Dietary Factors and the Risk of Thyroid Diseases: A Review

Wook Jin Choi; Jeongseon Kim

doi:10.11106/ijt.2015.8.2.137

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Choi and Kim: Dietary Factors and the Risk of Thyroid Diseases: A Review

Review Article

Int J Thyroidol 2015;8(2):137-146.

Published online: 20 November 2015

DOI: https://doi.org/10.11106/ijt.2015.8.2.137

Dietary Factors and the Risk of Thyroid Diseases: A Review

Wook Jin Choi, Jeongseon Kim

Molecular Epidemiology Branch, Division of Cancer Epidemiology and Prevention, Research Institute, National Cancer Center, Goyang, Korea

Correspondence: Jeongseon Kim, PhD, Molecular Epidemiology Branch, Division of Cancer Epidemiology and Prevention, National Cancer Center, 323 Ilsan-ro, Ilsandong-gu, Goyang 10408, Korea Tel: 82-31-920-2570, Fax: 82-31-920–2579, E-mail: jskim@ncc.re.kr

Received 2 May 2015 Revised 21 July 2015 Accepted 3 August 2015

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Background and Objectives

Diet is one of the major risk factors for thyroid diseases. It has been shown that high or excessive iodine intake is more likely to be a health concern in iodine-sufficient regions or regions where iodine deficiency previously existed due to the emergence of iodine-induced hypothyroidism or hyperthy-roidism. Therefore, this review investigates the occurrence of thyroid diseases, and particularly hypothyroidism and hyperthyroidism, in populations with different levels of iodine intake and other dietary factors in various geographic regions.

Materials and Methods

A total of 856 articles published between January 1st, 1990 and March 31st, 2015, were identified. Epidemiological studies that showed an association between dietary factors and thyroid diseases were selected, yielding a total of 21 articles.

Results

Due to a sudden increase in iodine supplementation (i.e., via salt iodization), regions such as Denmark and China, where insufficient iodine intake previously existed, showed a significant increase in the occurrence of hypothyroidism compared with that of hyperthyroidism. Other dietary factors, such as nitrate intake, may increase the risk of the diseases, whereas a vegan diet and alcohol intake may lower the risk.

Conclusion

The level of iodine intake is quite variable between individuals in different geographic regions, and the risk of thyroid diseases may also vary by age and gender. Therefore, monitoring of safe levels of iodine intake should be performed to prevent iodine-induced thyroid diseases.

Keywords: Thyroid disease, Hypothyroidism, Hyperthyroidism, Iodine, Diet

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Fig. 1.

Flow chart for selection of eligible studies.

Table 1.

