Professor Tam Wing Hung
Department of Obstetrics and Gynecology
The Chinese University of Hong Kong
- Why is iodine important for pregnant women?
- What is the prevalence of iodine deficiency among pregnant women in Hong Kong?
- How can healthcare professionals identify pregnant women with iodine deficiency
- What are the causes of inadequate iodine intake in local pregnant women?
- What dietary advice should healthcare professionals give to women planning for pregnancy and during pregnancy to ensure sufficient iodine intake?
- How common are thyroid disorders among pregnant women in Hong Kong?
- Should pregnant women with thyroid problems avoid foods rich in iodine and nutritional supplements?
- What other nutritional problems do you think local pregnant women may have?
- Practical tips for healthcare professionals
TWH: Professor Tam Wing Hung
R: Why is iodine important for pregnant women?
TWH: Iodine is a micronutrient that is needed for thyroid hormone synthesis4. These hormones play a pivotal role in growth and development, especially for the brain during the fetal period and the first two years of life5,6. Deficiency of thyroid hormones during pregnancy can impair fetal neurodevelopment and may have a long-term impact on children’s brain development. In severely iodine-deficient communities, cretinism, stunted growth and mental retardation can occur in children7.
R: What is the prevalence of iodine deficiency among pregnant women in Hong Kong?
TWH: The World Health Organization (WHO) recommended daily iodine intake of 250 μg for pregnant women8. We recently conducted a study on 600 local pregnant women and investigated their iodine status during early gestation. Among mothers who completed a food frequency questionnaire, the median daily iodine intake was 69.5 μg, and 84% of them had an insufficient dietary iodine intake.
Moreover, the median urinary iodine concentration (UIC) of the study population was 100 μg/L and was below the WHO definition 150 μg/L for iodine sufficiency. Based on the UIC, 70% of the subjects can be classified as iodine-deficient. However, the median UIC of those who had taken iodine-containing supplements was above the WHO recommended level. Nonetheless, only 27% of the women were taking iodine containing nutritional supplements. The findings suggest that most of our local pregnant women are not aware of the importance of iodine intake in their diet during early pregnancy8,9. In fact, this is not a new finding but has been known for 20 years10.
R: How can healthcare professionals identify pregnant women with iodine deficiency?
TWH: The median UIC is a good indicator of iodine status in a population, but this is not meant for individuals as it can vary from time to time. The best way to identify iodine deficiency is to have dietary assessment by dietitians and nutritionists.
R: What are the causes of inadequate iodine intake in local pregnant women?
TWH: It is likely attributed to inadequate public awareness and a misconception that we are living near the coast with plenty of seafood available so our diet should have adequate iodine. Only certain kinds of foods like seaweed, fish and iodized salt are rich in iodine but most mothers are not taking enough of them (Table 1).
In fact, many developed countries have the policy of salt iodization. Mainland China has also introduced this policy since early 90’s.
Table 1. Iodine content in different food groups
|Food||Portion||Mean iodine content|
|Seaweeds11||100 g||46,000 μg|
|Prepackaged seaweed snacks11||1 pack (2.5 g)||85 μg|
|Iodized table salt11||100 g||3,000 μg|
|1 teaspoon (5 g)||150 μg|
|Crustaceans and mollusks11||100 g||97 μg|
|Eggs and egg products11||100 g||49 μg|
|Milk and milk products including frozen confections11||100 g||34 μg|
|Fish11||100 g||19 μg|
|Sashimi and sushi11||100 g||8.6 μg|
Table reformatted from Risk Assessment Studies Report11
R: What dietary advice should healthcare professionals give to women planning for pregnancy and during pregnancy to ensure sufficient iodine intake?
TWH: I would suggest a healthy eating habit and including natural foods rich in iodine and choosing iodized salt. Antenatal classes should emphasize on diet and nutrition, and these topics should be covered by experienced nutritionists and dietitians. It is also important to build up sufficient iodine storage some time before pregnancy. In other words, we shouldaim for adequate dietary iodine for all women at their reproductive age.
R: How common are thyroid disorders among pregnant women in Hong Kong?
TWH: Thyroid disease is the second most common endocrine problem affecting local pregnant women. About 2-5% of our pregnant population has thyroid problems during pregnancy, either hyperthyroidism (mostly from Grave’s disease) or hypothyroidism (from previous thyroidectomy, radioactive iodine and autoimmune thyroiditis).
Hyperemesis gravidarum associated with the high human chorionic gonadotropin also commonly causes transient hyperthyroxemia that is usually self-limiting.
R: Should pregnant women with thyroid problems avoid foods rich in iodine and nutritional supplements?
TWH: In mothers who have hyperthyroidism not under control, iodinerich foods like seaweed and iodized salt should be avoided and iodine-containing nutritional supplements should be withheld. Once the disease is under control, they should resume adequate iodine intake. Given the fact that our pregnant population is iodine deficient, it is absolutely unnecessary to advise on an iodinedepleted diet to any women with hypothyroidism or well controlled hyperthyroidism.
R: What other nutritional problems do you think local pregnant women may have?
TWH: Iron deficiency is another common problem. The British Society for Haematology defines iron deficiency as ferritin level <15 μg/L and recommends women to take iron supplements for the whole pregnancy when ferritin level is <30 μg/L12. In a recent local study, the mean ferritin levels of mothers without iron supplements were 21.87 μg/L and 21.85 μg/L at 28 weeks and 36 weeks respectively13. This is consistent with our clinical experience.
Our preliminary data from a recent study also showed that pregnant women in Hong Kong tend to have high-fat but low-fiber diet14. In another ongoing study on vitamin D status, local mothers had low levels of vitamin D but the main determining factor is the exposure to sunlight rather than a dietary problem. Obviously, maternal blood vitamin D levels are lower in winter.
Last but not the least, it is also important for lactating mothers to have sufficient micronutrients, especially for those mothers who are exclusively breastfeeding.
Practical tips for healthcare professionals
- Krehl WA. Health Values 1983;7:9-13.
- Diemert A, et al. BMC Pregnancy Childbirth 2016;16:224.
- Kedir H, et al. J Nutr Metab 2014;2014:878926.
- Leung AM, Braverman LE. Nat Rev Endocrinol 2014;10:136-142.
- Alemu A, et al. Int J Reprod Biomed (Yazd) 2016;14:677-686.
- Porterfield SP. Environ Health Perspect 2000;108 Suppl 3:433-438.
- Skeaff SA. Nutrients 2011;3:265-273.
- World Health Organization. Assessment of Iodine Deficiency Disorders and Monitoring Their Elimination. A Guide for Programme Managers. 3rd Edition. Available at: http://www.who.int/nutrition/publications/micronutrients/iodine_deficiency0/9789241595827/en/ (Accessed on 2 March 2018).
- Tam WH, et al. Hong Kong Med J 2017;23:586-593.
- Kung AW, et al. Clin Endocrinol (Oxf ) 1997;46:315-319.
- Centre for Food Safety, Food and Environmental Hygiene Department, HKSAR. Risk Assessment Studies Report - Dietary Iodine Intake in Hong Kong Adults. July 2011
- Pavord S, et al. Br J Haematol 2012;156:588-600.
- Chan KK, et al. BJOG 2009;116:789-798; discussion 797-788.
- Chan R, et al. Poster presented at 12th Asian Congress of Nutrition, Yokohama, Japan; 14-18 May 2015. Abstract #229.