Thyroid strain in pregnancy: a nationwide study of iodine nutrition and thyroglobulin among Faroese pregnant women

TL;DR

A study of iodine levels in pregnant women in the Faroe Islands found thyroid strain only at very low iodine levels, suggesting current guidelines may be too strict.

Contribution

The study provides new evidence that thyroid strain in pregnant women occurs only when iodine levels are extremely low, challenging current WHO recommendations.

Findings

Thyroglobulin levels increased only in pregnant women with urinary iodine concentration (UIC) below 50 µg/L.

Thyroid-stimulating hormone (TSH) increased with higher UIC, indicating a possible inverse relationship.

No elevated TSH was observed in any participants, suggesting overall normal thyroid function.

Abstract

Abnormal thyroid function is particularly problematic in pregnant women. Iodine is important to maintain normal thyroid function, and the World Health Organization (WHO) recommends a raised iodine intake in pregnant compared with non-pregnant adults. The raised iodine intake level from 100 to 150 µg/L includes a safety margin, and we hypothesized that the thyroid is not strained until urinary iodine concentration (UIC) is below 100 μg/L.

Nationwide cross-sectional study.

Routine prenatal care at the National Hospital System of the Faroe Islands, 2020-2022.

A total of 623 pregnant women, representing 63% of all pregnancies in the Faroe Islands during the study period, with no known thyroid disease.

Iodine-containining dietary intake was assessed indirectly through urinary iodine concentration (UIC) measured on spot urine using the Sandell–Kolthoff reaction.

The primary outcomes were serum Thyroglobulin (s-Tg) and thyrotropin (TSH) concentration serving as indicators of potential thyroid strain and thyroid function.

UIC and TSH was measured in all participants. sS-Tg was measured in a randomly selected subset of 236 participants.

Women were seen in median gestational week 20. None had elevated TSH; the median UIC was 108 µg/L, and the median s-Tg was 10.3 µg/L. Serum Tg differed only for the group with UIC below 50 µg/L (P = .02), but not when UIC was above 50 µg/L. TSH increased with higher UIC (P < .001).

Thyroglobulin levels increased only in the group of Faroese pregnant women with UIC below 50 µg/L, indicating that strain on the thyroid gland was seen with low UIC levels parallel to that of non-pregnant adults. Our results suggest that the UIC limit recommended in pregnancy may be overly strict and warrant reconsideration to balance health efficacy.

Graphical Abstract

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