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Kim, Choi, and Park: Dietary iodine intake and the association with subclinical thyroid dysfunction in male workers∗
Original Article

Korean J Nutr 2012;45(3):218-228.

Published online: 8 January 2012

DOI: https://doi.org/10.4163/kjn.2012.45.3.218

Dietary iodine intake and the association with subclinical thyroid dysfunction in male workers∗

Eun Hye Kim1, Tae-in Choi2, Yoo Kyoung Park1, 3, §

1Deparment of Medical Nutrition, Kyunghee University, Yongin 446-701, Korea

2Radiation Health Research Institute, Korea & Hydro Nuclear Power Co., Ltd, Seoul 132-884, Korea

3Research Institute of Medical Nutrition, Kyunghee University, Seoul 130-784, Korea

∗ This work was supported by Radiation Health Research Institute, Korea & Hydro Nuclear Power Co., Ltd.

§To whom correspondence should be addressed. E-mail: ypark@khu.ac.kr

      Revised 1 December 20111 January 2012       Accepted 19 March 2012

Copyright © 2012 The Korean Nutrition Society

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The prevalence rate of thyroid dysfunction (hypothyroidism and hyperthyroidism) has increased within the Korean population and seems to be affected by iodine dietary habits. Some studies reported that the prevalence of thyroid dysfunction increase both in the area of iodine deficiency and excess. In this study, we tried to discover the difference in iodine intake, anthropometric measurements, and blood parameters between male subjects with or without subclinical thyroid dysfunction. A total of 5,249 subjects (Euthyroid: 4706, SubHypo: 454, SubHyper: 89) were used in this study. There were no significant differences in BMI, body fat, visceral fat, waist circumference, SBP, DBP, total cholesterol, HDL-cholesterol, LDL-cholesterol, TG, fasting serum glucose, HbA1c, alcohol intake, however significant differences were noticed in both age and smoking status. Through a food frequency questionnaire (FFQ), iodine intake per day was estimated. The average iodine intake was similar (SubHypo 392.9 ± 279.0 µg, Euthyroid 376.5 ± 281.7 µg, SubHyper 357.3 ± 253.8 µg) among groups. The main source of iodine intake was eggs (52.8%, 54.2%, 52.4%) followed by milk (16.3%, 15.8%, 17.8%), then sea mustard & sea tangle (12.4%, 11.9%, 11.6%). The prevalence of subclinical hypothyroidism and subclinical hyperthyroidism was higher in subjects whose intake was higher than the recommended nutrient intake (RNI). These results suggest that the excess consumption of iodine intake may act as one of the risk factors regarding thyroid dysfunction in Korea. Therefore, an adequate amount of iodine intake is necessary in order to prevent subclinical thyroid dysfunction and clinical thyroid dysfunction.

Keywords: subclinical hypothyroidism, subclinical hyperthyroidism, iodine content food, iodine

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Fig. 1.
Difference of free thyroxine hormone according to iodine intake.
kjn-45-218f1.tif
Fig. 2.
Difference of thyroid stimulating hormone according to iodine intake.
kjn-45-218f2.tif
Table 1.
Diagnosis by thyroid function tests
Criteria by KAHP
Normal FT4 2) 0.8-1.9 ng/dL SubHypo 3) (TSH 1) ＞ 4.0 uIU/mL) n (%) Euthyroid 4) (0.4 ≤ TSH ≥ 4.0 uIU/mL) n (%) SubHyper 5) (TSH ＜ 0.4 uIU/mL) n (%)
454 (8.6) 4706 (89.7) 89 (1.7)

Criteria of Subclinical hypothyroidism, Subclinical hyperthyroidism and Euthyroid by KAHP

