Endocrinol Metab > Volume 32(2); 2017 > Article
Lee and Shin: Letter: Thyroid Stimulating Hormone Reference Range and Prevalence of Thyroid Dysfunction in the Korean Population: Korea National Health and Nutrition Examination Survey 2013 to 2015 (Endocrinol Metab 2017;32:106-14, Won Gu Kim et al.)
Clinically, whether a patient has thyroid dysfunction is primarily screened for based on whether his or her thyroid stimulating hormone (TSH) level is within the normal range [1]. Therefore, the validity of the reference range of TSH is very important for ensuring informed clinical judgments. A previous report indicated that the reference range of TSH was higher in areas with high iodine intake [2]. Since iodine intake has been reported to be high in Koreans [3], it seems plausible that the serum TSH reference range would be somewhat high in the Korean population. However, no nationwide study had previously been conducted on the reference range of TSH in Korea.
Recently, Kim et al. [4] evaluated the Korean reference range of serum TSH using Korea National Health and Nutrition Examination Survey (KNHANES) data from 2013 to 2015 and reported that the serum TSH reference range in Korea (0.62 to 6.86 mIU/L) was higher than the corresponding ranges in Western countries. This result was in line with that of a previous study by Kim et al. [5], in which the reference interval for TSH was reported to be 0.73 to 7.06 mIU/L. These results suggest that subclinical hypothyroidism in Korea may be overdiagnosed due to an inappropriately low TSH reference range. Nevertheless, applying the TSH reference values reported in this study directly to real clinical practice must be prudent with the following considerations.
The National Academy of Clinical Biochemistry (NACB) suggested that the serum TSH reference range should be established in rigorously screened euthyroid volunteers who do not have goiter, anti-thyroid peroxidase antibody (TPOAb), or thyroglobulin antibody (TgAb) [1]. The KNHANES data provided information on TPOAb, but information on the results of ultrasonography or TgAb was not available. Therefore, it is possible that the high reference range could be attributed to the unintentional inclusion of persons with mild thyroid dysfunction induced by TPOAb-negative occult thyroiditis in this population. In this study, the mean TSH level was higher in females than in males. In contrast, in a previous United States study on this topic, the gender difference in the mean TSH level, which was significant in the disease-free population, disappeared in the reference population. These discrepancies may also indicate the possibility that mild thyroid dysfunction was not sufficiently excluded in this study [6]. The NACB also suggested that age-related normal reference limits should be used in children [1], but subjects aged 10 to 19 years were analyzed together with adults.
In addition, the results of this study raise another question. If high iodine intake increases the estimated reference value of TSH, can we be sure that this is not because of the increased prevalence of iodine-associated hypothyroidism? Excess iodine intake may lead to hypothyroidism and autoimmune thyroiditis [7], and because we do not have a perfect way to screen for thyroid dysfunction, if the prevalence of thyroid dysfunction in the population is high, the proportion of misclassified subjects with occult thyroid dysfunction may also be high, even in the selected reference population.
Therefore, although this study is valuable and important for understanding the patterns of thyroid hormone profiles in Korea, its results should be interpreted with caution. If we cannot suggest a complete explanation for the reason for the higher cut-off levels in the TSH reference interval of the Korean population, further studies with a more thorough exclusion of occult thyroid disease will be required to “re-define” the normal TSH level in Korea.

NOTES

CONFLICTS OF INTEREST: No potential conflict of interest relevant to this article was reported.

REFERENCES

1. Baloch Z, Carayon P, Conte-Devolx B, Demers LM, Feldt-Rasmussen U, Henry JF, et al. Laboratory medicine practice guidelines. Laboratory support for the diagnosis and monitoring of thyroid disease. Thyroid 2003;13:3-126.
[CROSSREF]  [PUBMED] 
2. Guan H, Shan Z, Teng X, Li Y, Teng D, Jin Y, et al. 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. Clin Endocrinol (Oxf) 2008;69:136-141.
[CROSSREF]  [PUBMED] 
3. Han MR, Ju DL, Park YJ, Paik HY, Song Y. An iodine database for common Korean foods and the association between iodine intake and thyroid disease in Korean adults. Int J Thyroidol 2015;8:170-182.
[CROSSREF]  [PDF]
4. Kim WG, Kim WB, Woo G, Kim H, Cho Y, Kim TY, et al. Thyroid stimulating hormone reference range and prevalence of thyroid dysfunction in the Korean population: Korea National Health and Nutrition Examination Survey 2013 to 2015. Endocrinol Metab (Seoul) 2017;32:106-114.
[CROSSREF]  [PUBMED]  [PMC] 
5. Kim M, Kim TY, Kim SH, Lee Y, Park SY, Kim HD, et al. Reference interval for thyrotropin in a ultrasonography screened Korean population. Korean J Intern Med 2015;30:335-344.
[CROSSREF]  [PUBMED]  [PMC]  [PDF]
6. Hollowell JG, Staehling NW, Flanders WD, Hannon WH, Gunter EW, Spencer CA, et al. Serum TSH, T(4), and thyroid antibodies in the United States population (1988 to 1994): National Health and Nutrition Examination Survey (NHANES III). J Clin Endocrinol Metab 2002;87:489-499.
[CROSSREF]  [PUBMED] 
7. Teng W, Shan Z, Teng X, Guan H, Li Y, Teng D, et al. Effect of iodine intake on thyroid diseases in China. N Engl J Med 2006;354:2783-2793.
[CROSSREF] 


Editorial Office
101-2503, Lotte Castle President, 109 Mapo-daero, Mapo-gu, Seoul 04146, Korea​
Tel: +82-2-716-2428    Fax: +82-2-714-5103    E-mail: journal@endocrinology.or.kr                

Copyright © 2020 by Korean Endocrine Society. All rights reserved.

Developed in M2community

Close layer
prev next