Long-term iodised salt and thyroid failure in pregnancy
What it found
In 100 pregnant women in a mildly iodine-deficient area of Sicily, those who had used iodised salt for at least 2 years before conceiving were much less likely to develop maternal thyroid failure than those who started when they became pregnant.
The study followed the women through all three trimesters.
What they found
Studies in people
Thyroid failure risk
Over the whole study, 36.8% of short-term women developed maternal thyroid failure compared with 6.4% of long-term women. The authors report this as a relative risk of 5.7, meaning the short-term group had almost 6 times the risk.
Timing of thyroid failure
In the short-term group, thyroid failure appeared as early as the first trimester and continued through the second and third. In the long-term group, it occurred almost only in late pregnancy and only as isolated hypothyroxinemia.
Thyroid hormone levels
Long-term women had consistently higher free T4 and lower TSH levels throughout pregnancy than short-term women. Their median urinary iodine was 115 micrograms per litre, consistent with an estimated daily intake of about 190 micrograms.
Type of thyroid failure
Of the 18 women who developed maternal thyroid failure, 16 had isolated hypothyroxinemia, meaning their TSH stayed within the normal range but their free T4 dropped below it.
Risk reduction
The authors calculated that long-term iodised salt use reduced the risk of maternal thyroid failure by 82.5% compared with starting during pregnancy.
What the authors conclude
“Prolonged iodized salt significantly improves maternal thyroid economy and reduces the risk of maternal thyroid insufficiency during gestation, probably because of a nearly restoring intrathyroidal iodine stores.”
Also in their conclusions
- They conclude that prolonged iodised salt use significantly improves maternal thyroid function and reduces the risk of maternal thyroid insufficiency during pregnancy.
- They say this is probably because it nearly restores the thyroid's iodine stores.
- They advise that women in countries with low iodine intake should be screened and their free T4 monitored early in pregnancy so treatment can start promptly if needed.
How it was done
Mild to moderate iodine deficiency during pregnancy can cause short-term underactive thyroid in the mother and impaired mental development in the child. In Europe, fewer than half of pregnant women take iodine supplements, so iodised salt is the main iodine source for most of them.
They followed 100 pregnant women from a mildly iodine-deficient area in northeastern Sicily. Sixty-two had used iodised salt regularly for at least 2 years before pregnancy (long-term), and 38 started using it when they became pregnant (short-term). Thyroid function was checked twice in each trimester.
What it can’t tell you
- This study followed women over time, so it can show a link between long-term iodised salt use and lower thyroid failure risk, but it cannot prove that iodised salt alone caused the difference.
- The women were not randomly assigned to long-term or short-term salt use, so other differences between the groups could have played a role.
- The findings come from one mildly iodine-deficient area in Sicily and may not apply to women in other regions or with different iodine intake.
Who paid
- Authors work at
- Policlinico Universitario, Italy
The paper
- Title
- Iodine prophylaxis using iodized salt and risk of maternal thyroid failure in conditions of mild iodine deficiency
- Type
- Study
- Evidence
- Studies in people
- Summarised from
- Full text
- Cite
- Moleti M, Lo Presti VP, Campolo MC, et al (2008). Iodine prophylaxis using iodized salt and risk of maternal thyroid failure in conditions of mild iodine deficiency. The Journal of clinical endocrinology and metabolism. doi:10.1210/jc.2008-0352Free full textPubMed 18413422DOI
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