1) TSH: Thyroid stimulating hormone

2) FT4: Free thyroxine

3) SubHypo: Subclinical hypothyroidism

4) Euthyroid: The state of having normal thyroid gland function

5) SubHyper: Subclinical hyperthyroidism

Table 2.
Anthropometric parameter and characteristics of the subjects
Variables SubHypo (n = 454) 1) Euthyroid (n = 4706) SubHyper (n = 89) 2) p 3) p 4)
Age (Yr) 40.1 ± 8.3 5)a 40.4 ± 8.1 ab 42.4 ± 8.3 c 0.044  
Anthropometrics          
 BMI (kg/m 2) 6) 24.1 ± 2.5 24.1 ± 2.7 23.8 ± 2.6 0.516 0.497
 Body fat (%) 21.1 ± 5.2 21.3 ± 5.3 21.3 ± 5.3 0.760 0.783
 Visceral fat (cm 2) 83.4 ± 24.8 85.5 ± 26.1 86.6 ± 29.4 0.236 0.333
 WC (cm) 7) 83.1 ± 8.2 83 ± 6.9 82.7 ± 7.3 0.883 0.776
 SBP (mmHg) 8) 122.7 ± 13.1 122.2 ± 13.5 122.9 ± 13.3 0.752 0.762
 DBP (mmHg) 9) 80.1 ± 10.4 79.8 ± 10.4 81.2 ± 12.6 0.406 0.548
Characteristics
  n (%) n (%) n (%) p 1-)
Alcohol          
 Yes 422 (93.6) 4442 (95.8) 83 (95.4)
 No 029 (06.4) 0196 (04.2) 04 (04.6) 0.094
 Total 451 (100) 4638 (100) 87 (100)
Smoking status
 Non smoker 185 (41.0) 1279 (27.6) 21 (24.1)
 Ex-Smoker 173 (38.4) 1585 (34.2) 31 (35.6) 0.000
 Smokers 093 (20.6) 1774 (38.2) 35 (40.2)
 Total 456 (100) 4638 (100) 87 (100)

1) SubHypo: Subclinical hypothyroidism

2) SubHyper: Subclinical hyperthyroidism

3) Differences were evaluated by ANOVA, p values by using LSD significances test, p ＜ 0.05

4) Differences were evaluated by ANCOVA, the latter when data were adjusted by age, p values by using LSD significances test, p ＜ 0.05

5) Mean ± SD

6) BMI: Body Mass Index

7) WC: Waist circumference

8) SBP: Systolic Blood Pressure

9) DBP: Diastolic Blood Pressure 10)P values by using χ 2, p ＜ 0.05

Table 3.
Serum biochemical parameter of the subjects
Variables SubHypo 1) (n = 454) Euthyroid (n = 4706) SubHyper 2) (n = 89) p3) p4)
TC (mg/dL) 6) 190.6 ± 31.6 5) 190.0 ± 32.7 186.1 ± 31.7 0.490 0.243
HDL-C (mg/dL) 7) 50.2 ± 11.8 50.2 ± 12.6 50.1 ± 11.4 0.999 1.000
LDL-C (mg/dL) 8) 127.0 ± 31.1 125.5 ± 31.7 122.5 ± 34.3 0.373 0.197
TG (mg/dL) 9) 130.0 ± 78.6 133.3 ± 80.5 131.3 ± 66.4 0.704 0.702
FSG (mg/dL) 10) 91.9 ± 10.3 92.8 ± 13.5 94.4 ± 14.0 0.192 0.376
HbA1c (%) 11) 5.4 ± 0.3 5.4 ± 0.4 5.4 ± 0.5 0.130 0.224
FT4 (ng/dL) 12) 1.3 ± 0.2 a 1.4 ± 0.3 b 1.5 ± 0.3 c 0.000 0.000
TSH (uIU/mL) 13) 5.9 ± 2.9 a 1.8 ± 0.8 b 0.2 ± 0.1 c 0.000 0.000

1) SubHypo: Subclinical hypothyroidism

2) SubHyper: Subclinical hyperthyroidism

3) Differences were evaluated by ANOVA, p values by using LSD significances test, p < 0.05

4) Differences were evaluated by ANCOVA, the latter when data were adjusted by age, p values by using LSD significances test, p < 0.05

