Introduction: Pregnancy nutrition is crucial to the health of offspring, and some nutrients are closely related to offspring's intelligence. Iodine is particularly noteworthy; this trace element plays a vital role in enhancing offspring intelligence. Besides participating in brain development, it also has important physiological functions, such as promoting growth and development, regulating metabolism, and influencing the function of other organs and systems. The guidelines for the diagnosis and treatment of thyroid diseases during pregnancy and postpartum specifically discuss iodine nutrition during pregnancy.
What is the recommended iodine intake during pregnancy and lactation?
The recommended daily intake of iodine for healthy adults is 150 ug/day.
Iodine requirements increase during pregnancy, and infants obtain their iodine needs from breast milk; therefore, breastfeeding women also need to increase their iodine intake. The WHO recommends an iodine intake of 250 ug/day for pregnant and breastfeeding women, while the Chinese Nutrition Society recommends 230 ug/day for pregnant women and 240 ug/day for breastfeeding women.
Iodine deficiency can lead to a deficiency of thyroid hormones. Studies involving 7,190 pregnant Chinese women have found that the prevalence of subclinical hypothyroidism and hypothyroidism is lowest when the urinary iodine concentration (UIC) is 150-249 ug/L; when the UIC is greater than 250 ug/L, the risk of both diseases increases.
How to supplement iodine scientifically during pregnancy?
Severe iodine deficiency in pregnant women can affect the synthesis of thyroid hormones in both the mother and fetus. Before the implementation of the iodized salt policy, severe iodine deficiency in pregnant women could lead to cognitive decline and cretinism in their offspring. However, with the implementation of the iodized salt policy over the years, cretinism is now rare. Nevertheless, according to the WHO's assessment criteria for iodine nutrition in pregnant women, approximately 50% of pregnant women in China are iodine deficient, with about 60% experiencing mild iodine deficiency. Therefore, iodine nutrition during pregnancy still requires attention.
The "Guidelines for the Diagnosis and Treatment of Thyroid Diseases in Pregnancy and Postpartum" recommends that women preparing for pregnancy, pregnant women, and breastfeeding women ensure a daily iodine intake of at least 250 μg (Recommendation Level A). Different iodine supplementation strategies should be developed based on different regions. In iodine-deficient areas, if iodized salt is consumed daily, no additional iodine supplementation is needed during pregnancy. If iodized salt is not consumed, an additional 150 μg of iodine is required daily during pregnancy. Potassium iodide is the preferred form of iodine supplementation (or a multivitamin containing the same dose of potassium iodide). The optimal time to begin supplementation is at least 3 months before pregnancy (Recommendation Level A).
The "Dietary Guidelines for Pregnant Women (2016)" formulated by the Chinese Nutrition Society recommends that pregnant women, in addition to using iodized salt, should consume iodine-rich seafood, such as kelp and seaweed, 1-2 times per week. Besides these, other foods also contain some iodine. Generally, seafood has a higher iodine content than land-based foods, such as kelp, seaweed, mussels, fresh ribbonfish, dried scallops, dried clams, sea cucumber, jellyfish, and lobster. Animal-based foods have a higher iodine content than plant-based foods, with eggs and dairy products having the highest iodine content, followed by meat and freshwater fish. Fruits and vegetables have the lowest iodine content.
What is the safe upper limit for iodine intake during pregnancy and lactation?
While iodine supplementation during pregnancy is important, this micronutrient is not a case of the more the better. The Chinese Nutrition Society recommends a tolerable upper intake level of iodine of 600 ug/day for pregnant and lactating women. In a normal body, acute iodine overdose produces the Wolff-Chaikoff effect, which reduces the synthesis and release of thyroid hormones. When iodine overdose persists, the body produces an iodine escape response, restoring the synthesis and secretion of thyroid hormones. However, the fetal thyroid's ability to escape the acute iodine-blocking effect does not fully develop until after 36 weeks of gestation, so iodine overdose can easily cause fetal hypothyroidism.
Iodine excess typically originates from iodine-containing medications, such as amiodarone and iodine-containing contrast agents. Surveys in my country from areas with high iodine levels in their water sources show that excessive iodine intake in pregnant women increases the prevalence of subclinical hypothyroidism (SCH) and hyperthyroidism-stimulating hormone (TSH) levels. A survey in iodine-sufficient areas found that excessive iodine intake in early pregnancy increases the risk of hypothyroidism. The "Guidelines for the Diagnosis and Treatment of Thyroid Diseases in Pregnancy and Postpartum" states that daily iodine intake >500 μg during pregnancy and lactation carries a risk of fetal hypothyroidism (Recommendation Level C). Therefore, pregnant women should also be mindful of avoiding excessive iodine intake, especially from iodine-containing medications.
Iodine supplementation during pregnancy should begin early.
Studies suggest that iodine supplementation during early pregnancy in women in areas with mild to moderate iodine deficiency can improve the neurodevelopment of their children. The timing of iodine supplementation is crucial. If supplementation occurs after 12-20 weeks of gestation, the benefits to offspring's neurodevelopment disappear. Furthermore, if iodine-deficient pregnant women are only supplemented with levothyroxine (LT4) without additional iodine, the offspring's intelligence will not show significant improvement. Therefore, a balanced diet should be emphasized from the preconception stage, including seafood (kelp, seaweed, fresh ribbonfish, etc.) 1-2 times per week.
Scientific assessment of iodine nutrition during pregnancy
The WHO's 2007 standards for iodine nutrition during pregnancy and lactation are: (1) Iodine deficiency: urinary iodine concentration (UIC) <150ug/L. (2) Iodine adequacy: UIC 150-249ug/L. (3) Iodine excess: UIC 250-500ug/L. (4) Iodine excess: UIC ≥500ug/L. However, because urine volume and urinary iodine excretion fluctuate during pregnancy, UIC often cannot reflect the true urinary iodine level of pregnant women. The "Guidelines for the Diagnosis and Treatment of Thyroid Diseases in Pregnancy and Postpartum" recommends that when assessing iodine nutrition in pregnant women, the ratio of a single UIC to urinary creatinine (ug/g) is better than a single UIC concentration (ug/L) (Recommendation Level B).
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