Iodine intake and thyroid diseases in Europe

Reference	Country (region)	Survey year	Study type	Age (years)	Iodine source	MUI (μ g/l)	Results (rate [%]/risk [95% CI])
Laurberg et al. (1998)¹⁰⁾	Denmark (Jutland) Iceland	–	Cross-sectional	68 (66–70)	Iceland (dairy products)	Jutland (38) Iceland (150)	Female/Male 1) Jutland (hyperfunction): PR=12.2/3.2% 2) Iceland (hypofunction): PR=1.9/2.2%
Bü low Pedersen et al. (2002)¹²⁾	Denmark (Aalborg, Copenhagen)	1998–2000	DanThyr (cohort, 2-year f/u)	0–80+	Salt IF	Aalborg (45) Copenhagen (61)	Aalborg/Copenhagen 1) Hyperthyroidism: SRR=1.49 [1.22–1.81] 2) Hypothyroidism: SRR=0.73 [0.55–0.97]
Carle et al. (2006)¹¹⁾	Denmark (Aalborg, Copenhagen)	1997–2000	DanThyr (cohort)	0–80+	Salt IF	Aalborg (45) Copenhagen (61)	Copenhagen/Aalborg 1) Hypothyroidism: SIR=1.33 [1.15–1.55]; p＜0.05 2) Spontaneous hypothyroidism: SIR=1.53 [1.29–1.80]; p＜0.05 3) Non-spontaneous hypothyroidism: SIR=0.64 [0.43–0.96]; p＜0.05
Bü low Pedersen et al. (2006)¹³⁾	Denmark (Aalborg, Copenhagen)	1997–2004	DanThyr (cohort, 6-year f/u)	0–60+	Salt IF	Aalborg (45) Copenhagen (61)	Hyperthyroidism (age, years) 1) 20–39: RR=2.61 [2.14–3.19] 2) 40–59: RR=1.29 [1.10–1.52] 3) ＞60: RR=1.13 [1.0–1.28]
Bü low Pedersen et al. (2007)¹⁴⁾	Denmark (Aalborg, Copenhagen)	1997–2005	DanThyr (cohort, 7-year f/u)	0–60+	Salt IF	Baseline/Late IF Aalborg (45/86) Copenhagen (61/99)	Hypothyroidism (late mandatory vs. baseline IF) 1) Overall: RR=1.23 [1.07–1.42] 2) Aalborg: RR=1.11 [1.11–1.66] 3) Copenhagen: RR=1.10 [0.90–1.34] 4) 20–39 years: RR=1.90 [1.28–2.84]
Vejbjerg et al. (2009)¹⁵⁾	Denmark (Aalborg, Copenhagen)	1997–1998,2000	Cross-sectional (before vs. after IF)	18–65	Salt IF	Aalborg (53) Copenhagen (68)	Mild hypothyroidism (age, years female; p=0.01) 1) 18–22: OR=1.7 [1.0–2.8] 2) 25–30: OR=2.0 [1.2–3.2] 3) 40–45: OR=1.4 [0.8–2.3] 4) 60–65: OR=0.7 [0.5–1.1]
Garcí a-Garcí a et al. (2012)¹⁶⁾	Spain (Almerí a)	2007–2010	Cross-sectional	1–16	Dairy products	199.5	Thyroid autoimmunity/Autoimmune thyroiditis 1) Deficient: PR=2.6% [1.3–5.1]/1.3% [0.0–4.5] 2) Adequate: PR=2.9% [1.3–6.3]/1.3% [0.1–3.1] 3) Above requirements: PR=4.6% [2.9–7.8]/1.6% [0.1–3.7] 4) Excessive: PR=5.0% [2.6–9.6]/1.4% [0.0–4.1]

CI: confidence interval, f/u: follow-up, IF: iodine fortification, IR: incidence rate, MUI:median urinary iodine, PR: prevalence rate, RR: relative risk, SIR: standardized incidence rate, SRR: standardized rate ratio

Table 2.

Iodine intake and thyroid diseases in Asia

Reference	Country (region)	Survey year	Study type	Age (years)	Iodine source	MUI (μ g/l)	Results (rate [%]/risk [95% CI])
Konno et al. (1994)²³⁾	Japan	1992	Cross-sectional	≥18	Seaweed (kelp)	≥75 μ mol/L (high)	Hypothyroidism: Thyroid AB-in subjects with high UI (12.1%) vs. normal UI (2.3%) (p＜0.001)
Yang et al. (2002)¹⁷⁾	China	1991–19951996–1999	Cross-sectional & longitudinal survey	≥14	USI	Panshan (103) Zhangwu (375) Huanghua (615)	Subclinical hyperthyroidism 1) PR=3.7% (P vs. H); p＜0.001 2) PR=3.9% (Z vs. H); p＜0.001 3) PR=1.1% (H)
Teng et al. (2006)¹⁹⁾	China	1999–2004	Cross-sectional (5-year f/u)	≥14	USI	Panshan (84) Zhangwu (243) Huanghua (651)	Subclinical hypothyroidism 1) CI=0.2(P)/2.6(Z)/2.9(H)%; p＜0.001 2) P vs. Z; p＜0.001 3) P vs. H; p＜0.001
Yang et al. (2007)¹⁸⁾	China	1999–2004	Cross-sectional (5-year f/u)	≥14	USI	Panshan (88) Zhangwu (214) Huanghua (634)	Hyperthyroidism 1) CI=1.4(P)/0.9(Z)/0.8(H)%; p＞0.05
Aminorroaya et al. (2009)²⁴⁾	Iran (Isfahan)	2006	Cross-sectional (15 years after USI)	＞20	USI	Male (20.0 μ g/dL) Female (20.6 μ g/dL)	Male (ref.) vs. Female 1) Subclinical hypothyroidism: OR=2.52 [1.70–3.73]; p＜0.001 2) Overt hypothyroidism: OR=2.31 [1.32–4.04]; p＜0.001
Chung et al. (2009)²⁵⁾	Korea	2007–2008	Case-control	≤34 week	Breast milk	75	Subclinical hypothyroidism: EDi (＜30 vs. 100 μ g/kg/day) 1) Week 3 (p=0.033, 0 vs. 67%): 51.2±45.5 vs. 149.0±103.8 μ g/kg/day 2) Week 6 (p=0.032, 20 vs. 100%): 32.8±35.5 vs. 92.1±51.2 μ g/kg/day
Teng et al. (2011)²⁰⁾	China	2007	Cross-sectional	＞14	USI	(school children: 145/ 261; general population: 164/272) Chengshan/Rongxing	Chengshan/Rongxing (%) 1) Hypothyroidism: PR=2.15/5.35%; p＜0.000 2) Subclinical hypothyroidism: PR=1.99/5.03%; p＜0.001 3) Hyperthyroidism: PR=3.52/1.89%; p=0.002 4) Subclinical hyperthyroidism: PR=1.73/0.63%; p=0.002
Zou et al. (2012)²¹⁾	China	2009 (Sep-Dec)	Cross-sectional	5–69	USI	146.7	Age (15–69 years, male/female (%)) 1) Hypothyroidism: PR=0.44/0.98%; p＜0.05 2) Subclinical hypothyroidism: PR=7.3/11.03%; p＜0.05 Age (15–39 vs. 40–69 years) 1) Subclinical hyperthyroidism: PR=0.05/0.41%; p＜0.05
Du et al. (2014)²²⁾	China	–	Cross-sectional	≥18	Iodized table salt	Excess (＞400) Sufficient (100–300) ID (＜100)	Excess/Sufficient/ID (%) 1) Overt hypothyroidism: PR=2.56/1.18/1.05%; p=0.045 2) Subclinical hyperthyroidism: PR=0.58/0.20/1.95%; p=0.003