5) Mean ± SD

6) TC: Total cholesterol

7) HDL-C: High density lipoprotein-cholestero

8) LDL-C: Low density lipoprotein-cholesterol

9) TG: Triglyceride

10) FSG: Fasting Serum Glucose

11) HbA1c: Hemoglobin A1c

12) FT4: Free Thyroxine

13) TSH: Thyroid stimulating hormone

Table 4.
Iodine intake and distribution in SubHypo, Euthyroid, SubHyper group
SubHypo 1) Euthyroid SubHyper 2)
        p 4)
Daily intake (µg) 392.9 ± 279.0 3) 376.5 ± 281.7 357.3 ± 253.8 0.513
Range (µg) 30.9-1651.3 10.2-2861.9 20.4-1053.9  
Excess or deficit compared to iodine DRI 5) (n, %)     p 8)
≤ 95 µg (EAR 6)) 030 (06.6) 0353 (07.5) 10 (11.2)  
＞ 95 µg-≤ 150 µg 054 (11.9) 0495 (10.5) 11 (12.4) 0.484
＞ 150 µg (RNI 7)) 370 (81.5) 3858 (82.0) 68 (76.4)  
Total 454 (100) 4706 (100) 89 (100)  

1) SubHypo: Subclinical hypothyroidism

2) SubHyper: Subclinical hyperthyroidism

3) Mean ± SD

4) p values by using LSD significances test, p ＜ 0.05

5) DRI: Dietary Reference Intakes

6) EAR: Estimated Average Requirement

7) RNI: Recommended Nutrient Intake

8) p values by using χ 2, p ＜ 0.05

Table 5.
Contribution of food sources to the total iodine intake
SubHypo(n = 454) Euthyroid (n = 4706) SubHyper(n = 89)
Food item µg (%) 1) Food item µg (%) 1) Food item µg (%) 1)
01 Egg 207.3 (52.8) Egg 204.0 (54.2) Egg 187.2 (52.4)
02 Milk 64.2 (16.3) Milk 59.3 (15.8) Milk 63.7 (17.8)
03 Sea mustard/ Sea tangle 48.6 (12.4) Sea mustard/ Sea tangle 45.0 (11.9) Sea mustard/ Sea tangle 41.4 (11.6)
04 Yoplait 21.4 (05.4) Yoplait 18.2 (04.8) Rice 15.3 (04.3)
05 Rice 16.5 (04.2) Rice 16.5 (04.4) Yoplait 16.2 (04.5)
06 Beef 11.5 (02.9) Beef 11.0 (02.9) Beef 11.4 (03.2)
07 Anchovy 4.7 (01.2) Anchovy 4.2 (01.1) Anchovy 4.3 (01.2)
08 Apple 4.1 (01.1) Apple 4.1 (01.1) Apple 4.1 (01.1)
09 Laver 3.6 (00.9) Laver 3.6 (00.9) Laver 3.9 (01.1)
10 Extra total 11.0 (02.8) Extra total 10.6 (02.9) Extra total 9.6 (02.8)

1) µg and % of total iodine intake

Table 6.
Differences according to DRI of iodine intake in subclinical hypothyroidism
Subclinical hypothyroidism
Iodine intake (µg) EAR ≤ 95 (N = 30) EAR/RNI ＞ 95-≤ 150 (N = 54) RNI ＞ 150 (N = 370) p 1) p 2)
Age (yr) 44.0 ± 9.6 a3) 42.1 ± 8.9 bc 39.5 ± 8.0 b 0.003 -
Anthropometrics          
 BMI (kg/m2)4) 24.4 ± 2.3 23.7 ± 2.5 24.1 ± 2.5 0.399 0.390
 Body fat (%) 22.2 ± 3.9 21.0 ± 5.6 21.1 ± 5.2 0.509 0.608
 Visceral fat (cm 2) 94.0 ± 22.0 a 85.0 ± 25.5 ab 82.3 ± 24.8 b 0.039 0.357
 WC (cm) 5) 84.2 ± 5.9 82.5 ± 6.2 83.0 ± 8.6 0.676 0.711
 SBP (mmHg)6) 124.0 ± 13.4 119.8 ± 12.9 123.0 ± 13.1 0.218 0.219
 DBP (mmHg) 7) 79.4 ± 11.26 80.5 ± 10.25 80.1 ± 10.33 0.894 0.767
Blood test          
 TC (mg/dL) 8) 190.9 ± 35.5 191.6 ± 38.6 190.4 ± 30.3 0.964 0.887
 HDL-C (mg/dL)9) 46.4 ± 8.9 51.2 ± 11.5 50.3 ± 12.0 0.164 0.199
 LDL-C (mg/dL) 10) 126.9 ± 33.9 128.1 ± 37.5 127.0 ± 29.8 0.973 0.840
 TG (mg/dL) 11) 151.3 ± 91.3 136.7 ± 124.7 127.3 ± 68.2 0.221 0.333
 FSG (mg/dL) 12) 91.9 ± 9.4 91.7 ± 9.3 91.9 ± 10.6 0.988 0.548
 HbA1c (%)13) 5.4 ± 0.3 5.3 ± 0.3 5.4 ± 0.3 0.215 0.212
 FT4 (ng/dL) 14) 1.2 ± 0.3 1.3 ± 0.2 1.3 ± 0.2 0.116 0.349
 TSH (uIU/mL) 15) 6.6 ± 2.7 6.1 ± 2.7 5.8 ± 3.0 0.324 0.332
Iodine intake          
 Iodine (µg/day) 68.4 ± 20.0a 121.9 ± 13.5ab 458.8 ± 268.0c 0.000 0.000

1) Differences were evaluated by ANOVA, p values by using LSD significances test, p ＜ 0.05

2) Differences were evaluated by ANCOVA, the latter when data were adjusted by age, p values by using LSD significances test, p ＜ 0.05

3) Mean ± SD

4) BMI: Body Mass Index

5) WC: Waist circumference

6) SBP: Systolic Blood Pressure

7) DBP: Diastolic Blood Pressure

8) TC: Total cholesterol

9) HDL-C: High density lipoprotein-cholesterol

10) LDL-C: Low density lipoprotein-cholesterol

11) TG: Triglyceride

12) FSG: Fasting Serum Glucose

13) HbA1c: Hemoglobin A1c

14) FT4: Free Thyroxine

15) TSH: Thyroid stimulating hormone

Table 7.
Differences according to DRI of iodine intake in euthyroid
Euthyroid
Iodine intake (µg) EAR ≤ 95 (n = 353) EAR/RNI ＞ 95-≤ 150 (n = 495) RNI ＞ 150 (n = 3858) p 1) p 2)
Age (yr) 44.5 ± 7.7 a3) 42.2 ± 8.0 b 39.8 ± 8.0 c 0.000 -
Anthropometrics          
 BMI (kg/m 2) 4) 24.1 ± 2.8 24.1 ± 2.6 24.1 ± 2.7 0.918 0.841
 Body fat (%) 21.5 ± 5.3 21.8 ± 5.1 21.3 ± 5.3 0.094 0.321
 Visceral fat (cm 2) 90.4 ± 25.7 a 87.8 ± 25.7 ac 84.7 ± 26.1 b 0.000 0.982
 WC (cm)5) 82.9 ± 7.0 83.1 ± 6.9 82.9 ± 6.9 0.864 0.552
 SBP (mmHg) 6) 122.6 ± 14.3 122.9 ± 13.7 122.1 ± 13.4 0.434 0.746
 DBP (mmHg) 7) 80.9 ± 10.9 80.1 ± 10.6 79.7 ± 10.4 0.088 0.949
Blood test          
 TC (mg/dL)8) 192.7 ± 32.7 190.5 ± 31.6 189.7 ± 32.9 0.233 0.828
 HDL-C (mg/dL) 9) 49.2 ± 13.0 49.5 ± 13.3 50.3 ± 12.5 0.134 0.128
 LDL-C (mg/dL) 10) 130.6 ± 31.8 a 125.8 ± 30.7 b 125.1 ± 31.8 bc 0.008 0.260
 TG (mg/dL) 11) 135.2 ± 74.5 136.6 ± 83.8 132.6 ± 80.6 0.527 0.818
 FSG (mg/dL)12) 94.5 ± 15.7a 92.4 ± 12.5b 92.7 ± 13.4bc 0.041 0.199
 HbA1c (%) 13) 5.5 ± 0.5 a 5.4 ± 0.4 ac 5.4 ± 0.4 b 0.000 0.308
 FT4 (ng/dL) 14) 1.4 ± 0.3 ab 1.4 ± 0.3 a 1.4 ± 0.3 b 0.034 0.427
 TSH (uIU/mL) 15) 1.7 ± 0.9 1.8 ± 0.8 1.8 ± 0.8 0.106 0.225
Iodine intake          
 Iodine (µg/day) 65.9 ± 20.5 a 123.4 ± 15.3 b 437.4 ± 275.6 c 0.000 0.000

