Optimal and safe upper limits of iodine intake for early pregnancy in iodine-sufficient regions: a cross-sectional study of 7190 pregnant women in China

J Clin Endocrinol Metab. 2015 Apr;100(4):1630-8. doi: 10.1210/jc.2014-3704. Epub 2015 Jan 28.


Context: The WHO Technical Consultation recommends urinary iodine concentrations (UIC) from 250 to 499 μg/L as more-than-adequate iodine intake and UIC ≥ 500 μg/L as excessive iodine for pregnant and lactating women, but scientific evidence for this is weak.

Objective: We investigated optimal and safe ranges of iodine intake during early pregnancy in an iodine-sufficient region of China.

Method: Seven thousand one hundred ninety pregnant women at 4-8 weeks gestation were investigated and their UIC, serum thyroid stimulating hormone (TSH), free thyroxine (FT4), thyroid-peroxidase antibody (TPOAb), thyroglobulin antibody (TgAb), and thyroglobulin (Tg) were measured.

Results: The prevalence of overt hypothyroidism was lowest in the group with UIC 150-249 μg/L, which corresponded to the lowest serum Tg concentration (10.18 μg/L). Prevalences of subclinical hypothyroidism (2.4%) and isolated hypothyroxinemia (1.7%) were lower in the group with UIC 150-249 μg/L. Multivariate logistic regression indicated that more-than-adequate iodine intake (UIC 250-499 μg/L) and excessive iodine intake (UIC ≥ 500 μg/L) were associated with a 1.72-fold and a 2.17-fold increased risk of subclinical hypothyroidism, respectively. Meanwhile, excessive iodine intake was associated with a 2.85-fold increased risk of isolated hypothyroxinemia. Moreover, the prevalence of TPOAb positivity and TgAb positivity presented a U-shaped curve, ranging from mild iodine deficiency to iodine excess.

Conclusion: The upper limit of iodine intake during early pregnancy in an iodine-sufficient region should not exceed UIC 250 μg/L, because this is associated with a significantly high risk of subclinical hypothyroidism, and a UIC of 500 μg/L should not be exceeded, as it is associated with a significantly high risk of isolated hypothyroxinemia.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • China / epidemiology
  • Cross-Sectional Studies
  • Female
  • Humans
  • Hypothyroidism / epidemiology*
  • Iodine / administration & dosage*
  • Iodine / adverse effects
  • Iodine / urine
  • Lactation / metabolism
  • Maternal Nutritional Physiological Phenomena*
  • Pregnancy
  • Pregnancy Complications / epidemiology
  • Pregnancy Complications / prevention & control
  • Prevalence
  • Recommended Dietary Allowances*
  • Thyroglobulin / blood
  • Thyrotropin / blood
  • Thyroxine / blood
  • Young Adult


  • Thyrotropin
  • Thyroglobulin
  • Iodine
  • Thyroxine