample, after adjustment for age, sex, and body surface area, log(Tvol) began to rise at a log(UI concentration) >2.7, which, when transformed back to the linear scale, corresponded to a UI concentration of
500 µg/L.
Conclusions: Chronic iodine intakes approximately twice those recommendedindicated by UI concentrations in the range of 300500 µg/Ldo not increase Tvol in children. However, UI concentrations
500 µg/L are associated with increasing Tvol, which reflects the adverse effects of chronic iodine excess.
Key Words: Thyroid volume goiter children urinary iodine excess iodine
| INTRODUCTION |
|---|
|
|
|---|
300 µg/L (12),. The World Health Organization/International Council for Control of Iodine Deficiency Disorders (WHO/ICCIDD) cautioned that a median UI concentration >300 µg/L in 612-y-old children is excessive (1). By extrapolating from studies in adults, the Institute of Medicine has set the tolerable upper intake level (UL) for iodine in 48-y-old and 913-y-old children at 300 and 600 µg/d, respectively (13). Experts have highlighted the need for more research on the safety in children of iodine intakes between 2001000 µg/d (13, 14).
Excess dietary iodine may increase the risk of thyroiditis, hyperthyroidism, hypothyroidism, and goiter (14). In healthy adults, short-term iodine intakes of 5001500 µg/d have mild inhibitory effects on thyroid function (1517). The consequences of prolonged exposure to high intakes of iodine, particularly in children, are less clear. Endemic goiter in children has been described in coastal Japan, where iodine intake from seaweed was >10 000 µg/d (5). Lower intakes, in the range of 4001300 µg/d, from iodine-rich drinking water, were associated with increased serum thyrotropin (TSH) and thyroid volume (Tvol) in a small sample of Chinese children (7).
In this study, we analyzed a broad range of UI concentration and Tvol data from a recent international study of school-age children (18), and we included new data from Hokkaido, one of the islands of Japan, on which there traditionally is a high iodine intake. Our aim was to determine whether chronic high iodine intakes are associated with greater thyroid size in school-age children.
| SUBJECTS AND METHODS |
|---|
|
|
|---|
|
Methods
Height and weight were measured by using standard anthropometric technique (19). Heights were recorded to the nearest millimeter and weights to the nearest 100 g. Tvol was measured by using an Aloka SSD-500 echocamera (Aloka, Mure, Japan) equipped with 7.5-MHz linear transducers. Measurements were performed while subjects sat upright in a straight-backed chair with the neck extended. For each thyroid lobe, the maximum perpendicular anteroposterior and mediolateral dimensions were measured on a transverse image of the largest diameter, without including the isthmus. The maximum craniocaudal diameter of each lobe was then measured on a longitudinal image. The thyroid capsule was not included. The ultrasound measurements were done by one of us (MZ or SH). Intraobserver variability, as defined by limits of agreement (20) for repeat measurements done in
4% of the sample by one of us (MZ or SH), were 0.087, 0.134 and 0.114, 0.165. For interobserver variability, the limits of agreement for duplicate measurements done in
6% of the sample were 0.202, 0.246.
Spot urine samples were collected, and aliquots were stored at 20 °C until they were analyzed. Measurement of UI concentrations was done in Zürich by using the Pino modification of the Sandell-Kolthoff reaction (21), which was validated against the results of inductively coupled plasma mass spectrometry (22). External control was provided by the Ensuring the Quality of Urinary Iodine Procedures (EQUIP) round-robin program of the Centers for Disease Control and Prevention. The intraassay CV of the Sandell-Kolthoff method in our laboratory was 9.1% at 45.7 ± 4.5 µg/L, 2.8% at 100.1 ± 2.8 µg/L, and 1.2% at 584 ± 6.8 µg/L.
Data and statistical analyses
Data processing and statistical analyses were done by using S-PLUS-2000 (version 2000; Insightful Corporation, Seattle, WA) and EXCEL (XP Edition; Microsoft, Seattle, WA) software. Body surface area (BSA) was calculated as
![]() | (1) |
![]() | (2) |
| RESULTS |
|---|
|
|
|---|
|
500 µg/L.
|
| DISCUSSION |
|---|
|
|
|---|
500 µg/d over several weeks induce subtle, reversible changes in pituitary-thyroid function in adults (with values remaining within the normal range), probably by inhibiting thyroid hormone release (1517).
Several studies have reported that excess iodine has a goitrogenic effect in adults. In Peace Corps volunteers, ingestion for up to 32 mo of
50 mg iodine/d from iodine-resin water filters increased mean (±SD) UI concentrations to 11.1 ± 19.1 mg/L (28). Goiter by palpation was found in 44% of the subjects; 30 ± 11 wk after removal of excess iodine, the goiter prevalence decreased to 30% (29). LeMar et al (30) reported a reversible, TSH-dependent thyroid enlargement in response to iodine excess from tetraglycine hydroperiodide water-purification tablets. A dose of 32 mg iodine/d for 3 mo given to 8 healthy adults increased mean UI concentrations from 0.28 to 40 mg/d. Mean Tvol, determined by ultrasound, increased by 37% after 3 mo in those subjects but returned to baseline an average of 7.1 mo afterward. Namba et al (31) gave 10 healthy men 27 mg iodine/d for 28 d. Mean Tvol increased significantly (16%) after 4 wk and returned to baseline 4 wk after iodine withdrawal.