1) Differences were evaluated by ANOVA, p values by using LSD significances test, p < 0.05

2) Differences were evaluated by ANCOVA, the latter when data were adjusted by age, p values by using LSD significances test, p < 0.05

3) Mean ± SD

4) BMI: Body Mass Index

5) WC: Waist circumference

6) SBP: Systolic Blood Pressure

7) DBP: Diastolic Blood Pressure

8) TC: Total cholesterol

9) HDL-C: High density lipoprotein-cholesterol

10) LDL-C: Low density lipoprotein-cholesterol

11) TG: Triglyceride

12) FSG: Fasting Serum Glucose

13) HbA1c: Hemoglobin A1c

14) FT4: Free Thyroxine

15) TSH: Thyroid stimulating hormone

Table 8.
Differences according to DRI of iodine intake in subclinical hyperthyroidism
Subclinical hyperthyroidism
Iodine intake (µg) EAR ≤ 95 (n = 10) EAR/RNI ＞ 95-≤ 150 (n = 11) RNI ＞ 150 (n = 68) p1) p2)
Age (yr) 46.6 ± 7.2 ab3) 46.5 ± 9.8 b 41.2 ± 7.9 a 0.032 -
Anthropometrics          
 BMI (kg/m2) 4) 23.6 ± 2.4 25.3 ± 2.1 23.6 ± 2.6 0.118 0.138
 Body fat (%) 20.3 ± 5.2 a 25.0 ± 4.6 b 20.9 ± 5.3 ac 0.043 0.055
 Visceral fat (cm 2) 83.2 ± 21.7 104.9 ± 27.2 84.1 ± 30.0 0.085 0.163
 WC (cm) 5) 82.5 ± 6.5 85.9 ± 6.2 82.1 ± 7.6 0.292 0.368
 SBP (mmHg)6) 120.5 ± 11.4 127.3 ± 16.2 122.5 ± 13.1 0.461 0.485
 DBP (mmHg) 7) 77.9 ± 7.9 88.4 ± 23.0 80.5 ± 10.6 0.109 0.128
Blood test          
 TC (mg/dL) 8) 187.9 ± 30.2 181.7 ± 16.7 186.5 ± 34.0 0.884 0.759
 HDL-C (mg/dL)9) 50.8 ± 12.7 45.8 ± 10.9 50.7 ± 11.3 0.414 0.440
 LDL-C (mg/dL) 10) 117.2 ± 31.0 120.5 ± 27.8 123.4 ± 36.2 0.855 0.659
 TG (mg/dL) 11) 157.1 ± 115.0 160.6 ± 66.8 122.8 ± 55.2 0.091 0.101
 FSG (mg/dL) 12) 91.9 ± 14.6 a 105.0 ± 23.6 b 93.0 ± 11.1a c 0.024 0.046
 HbA1c (%) 13) 5.4 ± 0.2ab 5.8 ± 1.3 b 5.4 ± 0.3 a 0.023 0.072
 FT4 (ng/dL) 14) 1.3 ± 0.3ab 1.4 ± 0.2 a 1.5 ± 0.2 b 0.026 0.045
 TSH (uIU/mL) 15) 0.3 ± 0.1 0.2 ± 0.1 0.2 ± 0.1 0.188 0.212
Iodine intake          
 Iodine (µg/day) 54.8 ± 20.3 a 118.6 ± 19.5 ab 440.4 ± 233.3 c 0.000 0.000

1) Differences were evaluated by ANOVA, p values by using LSD significances test, p ＜ 0.05

2) Differences were evaluated by ANCOVA, the latter when data were adjusted by age, p values by using LSD significances test, p ＜ 0.05

3) Mean ± SD

4) BMI: Body Mass Index

5) WC: Waist circumference

6) SBP: Systolic Blood Pressure

7) DBP: Diastolic Blood Pressure

8) TC: Total cholesterol

9) HDL-C: High density lipoprotein-cholesterol

10) LDL-C: Low density lipoprotein-cholesterol

11) TG: Triglyceride

12) FSG: Fasting Serum Glucose

13) HbA1c: Hemoglobin A1c

14) FT4: Free Thyroxine

15) TSH: Thyroid stimulating hormone

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