CI: cumulative incidence, CIR:cumulative incidence rate, Edi:estimated daily iodine intake, f/u: follow-up, ID: iodine deficiency, IR: incidence rate, MUI: median urinary iodine, OR: odds ratio, PR: prevalence rate, UI: urinary iodide, USI: universal salt iodization

Table 3.

Other dietary factors and thyroid diseases

Reference	Country (region)	Study type	Survey year	Dietary factor	Age (years)	Result (OR [95% CI])
Ward et al. (2010)²⁶⁾	US (Iowa)	Cohort	1955–1988	Nitrate (diet & water supplies)	55–69	Dietary nitrate (≤17.4 vs. 41.1 mg nitrate-N/day) 1) Hypothyroidism: OR=1.24 [1.10–1.40]; p=0.001
Carlé et al. (2012)²⁸⁾	Denmark	DanThyr (population based casecontrol)	1997–2001	Alcohol	Female (18–65) Male (60–65)	Overt hypothyroidism (1–10 units/week vs.) 1) 0 unit/week: OR=1.68 [1.02–2.77]; p＜0.05 2) 11–20 units/week: OR=0.46 [0.25–0.83]; p＜0.01 3) ≥21 units/week: OR=0.52 [0.25–1.09]
Effraimidis et al. (2012)³⁰⁾	Netherlands	Amsterdam AITD (nested casecontrol)	–	Alcohol	18–65	Overt hypothyroidism (＞10 units of alcohol/week vs.) 1) Baseline: OR=0.54 [0.14–2.06] 2) 1 year before event: OR=0.23 [0.05–1.04]; p=0.041 3) At event: OR=0.23 [0.05–1.06]; p=0.044
Carlé et al. (2013)²⁹⁾	Denmark	DanThyr (population based casecontrol)	1997–2000	Alcohol	≥18	Grave' s hyperthyroidism (maximum 1–2 units/week vs.) 1) 0 unit/week: OR=1.59 [1.02–2.48]; p＜0.05 2) 3–10 units/week: OR=0.58 [0.40–0.85]; p＜0.01 3) 11–20 units/week: OR=0.57 [0.36–0.91]; p＜0.05 4) ≥21 units/week: OR=0.29 [0.15–0.56]; p＜0.001
Tonstad et al. (2013)²⁷⁾	US, Canada	AHS-2 (longitudinal study)	2002–2008	Plant-based diet	≥30	Omnivore vs. lacto-ovo 1) Hypothyroidism: OR=1.09 [1.01–1.18]

AHS-2: the Adventist Health Study-2, AITD: autoimmune thyroid disease, CI: confidence interval, DanThyr: the Danish Investigation of Iodine Intake and Thyroid Diseases, f/u: follow-up, OR: odds ratio

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