In children, excess dietary iodine has been associated with goiter and thyroid dysfunction. In a report of what the authors called "endemic coastal goiter" in Hokkaido, Japan (5), the traditional local diet was high in iodine-rich seaweed. UI excretion in children consuming the local diet was
23 000 µg/d. The overall prevalence of visible goiter in children was 39%, but, in several villages,
25% of the children had visible goiter. Most of the goiters responded to the administration of thyroid hormone, restriction of dietary iodine intake, or both. TSH assays were not available, but it was suggested that an increase in serum TSH was involved in the generation of goiter. No cases of clinical hypothyroidism or hyperthyroidism were reported.
Goiter in children may also be precipitated by iodine intake well below the high amount in the studies from Hokkaido. Li et al (7) examined thyroid status in 171 Chinese children from 2 villages where the iodine concentrations in drinking water were 462 and 54 µg/L, and the children's mean UI concentrations were 1235 and 428 µg/g creatinine, respectively. The mean serum TSH concentration (7.8 mU/L) was high in the first village and high-normal (3.9 mU/L) in the second village. In the first village, the goiter rate was >60% and mean (±SD) Tvol was 13.3 ± 2.7, whereas the goiter rate was 1520% and mean (±SD) Tvol was 5.9 ± 1.8 in the second village. There were no signs of neurologic deficits in the children. In other reports from China, drinking water with iodine concentrations >300 µg/L resulted in UI concentrations >900 µg/L and a goiter rate of >10% (8). Although the mechanism remains unclear, increased thyroid size associated with high iodine intake may be due to autoimmune-mediated lymphoid infiltration of the thyroid (32, 33), inhibition of thyroid hormone release that increases serum TSH and thyroid stimulation (7, 27), or both. Taken together, the Chinese studies suggest that goiter and thyroid dysfunction may occur in children at iodine intakes in the range of 4001300 µg/d.
Our data support previous findings of thyroid sensitivity to high iodine intakes and suggest that chronic iodine intakes
500 µg/d in children increase thyroid size. However, this possibility is based mainly on the data from coastal Hokkaido; in central Hokkaido and in the United Statesthe 2 other sites with a high prevalence of UI concentrations >500 µg/Lthere was no significant increase in Tvol at higher UI concentrations. This difference could be due to dietary or environmental factors (or both) in coastal Hokkaido that potentiate the effects of high iodine intake. A limitation of our study was that thyroid function tests and antithyroid antibodies were not measured in the sample. It is possible that children with high iodine intakes could have subtle changes in pituitary-thyroid function that were not reflected by increases in thyroid size. However, in previous studies of iodine excess in adults and children that measured Tvol by ultrasound (most of which reported iodine intakes much higher than those in our sample), nearly all detected an increase in Tvol (5, 7, 8, 2831). This suggests that an increased Tvol is a reasonable marker of thyroid dysfunction in response to iodine excess. Although our findings support the contention that moderately high dietary intakes of iodine in the range of 300500 µg/dare well tolerated by healthy children, iodine intakes in this range are of no benefit and may have adverse effects not detected in this study.
| ACKNOWLEDGMENTS |
|---|
The data were collected by MZ, YI, and SH; the statistical analyses were done by MZ and LM; the first draft of the manuscript was written by MZ; and each of the authors made substantial contributions to the study design, data analyses, and editing of the manuscript. None of the authors had a personal or financial conflict of interest.
| REFERENCES |
|---|
|
|
|---|
This article has been cited by other articles:
![]() |
M. B. Zimmermann Iodine Deficiency Endocr. Rev., June 1, 2009; 30(4): 376 - 408. [Abstract] [Full Text] [PDF] |
||||
![]() |
H.-L. Liu, L. T. Lam, Q. Zeng, S.-q. Han, G. Fu, and C.-c. Hou Effects of drinking water with high iodine concentration on the intelligence of children in Tianjin, China J. Public Health Med., March 1, 2009; 31(1): 32 - 38. [Abstract] [Full Text] [PDF] |
||||
![]() |
R. Y A Camargo, E. K Tomimori, S. C Neves, I. G S Rubio, A. L. Galrao, M. Knobel, and G. Medeiros-Neto Thyroid and the environment: exposure to excessive nutritional iodine increases the prevalence of thyroid disorders in Sao Paulo, Brazil Eur. J. Endocrinol., September 1, 2008; 159(3): 293 - 299. [Abstract] [Full Text] [PDF] |
||||
![]() |
H. I. Gozu, R. Bircan, K. Krohn, S. Muller, S. Vural, C. Gezen, H. Sargin, D. Yavuzer, M. Sargin, B. Cirakoglu, et al. Similar prevalence of somatic TSH receptor and Gs{alpha} mutations in toxic thyroid nodules in geographical regions with different iodine supply in Turkey. Eur. J. Endocrinol., October 1, 2006; 155(4): 535 - 545. [Abstract] [Full Text] [PDF] |
||||
![]() |
R. Prakash High thyroid volume in children with excess dietary iodine intakes Am. J. Clinical Nutrition, September 1, 2005; 82(3): 708 - 709. [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |